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  • 1.
    Albertsson, Dagur Ingi
    et al.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektronik, Integrerade komponenter och kretsar.
    Zahedinejad, Mohammad
    Department of Physics, University of Gothenburg.
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Department of Physics, University of Gothenburg.
    Rodriguez, Saul
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektronik, Integrerade komponenter och kretsar.
    Rusu, Ana
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektronik, Integrerade komponenter och kretsar.
    Compact Macrospin-Based Model of Three-Terminal Spin-Hall Nano Oscillators2019Ingår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 55, nr 10, artikel-id 4003808Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Emerging spin-torque nano oscillators (STNOs) and spin-Hall nano oscillators (SHNOs) are potential candidates for microwave applications. Recent advances in three-terminal magnetic tunnel junction (MTJ)-based SHNOs opened the possibility to develop more reliable and well-controlled oscillators, thanks to individual spin Hall-driven precession excitation and read-out paths. To develop hybrid systems by integrating three-terminal SHNOs and CMOS circuits, an electrical model able to capture the analog characteristics of three-terminal SHNOs is needed. This model needs to be compatible with current electric design automation (EDA) tools. This work presents a comprehensive macrospin-based model of three-terminal SHNOs able to describe the dc operating point, frequency modulation, phase noise, and output power. Moreover, the effect of voltage-controlled magnetic anisotropy (VCMA) is included. The model shows good agreement with experimental measurements and could be used in developing hybrid three-terminal SHNO/CMOS systems.

  • 2. Awad, A. A.
    et al.
    Durrenfeld, P.
    Houshang, A.
    Dvornik, M.
    Iacocca, E.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Long-range mutual synchronization of spin Hall nano-oscillators2017Ingår i: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 13, nr 3, s. 292-+Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The spin Hall effect in a non-magnetic metal with spin-orbit coupling injects transverse spin currents into adjacent magnetic layers, where the resulting spin transfer torque can drive spin wave auto-oscillations. Such spin Hall nano-oscillators (SHNOs) hold great promise as extremely compact and broadband microwave signal generators and magnonic spin wave injectors. Here we show that SHNOs can also be mutually synchronized with unprecedented efficiency. We demonstrate mutual synchronization of up to nine individual SHNOs, each separated by 300 nm. Through further tailoring of the connection regions we can extend the synchronization range to 4 mu m. The mutual synchronization is observed electrically as an increase in the power and coherence of the microwave signal, and confirmed optically using micro-Brillouin light scattering microscopy as two spin wave regions sharing the same spectral content, in agreement with our micromagnetic simulations.

  • 3. Balinsky, Michael
    et al.
    Ranjbar, Mojtaba
    Haidar, Mohammad
    Durrenfeld, Philipp
    Khartsev, Sergiy
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Slavin, Andrei
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Dumas, Randy K.
    Spin Pumping and the Inverse Spin-Hall Effect via Magnetostatic Surface Spin-Wave Modes in Yttrium-Iron Garnet/Platinum Bilayers2015Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 6, nr 3000604Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Spin pumping at a boundary between a yttrium-iron garnet (YIG) film and a thin platinum (Pt) layer is studied under conditions in which a magnetostatic surface spin wave (MSSW, or Damon-Eshbach mode) is excited in YIG by a narrow strip-line antenna. It is shown that the voltage created by the inverse spin-Hall effect (ISHE) in Pt is strongly dependent on the wavevector of the excited MSSW. For YIG film thicknesses of 41 and 0.9 mu m, the maximum ISHE voltage corresponds to the maximum of efficiently excited MSSW wavevectors and does not coincide with the maximum of absorbed microwave power. For a thinner (0.175 mu m) YIG film, the maximum of the ISHE voltage moves closer to the ferromagnetic resonance and almost coincides with the region of the maximum microwave absorption. We show that the effect is related to the change in the thickness profile and the wavenumber spectrum of the excited MSSW taking place when the YIG film thickness is increased.

  • 4.
    Banuazizi, S. Amir Hossein
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Microwave probe stations with throw-dimensional control of the magnetic field to study high-frequency dynamic in nanoscale devices2018Ingår i: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 89, nr 6, artikel-id 064701Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present two microwave probe stations with motorized rotary stages for adjusting the magnitude and angle of the applied magnetic field. In the first system, the magnetic field is provided by an electromagnet and can be adjusted from 0 to similar to 1.4 T while its polar angle (theta) can be varied from 0 degrees to 360 degrees. In the second system the magnetic field is provided by a Halbach array permanent magnet, which can be rotated and translated to cover the full range of polar (theta) and azimuthal (phi) angles with a tunable field magnitude up to similar to 1 T. Both systems are equipped with microwave probes, bias-Ts, amplifiers, and spectrum analyzers, to allow for microwave characterization up to 40 GHz, as well as software to automatically perform continuous large sets of electrical and microwave measurements.

  • 5.
    Banuazizi, Seyed Amir Hossein
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Sani, Sohrab R.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar.
    Naiini, Maziar M.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar.
    Mohseni, Seyed Majid
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg, Sweden.
    Durrenfeld, Philipp
    Malm, B. Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg, Sweden.
    Order of magnitude improvement of nano-contact spin torque nano-oscillator performance2017Ingår i: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 9, nr 5, s. 1896-1900Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Spin torque nano-oscillators (STNO) represent a unique class of nano-scale microwave signal generators and offer a combination of intriguing properties, such as nano sized footprint, ultrafast modulation rates, and highly tunable microwave frequencies from 100 MHz to close to 100 GHz. However, their low output power and relatively high threshold current still limit their applicability and must be improved. In this study, we investigate the influence of the bottom Cu electrode thickness (t(Cu)) in nano-contact STNOs based on Co/Cu/NiFe GMR stacks and with nano-contact diameters ranging from 60 to 500 nm. Increasing t(Cu) from 10 to 70 nm results in a 40% reduction of the threshold current, an order of magnitude higher microwave output power, and close to two orders of magnitude better power conversion efficiency. Numerical simulations of the current distribution suggest that these dramatic improvements originate from a strongly reduced lateral current spread in the magneto-dynamically active region.

  • 6.
    Banuazizi, Seyed
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Sani, S. R.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar.
    Naiini, Maziar Manouchehry
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar.
    Mohseni, S.
    Chung, S.
    Dürrenfeld, P.
    Malm, B. Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Order of magnitude improvement of nano-contact spin torque nano-oscillator performance2017Ingår i: 2017 IEEE International Magnetics Conference, INTERMAG 2017, Institute of Electrical and Electronics Engineers (IEEE), 2017, artikel-id 8007567Konferensbidrag (Refereegranskat)
    Abstract [en]

    Spin torque nano-oscillators [1,2] (STNO) represent a unique class of nano-scale microwave signal generators where spin transfer torque [3-5] (STT) from a direct spin-polarized current drives and controls the auto-oscillation of the local free layer magnetization, which through its oscillating magnetoresistance transforms the direct current into a tunable microwave voltage.

  • 7.
    Burgos-Parra, E.
    et al.
    Univ Exeter, Coll Engn Math & Phys Sci, Exeter EX4 4QL, Devon, England..
    Bukin, N.
    Univ Exeter, Coll Engn Math & Phys Sci, Exeter EX4 4QL, Devon, England..
    Redjai Sani, Sohrab
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Figueroa, A. I.
    Diamond Light Source, Magnet Spect Grp, Didcot, Oxon, England..
    Beutier, G.
    Univ Grenoble Alpes, CNRS, Genoble INP, SIMaP, Grenoble, France..
    Dupraz, M.
    Univ Grenoble Alpes, CNRS, Genoble INP, SIMaP, Grenoble, France..
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;Uppsala Univ, Dept Phys & Astron, S-75120 Uppsala, Sweden..
    Duerrenfeld, P.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Le, Q. Tuan
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;Uppsala Univ, Dept Phys & Astron, S-75120 Uppsala, Sweden..
    Mohseni, S. M.
    Shahid Beheshti Univ, Fac Phys, Tehran 19839, Iran..
    Houshang, A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, Electrum 205, S-16440 Kista, Sweden..
    Cavill, S. A.
    Univ York, Dept Phys, York YO10 5DD, N Yorkshire, England..
    Hicken, R. J.
    Univ Exeter, Coll Engn Math & Phys Sci, Exeter EX4 4QL, Devon, England..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, Electrum 205, S-16440 Kista, Sweden..
    van der Laan, G.
    Diamond Light Source, Magnet Spect Grp, Didcot, Oxon, England..
    Ogrin, F. Y.
    Univ Exeter, Coll Engn Math & Phys Sci, Exeter EX4 4QL, Devon, England..
    Investigation of magnetic droplet solitons using x-ray holography with extended references2018Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, artikel-id 11533Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A dissipative magnetic soliton, or magnetic droplet, is a structure that has been predicted to exist within a thin magnetic layer when non-linearity is balanced by dispersion, and a driving force counteracts the inherent damping of the spin precession. Such a soliton can be formed beneath a nano-contact (NC) that delivers a large spin-polarized current density into a magnetic layer with perpendicular magnetic anisotropy. Although the existence of droplets has been confirmed from electrical measurements and by micromagnetic simulations, only a few attempts have been made to directly observe the magnetic landscape that sustains these structures, and then only for a restricted set of experimental parameter values. In this work we use and x-ray holography technique HERALDO, to image the magnetic structure of the [ Co/ Ni] x4 multilayer within a NC orthogonal pseudo spin-valve, for different range of magnetic fields and injected electric currents. The magnetic configuration imaged at -33 mA and 0.3 T for devices with 90 nm NC diameter reveals a structure that is within the range of current where the droplet soliton exist based on our electrical measurements and have it is consistent with the expected size of the droplet (similar to 100 nm diameter) and its spatial position within the sample. We also report the magnetisation configurations observed at lower DC currents in the presence of fields (0-50 mT), where it is expected to observe regimes of the unstable droplet formation.

  • 8.
    Chen, Tingsu
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Rodriguez, Saul
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Malm, B. Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Rusu, Ana
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Integration of GMR-based spin torque oscillators and CMOS circuitry2015Ingår i: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 111, s. 91-99Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper demonstrates the integration of giant magnetoresistance (GMR) spin torque oscillators (STO) with dedicated high frequency CMOS circuits. The wire-bonding-based integration approach is employed in this work, since it allows easy implementation, measurement and replacement. A GMR STO is wire-bonded to the dedicated CMOS integrated circuit (IC) mounted on a PCB, forming a (GMR STO + CMOS IC) pair. The GMR STO has a lateral size of 70 nm and more than an octave of tunability in the microwave frequency range. The proposed CMOS IC provides the necessary bias-tee for the GMR STO, as well as electrostatic discharge (ESD) protection and wideband amplification targeting high frequency GMR STO-based applications. It is implemented in a 65 nm CMOS process, offers a measured gain of 12 dB, while consuming only 14.3 mW and taking a total silicon area of 0.329 mm2. The measurement results show that the (GMR STO + CMOS IC) pair has a wide tunability range from 8 GHz to 16.5 GHz and improves the output power of the GMR STO by about 10 dB. This GMR STO-CMOS integration eliminates wave reflections during the signal transmission and therefore exhibits good potential for developing high frequency GMR STO-based applications, which combine the features of CMOS and STO technologies.

  • 9.
    Chung, Sunjae
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Department of Physics, University of Gothenburg.
    Jiang, Sheng
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Eklund, Anders
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektronik, Integrerade komponenter och kretsar.
    Iacocca, Ezio
    Department of Applied Mathematics, University of Colorado.
    Le, Quang Tuan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Mazraati, Hamid
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Mohseni, Seyed Majid
    Department of Physics, Shahid Beheshti University, Tehran 19839, Iran.
    Sani, Sohrab Redjai
    Department of Physics and Astronomy, Uppsala University,.
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Effect of canted magnetic field on magnetic droplet nucleation boundariesManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    The influence on magnetic droplet nucleation boundaries by canted magnetic elds are investigated and reported. The nucleation boundary condition, In = αAH + BH + C, is determined at different canted angles (0°< θH<20°) using magnetoresistance (MR) and microwave measurements in nanocontact spintorque oscillators (NC-STOs). As θH increased, the nucleation boundary shifts gradually towards higher In and H. The coefficient B of the nucleation boundary equation also nearly doubled as θH increases. On theother hand, the coefficient αA remained constant for all values of θH. These observations can be explained by considering the drift instability of magnetic droplets and the different tilt behaviour of the Co fixed layer induced by different θH.

  • 10.
    Chung, Sunjae
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;Uppsala Univ, Dept Phys & Astron, S-75120 Uppsala, Sweden..
    Le, Q. Tuan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Ahlberg, Martina
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden..
    Awad, Ahmad A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden..
    Weigand, Markus
    Max Planck Inst Intelligent Syst, D-70569 Stuttgart, Germany..
    Bykova, Iuliia
    Max Planck Inst Intelligent Syst, D-70569 Stuttgart, Germany..
    Khymyn, Roman
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Dvornik, Mykola
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Mazraati, Hamid
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. NanOsc AB, S-16440 Kista, Sweden..
    Houshang, Afshin
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden..
    Jiang, Sheng
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Anh Nguyen, T. N.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.; Vietnam Acad Sci & Technol, Inst Mat Sci, Lab Magnetism & Superconduct, 18 Hoang Quoc Viet, Hanoi 122300, Vietnam..
    Goering, Eberhard
    Max Planck Inst Intelligent Syst, D-70569 Stuttgart, Germany..
    Schuetz, Gisela
    Max Planck Inst Intelligent Syst, D-70569 Stuttgart, Germany..
    Graefe, Joachim
    Max Planck Inst Intelligent Syst, D-70569 Stuttgart, Germany..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden..
    Direct Observation of Zhang-Li Torque Expansion of Magnetic Droplet Solitons2018Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 120, nr 21, artikel-id 217204Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Magnetic droplets are nontopological dynamical soli tons that can be nucleated in nanocontact based spin torque nano-oscillators (STNOs) with perpendicular magnetic anisotropy free layers. While theory predicts that the droplet should be of the same size as the nanocontact, its inherent drift instability has thwarted attempts at observing it directly using microscopy techniques. Here, we demonstrate highly stable magnetic droplets in all-perpendicular STNOs and present the first detailed droplet images using scanning transmission X-ray microscopy. In contrast to theoretical predictions, we find that the droplet diameter is about twice as large as the nanocontact. By extending the original droplet theory to properly account for the lateral current spread underneath the nanocontact, we show that the large discrepancy primarily arises from current-in-plane Zhang-Li torque adding an outward pressure on the droplet perimeter. Electrical measurements on droplets nucleated using a reversed current in the antiparallel state corroborate this picture.

  • 11. Durrenfeld, P.
    et al.
    Gerhard, F.
    Mohseni, S. M.
    Ranjbar, M.
    Sani, S. R.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Gould, C.
    Molenkamp, L. W.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Low-current, narrow-linewidth microwave signal generation in NiMnSb based single-layer nanocontact spin-torque oscillators2016Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 109, nr 22, artikel-id 222403Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on the fabrication of nano-contact spin-torque oscillators based on single layers of the epitaxially grown half-metal NiMnSb with ultralow spin wave damping. We demonstrate magnetization auto-oscillations at microwave frequencies in the 1-3 GHz range in out-of-plane magnetic fields. Threshold current densities as low as 3 x 10(11) A m(-2) are observed as well as minimum oscillation linewidths of 200 kHz, both of which are much lower than the values achieved in conventional metallic spin-valve-based devices of comparable dimensions. These results enable the fabrication of spin transfer torque driven magnonic devices with low current density requirements, improved signal linewidths, and in a simplified single-layer geometry. Published by AIP Publishing.

  • 12. Durrenfeld, Philipp
    et al.
    Xu, Yongbing
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Department of Physics, University of Gothenburg, .
    Zhou, Yan
    Controlled skyrmion nucleation in extended magnetic layers using a nanocontact geometry2017Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, nr 5, artikel-id 054430Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We propose and numerically simulate a spintronic device layout consisting of a nanocontact on top of an extended Co/Pt bilayer. The interfacial Dzyaloshinskii-Moriya interaction in such bilayer systems can lead to the possible existence of metastable skyrmions. A small dc current injected through the nanocontact enables the manipulation of the size as well as the annihilation of an initially present skyrmion, while ps-long current pulses allow for the controlled nucleation of single skyrmions underneath the nanocontact. The results are obtained from micromagnetic simulations and can be potentially used for future magnetic storage implementations.

  • 13. Dvornik, Mykola
    et al.
    Awad, Ahmad A.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Origin of Magnetization Auto-Oscillations in Constriction-Based Spin Hall Nano-Oscillators2018Ingår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 9, nr 1, artikel-id 014017Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use micromagnetic simulations to map out and compare the linear and auto-oscillating modes in constriction-based spin Hall nano-oscillators as a function of the applied magnetic field with a varying magnitude and out-of-plane angle. We demonstrate that, for all possible applied field configurations, the auto-oscillations emerge from the localized linear modes of the constriction. For field directions tending towards the plane, these modes are of the so-called edge type, i.e., localized at the opposite edges of the constriction. By contrast, when the magnetization direction approaches the film normal, the modes transform to the so-called bulk type, i.e., localized inside the constriction with substantially increased precession volume, consistent with the redistribution of the magnetic charges from the edges to the top and bottom surfaces of the constriction. In general, the threshold current of the corresponding auto-oscillations increases with the applied field strength and decreases with its out-of-plane angle, consistent with the behavior of the internal field and in good agreement with a macrospin model. A quantitative agreement is then achieved by taking into account the strongly nonuniform character of the system via a mean-field approximation. Both the Oersted (Oe) field and the spin-transfer torque from the drive current increase the localization and decrease the frequency of the observed mode. Furthermore, the antisymmetric Oe field breaks the lateral symmetry, favoring the localized mode at one of the two constriction edges, particularly for large out-of-plane field angles where the threshold current is significantly increased and the edge demagnetization is suppressed.

  • 14. Dürrenfeld, P.
    et al.
    Gerhard, F.
    Ranjbar, M.
    Gould, C.
    Molenkamp, L. W.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Spin Hall effect-controlled magnetization dynamics in NiMnSb2015Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, nr 17, artikel-id 17E103Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the influence of a spin current generated from a platinum layer on the ferromagnetic resonance (FMR) properties of an adjacent ferromagnetic layer composed of the halfmetallic half-Heusler material NiMnSb. Spin Hall nano-oscillator devices are fabricated, and the technique of spin torque FMR is used to locally study the magnetic properties as in-plane anisotropies and resonance fields. A change in the FMR linewidth, in accordance with the additional spin torque produced by the spin Hall effect, is present for an applied dc current. For sufficiently large currents, this should yield auto-oscillations, which however are not achievable in the present device geometry.

  • 15. Dürrenfeld, P.
    et al.
    Iacocca, E.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Muduli, P. K.
    Modulation-mediated unlocking of a parametrically phase-locked spin torque oscillator2014Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, nr 25, s. 252404-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Modulation of an oscillator is crucial for its application in communication devices. While the output power and linewidth of single magnetic tunnel junction-based spin-torque oscillators (MTJ-STO) are not yet adequate for practical uses, the synchronization of such devices can overcome these limitations. Here, we investigate the modulation behavior of a parametrically synchronized MTJ-STO and show experimentally that modulation of the synchronized state preserves synchronization as long as the modulation frequency, integral(mod), is above a characteristic frequency, integral(unlock). The unlocking frequency increases with the modulation amplitude in agreement with analytical estimates and numerical simulations. These phenomena are described as a non-resonant unlocking mechanism, whose characteristics are directly related to inherent parameters of the oscillator.

  • 16.
    Fazlali, Masoumeh
    et al.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Banuazizi, S. Amir Hossein
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Ahlberg, Martina
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Dvornik, Mykola
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Sani, Sohrab R.
    MIT, Dept Mat Sci & Engn, Cambridge, MA 02139 USA..
    Mohseni, Seyed Majid
    Shahid Beheshti Univ, Dept Phys, Tehran 19839, Iran..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Tuning exchange-dominated spin-waves using lateral current spread in nanocontact spin-torque nano-oscillators2019Ingår i: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 492, artikel-id UNSP 165503Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an efficient method to tailor propagating spin waves in quasi-confined systems. We use nanocontact spin-torque nano-oscillators based on NiFe/Cu/Co spin-valves and study the ferromagnetic and spin-wave resonances (FMR and SWR) of both layers. We employ homodyne-detected ferromagnetic resonance spectroscopy, resembling spin-torque FMR, to detect the magnetodynamics. The external field is applied in-plane, giving a parallel configuration of the magnetic layers, which do not provide any spin-transfer torque. Instead, the excitation is caused by the Oersted field. By varying the thickness of the bottom Cu electrode (t(Cu)) of the devices, we tune the current distribution in the samples, and thereby the Oersted field, which governs the spin wave characteristics. Both the average k-vector and the bandwidth of the SWR increases as t(Cu) increases.

  • 17. Franco, A. F.
    et al.
    Gonzalez-Fuentes, C.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Garcia, C.
    Anisotropy constant and exchange coupling strength of perpendicularly magnetized CoFeB/Pd multilayers and exchange springs2017Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, nr 14, artikel-id 144417Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A model describing the ferromagnetic resonance of multilayer structures is used to characterize the interface anisotropy constant and interlayer exchange coupling strength associated to individual components of [CoFeB/Pd](n) multilayers with perpendicular magnetic anisotropy and [CoFeB/Pd](5)/(CoFeB or Co) exchange spring structures by comparing with ferromagnetic resonance behavior measurements. We find that the effective perpendicular anisotropy increases with the number of repetitions of the multilayer, which we could explain only after accounting for a different anisotropy at the bottom repetition of the multilayer with perpendicular anisotropy. Similarly, the characterization of the exchange coupling in our structures was only possible after accounting for individual components, thus portraying the importance of using a multilayer model to properly describe the magnetic behavior and properties of a multilayer structure. We find that the perpendicular anisotropy constant in amorphous Pd/CoFeB/Pd structures increases slightly from 0.295 to 0.315 mJ/m(2) when increasing the thickness of the CoFeB from 3 to 4 angstrom. Furthermore, we find that the exchange coupling in CoFeB/Pd(10 A degrees)/CoFeB structures decreases from 4.899 to 3.268 mJ/m(2) when increasing the thickness of the CoFeB from 3 to 4 angstrom. Finally, we find that the magnitude of the anisotropy at Co/Pd interfaces is 65% larger than at CoFeB/Pd interfaces, and the exchange coupling at CoFeB/Pd/Co interfaces decreases approximately 30% when compared to a CoFeB/Pd/CoFeB structure.

  • 18. Haidar, M.
    et al.
    Durrenfeld, P.
    Ranjbar, M.
    Balinsky, M.
    Fazlali, M.
    Dvornik, M.
    Dumas, R. K.
    Khartsev, Sergiy
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Controlling Gilbert damping in a YIG film using nonlocal spin currents2016Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, nr 18, artikel-id 180409Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate the control of Gilbert damping in 65-nm-thick yttrium iron garnet (YIG) films using a spin-polarized current generated by a direct current through a nanocontact, spin filtered by a thin Co layer. The magnetodynamics of both the YIG and the Co layers can be excited by a pulse-modulated microwave current injected through the nanocontact and the response detected as a lock-in amplified voltage over the device. The spectra show three clear peaks, two associated with the ferromagnetic resonance (FMR) in each layer, and an additional Co mode with a higher wave vector proportional to the inverse of the nanocontact diameter. By varying the sign and magnitude of the direct nanocontact current, we can either increase or decrease the linewidth of the YIG FMR peak consistent with additional positive or negative damping being exerted by the nonlocal spin current injected into the YIG film. Our nanocontact approach thus offers an alternative route in the search for auto-oscillations in YIG films.

  • 19. Haidar, M.
    et al.
    Ranjbar, M.
    Balinsky, M.
    Dumas, R. K.
    Khartsev, Sergiy
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Thickness- and temperature-dependent magnetodynamic properties of yttrium iron garnet thin films2015Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, nr 17, artikel-id 17D119Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The magnetodynamical properties of nanometer-thick yttrium iron garnet films are studied using ferromagnetic resonance as a function of temperature. The films were grown on gadolinium gallium garnet substrates by pulsed laser deposition. First, we found that the damping coefficient increases as the temperature increases for different film thicknesses. Second, we found two different dependencies of the damping on film thickness: at room temperature, the damping coefficient increases as the film thickness decreases, while at T = 8 K, we find the damping to depend only weakly on the thickness. We attribute this behavior to an enhancement of the relaxation of the magnetization by impurities or defects at the surfaces.

  • 20. Haidar, Mohammad
    et al.
    Awad, Ahmad A.
    Dvornik, Mykola
    Khymyn, Roman
    Houshang, Afshin
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    A single layer spin-orbit torque nano-oscillator2019Ingår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 10, artikel-id 2362Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Spin torque and spin Hall effect nano-oscillators generate high intensity spin wave auto-oscillations on the nanoscale enabling novel microwave applications in spintronics, magnonics, and neuromorphic computing. For their operation, these devices require externally generated spin currents either from an additional ferromagnetic layer or a material with a high spin Hall angle. Here we demonstrate highly coherent field and current tunable microwave signals from nano-constrictions in single 15-20 nm thick permalloy layers with oxide interfaces. Using a combination of spin torque ferromagnetic resonance measurements, scanning micro-Brillouin light scattering microscopy, and micromagnetic simulations, we identify the auto-oscillations as emanating from a localized edge mode of the nano-constriction driven by spin-orbit torques. Our results pave the way for greatly simplified designs of auto-oscillating nano-magnetic systems only requiring single ferromagnetic layers with oxide interfaces.

  • 21.
    Houshangh, A.
    et al.
    Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden..
    Khymyn, R.
    Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden..
    Fulara, H.
    Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden..
    Gangwar, A.
    Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden..
    Haidar, M.
    Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden..
    Etesami, S. R.
    Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden..
    Ferreira, R.
    Int Iberian Nanotechnol Lab, P-4715330 Braga, Portugal..
    Freitas, P. P.
    Int Iberian Nanotechnol Lab, P-4715330 Braga, Portugal..
    Dvornik, M.
    Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden..
    Dumas, R. K.
    Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik. Univ Gothenburg, Phys Dept, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden..
    Spin transfer torque driven higher-order propagating spin waves in nano-contact magnetic tunnel junctions2018Ingår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, artikel-id 4374Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Short wavelength exchange-dominated propagating spin waves will enable magnonic devices to operate at higher frequencies and higher data transmission rates. While giant magnetoresistance (GMR)-based magnetic nanocontacts are efficient injectors of propagating spin waves, the generated wavelengths are 2.6 times the nano-contact diameter, and the electrical signal strength remains too weak for applications. Here we demonstrate nano-contact-based spin wave generation in magnetic tunnel junctions and observe large-frequency steps consistent with the hitherto ignored possibility of second-and third-order propagating spin waves with wavelengths of 120 and 74 nm, i.e., much smaller than the 150-nm nanocontact. Mutual synchronization is also observed on all three propagating modes. These higher-order propagating spin waves will enable magnonic devices to operate at much higher frequencies and greatly increase their transmission rates and spin wave propagating lengths, both proportional to the much higher group velocity.

  • 22.
    Jiang, Sheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Ahlberg, Martina
    Department of Physics, University of Gothenburg.
    Afshin, Houshang
    Department of Physics, University of Gothenburg.
    Ferreira, R.
    International Iberian Nanotechnology Laboratory (INL), Braga, Portugal.
    Freitas, P. P.
    International Iberian Nanotechnology Laboratory (INL), Braga, Portugal.
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Department of Physics, University of Gothenburg.
    Magnetodynamics in orthogonal nanocontact spin-torque nano-oscillators based on magnetic tunnel junctionsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We demonstrate the magnetodynamics of nanocontact spin-torque nano-oscillators based on magnetic tunnel junctions, with a strong perpendicular magnetic anisotropy free layer and an easy-plane reference layer. First, the static magnetic properties are studied by conducting hysteresis loop and magnetoresistance measurements. Then, we characterize the generated microwave signals by applying dc currents. Field-sweep signals at xed current show a typical FMR-like frequency dependence. Interestingly, for current-sweep measurements, the plot of frequency versus dc current can be fitted well with a symmetric parabolic and an asymmetric linear term. The symmetric behavior is ascribed to the current-induced Joule heating and Oersted elds. The asymmetric dependence is due to the linear perpendicular spin torque (or eld-like torque) and the voltage-controlled magnetic anisotropy. Our observation hints at a route to improving the frequency tunability in future spintronic applications by selectively adding all these contributions.

  • 23.
    Jiang, Sheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. NanOsc AB, S-16440 Kista, Sweden..
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Diez, Liza Herrera
    Univ Paris Saclay, Univ Paris Sud, CNRS, Inst Elect Fondamentale, F-91405 Orsay, France..
    Le, Quang Tuan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.
    Magnusson, Fredrik
    NanOsc AB, S-16440 Kista, Sweden..
    Ravelosona, Dafine
    Univ Paris Saclay, Univ Paris Sud, CNRS, Inst Elect Fondamentale, F-91405 Orsay, France.;Spin Ion Technol, 28 Rue Gen Leclerc, F-78000 Versailles, France..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. NanOsc AB, S-16440 Kista, Sweden.;Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Tuning the magnetodynamic properties of all-perpendicular spin valves using He+ irradiation2018Ingår i: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 8, nr 6, artikel-id 065309Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using He+ ion irradiation, we demonstrate how the magnetodynamic properties of both ferromagnetic layers in all-perpendicular [Co/Pd]/Cu/[Co/Ni] spin valves can be tuned by varying the He+ ion fluence. As the perpendicular magnetic anisotropy of both layers is gradually reduced by the irradiation, different magnetic configurations can be achieved from all-perpendicular (up arrow up arrow), through orthogonal (->up arrow), to all in-plane (paired right arrows). In addition, both the magnetic damping (alpha) and the inhomogeneous broadening (Delta H-0) of the Co/Ni layer improve substantially with increasing fluence. While the GMR of the spin valve is negatively affected, decreasing linearly from an original value of 1.14% to 0.4% at the maximum fluence of 50x10(14) He+/cm(2), most of the Co/Ni layer improvement is achieved already at a fluence of 10x10(14) He+/cm(2), for which GMR only reduces to 0.9%.

  • 24.
    Jiang, Sheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Department of Physics and Astronomy, Uppsala University.
    Le, Quang Tuan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Diez, Liza Herrera
    Institut d'Electronique Fondamentale, CNRS, Universite Paris-Sud, Universite Paris-Saclay.
    Houshang, Afshin
    Department of Physics, University of Gothenburg.
    Zahedinejad, Mohammad
    Department of Physics, University of Gothenburg.
    Ravelosona, Dafine
    Institut d'Electronique Fondamentale, CNRS, Universite Paris-Sud, Universite Paris-Saclay.
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Department of Physics, University of Gothenburg.
    Experimental evidence of tunable nonlinearity in He+ irradiated spin-torque oscillatorsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Spin-torque nano-oscillators (STNOs) are among the most promising candidates for nanoscale broadband microwave generators. Before this application can be realized, however, enormous efforts are required of researchers to meet the commercial requirements of high-frequency tunability, high power, and narrow linewidth. As the performance of STNOs is mainly attributed to the nonlinearity on the basis of nonlinear auto-oscillator theory, we here systematically study how this nonlinearity is affected by the free-layer's effective magnetization Meff (i.e., the perpendicular magnetic anisotropy, PMA) in STNOs. The PMA is gradually tuned by using different fluences of He+ irradiation. Consequently, the nonlinearity can be continuously tailored from positive to negative. In addition, due to the almost zero nonlinearity, the linewidth shows an improvement of more than two orders of magnitude. This experimental observation is in strong agreement with the theory of nonlinear auto-oscillators. Our study not only confirms the theoretical prediction of nonlinearity, but also indicates the route to be taken towards realizing commercial microwave generators.

  • 25.
    Jiang, Sheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Le, Quang Tuan
    Mazraati, Hamid
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Houshang, Afshin
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Using magnetic droplet nucleation to determine the spin torque effciency and asymmetry in Cox(NiFe)1-x thin filmsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We demonstrate how to extract the material dependent spin torque efficiency (ε) and asymmetry(λ) from the eld{current nucleation boundaries of magnetic droplet solitons in orthogonal nanocontacts in-torque oscillators with Cox(Ni80Fe20)1-x, (x=0{1), fixed layers. As the perpendicular component of the xed layer magnetization plays a central role in governing droplet nucleation, the nucleation boundaries exhibit monotonic shifts towards higher perpendicular magnetic elds when the xed layer magnetization μ0Ms,p is tuned from 1.04 to 1.7 T. We then extract ε and λ from tsto the nucleation boundaries and nd that while ε does not vary with composition, λ increases from1.5 to 3 with increasing Co content. The analysis of droplet nucleation boundaries is hence a useful tool for the systematic study of both ε and λ as functions of material composition.

  • 26.
    Jiang, Sheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. NanOsc AB, S-16440 Kista, Sweden.
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Uppsala Univ, Dept Phys & Astron, S-75120 Uppsala, Sweden..
    Le, Quang Tuan
    Mazraati, Hamid
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. NanOsc AB, S-16440 Kista, Sweden..
    Houshang, Afshin
    NanOsc AB, S-16440 Kista, Sweden.;Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Using Magnetic Droplet Nucleation to Determine the Spin Torque Efficiency and Asymmetry in Co-x(Ni,Fe)(1-x) Thin Films2018Ingår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 10, nr 5, artikel-id 054014Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate how to extract the material-dependent spin-torque efficiency (epsilon) and asymmetry (lambda) from the field-current nucleation boundaries of magnetic droplet solitons in orthogonal nano-contact spintorque oscillators with Co-x(Ni80Fe20)(1-x), (x = 0 -1), fixed layers. As the perpendicular component of the fixed-layer magnetization plays a central role in governing droplet nucleation, the nucleation boundaries exhibit monotonic shifts towards higher perpendicular magnetic fields when the fixed-layer magnetization mu M-0(s, p) is tuned from 1.04 to 1.7 T. We then extract epsilon and lambda from fits to the nucleation boundaries and find that while epsilon does not vary with composition,lambda increases from 1.5 to 3 with increasing Co content. The analysis of droplet nucleation boundaries is hence a useful tool for the systematic study of both epsilon and lambda as functions of material composition.

  • 27.
    Jiang, Sheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. NanOsc AB, S-16440 Kista, Sweden..
    Etesami, Seyyed Ruhollah
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Uppsala Univ, Dept Phys & Astron, S-75120 Uppsala, Sweden..
    Le, Quang Tuan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Houshang, Afshin
    NanOsc AB, S-16440 Kista, Sweden.;Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Impact of the Oersted Field on Droplet Nucleation Boundaries2018Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 9, artikel-id 3104304Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate how the Oersted field affects the magnetic droplet nucleation boundary in spin-torque nano-oscillators based on orthogonal spin-valve stacks with a perpendicular magnetic anisotropy Co/Ni free layer and an easy-plane anisotropy Ni80Fe20 fixed layer. The current-field nucleation boundary is determined experimentally using both microwave signal and dc resistance measurements. The Oersted field can, in principle, have an impact on droplet nucleation. This effect is considered approximately using an analytical equation for the nucleation boundary, which is extended to cover fields larger than the fixed-layer saturation field. We test the accuracy of this approach by comparing with micromagnetic simulations. Finally, we carry out a numerical fit to experimental data and find good agreement.

  • 28. Keatley, P. S.
    et al.
    Sani, Sohrab Redjai
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Uppsala University, Sweden.
    Hrkac, G.
    Mohseni, S. M.
    Durrenfeld, P.
    Loughran, T. H. J.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. University of Gothenburg, Sweden.
    Hicken, R. J.
    Direct observation of magnetization dynamics generated by nanocontact spin-torque vortex oscillators2016Ingår i: Physical Review B - Condensed Matter and Materials Physics, ISSN 2469-9950, Vol. 94, nr 6, artikel-id 060402Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Time-resolved scanning Kerr microscopy has been used to directly image the magnetization dynamics of nanocontact (NC) spin-torque vortex oscillators (STVOs) when phase locked to an injected microwave (rf) current. The Kerr images reveal free-layer magnetization dynamics that extend outside the NC footprint, where they cannot be detected electrically, but which are crucial to phase-lock STVOs that share common magnetic layers. For a single NC, dynamics were observed not only when the STVO frequency was fully locked to that of the rf current, but also for a partially locked state characterized by periodic changes in the core trajectory at the rf frequency. For a pair of NCs, we explore the correlation between the spatial character of injection-locked dynamics and the free-running spectra. Insight gained from these images may improve understanding of the conditions required for mutual phase locking of multiple STVOs, and hence enhanced microwave power emission.

  • 29. Keatley, Paul Steven
    et al.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Hrkac, Gino
    Mohseni, Seyed Majid
    Durrenfeld, Philipp
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Hicken, Robert James
    Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting2017Ingår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 50, nr 16, artikel-id 164003Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by the injected RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.

  • 30.
    Khymyn, Roman
    et al.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Lisenkov, Ivan
    Oregon State Univ, Dept Elect Engn & Comp Sci, Corvallis, OR 97331 USA.;Northeastern Univ, Elect & Comp Engn Dept, Boston, MA 02464 USA..
    Voorheis, James
    Oakland Univ, Dept Phys, Rochester, MI 48309 USA..
    Sulymenko, Olga
    Taras Shevchenko Natl Univ Kyiv, Fac Radiophys Elect & Comp Syst, UA-01601 Kiev, Ukraine..
    Prokopenko, Oleksandr
    Taras Shevchenko Natl Univ Kyiv, Fac Radiophys Elect & Comp Syst, UA-01601 Kiev, Ukraine..
    Tiberkevich, Vasil
    Oakland Univ, Dept Phys, Rochester, MI 48309 USA..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Slavin, Andrei
    Oakland Univ, Dept Phys, Rochester, MI 48309 USA..
    Ultra-fast artificial neuron: generation of picosecond-duration spikes in a current-driven antiferromagnetic auto-oscillator2018Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, artikel-id 15727Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate analytically and numerically, that a thin film of an antiferromagnetic (AFM) material, having biaxial magnetic anisotropy and being driven by an external spin-transfer torque signal, can be used for the generation of ultra-short "Dirac-delta-like" spikes. The duration of the generated spikes is several picoseconds for typical AFM materials and is determined by the inplane magnetic anisotropy and the effective damping of the AFM material. The generated output signal can consist of a single spike or a discrete group of spikes ("bursting"), which depends on the repetition (clock) rate, amplitude, and shape of the external control signal. The spike generation occurs only when the amplitude of the control signal exceeds a certain threshold, similar to the action of a biological neuron in response to an external stimulus. The "threshold" behavior of the proposed AFM spike generator makes possible its application not only in the traditional microwave signal processing but also in the future neuromorphic signal processing circuits working at clock frequencies of tens of gigahertz.

  • 31.
    Mazraati, Hamid
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Houshang, Afshin
    Dvornik, Mykola
    Piazza, Luca
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Qejvanaj, Fatjon
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Jiang, Sheng
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Le, Tuan Q.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Weissenrieder, Jonas
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Low operational current spin Hall nano-oscillators based on NiFe/W bilayers2016Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 109, nr 24, artikel-id 242402Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate highly efficient spin Hall nano-oscillators (SHNOs) based on NiFe/beta-W bilayers. Thanks to the very high spin Hall angle of beta-W, we achieve more than a 60% reduction in the auto-oscillation threshold current compared to NiFe/Pt bilayers. The structural, electrical, and magnetic properties of the bilayers, as well as the microwave signal generation properties of the SHNOs, have been studied in detail. Our results provide a promising path for the realization of low-current SHNO microwave devices with highly efficient spin-orbit torque from beta-W. Published by AIP Publishing.

  • 32.
    Mazraati, Hamid
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Etesami, Seyyed Ruhollah
    University of Gothenburg.
    Banuazizi, S. Amir Hossein
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Houshang, Afshin
    University of Gothenburg.
    Awad, Ahmad A.
    University of Gothenburg.
    Dvornik, Mykola
    University of Gothenburg.
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. University of Gothenburg.
    Auto-oscillating spin-wave modes of constriction-based spin Hall nano-oscillators in weak in-plane fieldsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We experimentally study the auto-oscillating spin-wave modes in Ni80Fe20/β-W constriction-based spin Hall nano-oscillators as a function of bias current, in-plane applied field strength, and azimuthal field angle, in the low-field range of 40-80 mT. We observe two different spin-wave modes: i) a linear-like mode confined to the minima of the internal field near the edges of the nanoconstriction, with weak frequency dependencies on the bias current and the applied field angle, and ii) a second, lower frequency mode that has significantly higher threshold current and stronger frequency dependencies on both bias current and the external eld angle. Our micromagnetic modeling qualitatively reproduces the experimental data and reveals that the second mode is a spin-wave bullet and that the SHNO mode hops between the two modes, resulting in a substantial increase in linewidths. In contrast to the linear-like mode, the bullet is localized in the middle of the constriction and shrinks with increasing bias current. Utilizing intrinsic frequency doubling at zero eld angle we can reach frequencies above 9 GHz in fields as low as 40 mT, which is important for the development of low-eld spintronic oscillators with applications in microwave signal generation and neuromorphic computing.

  • 33.
    Mazraati, Hamid
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. NanOsc AB, SE-16440 Kista, Sweden.
    Etesami, Seyyed Ruhollah
    Univ Gothenburg, Dept Phys, SE-41296 Gothenburg, Sweden..
    Banuazizi, Seyed Amir Hossein
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Houshang, Afshin
    NanOsc AB, SE-16440 Kista, Sweden.;Univ Gothenburg, Dept Phys, SE-41296 Gothenburg, Sweden..
    Awad, Ahmad A.
    NanOsc AB, SE-16440 Kista, Sweden.;Univ Gothenburg, Dept Phys, SE-41296 Gothenburg, Sweden..
    Dvornik, Mykola
    NanOsc AB, SE-16440 Kista, Sweden.;Univ Gothenburg, Dept Phys, SE-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. NanOsc AB, SE-16440 Kista, Sweden.;;Univ Gothenburg, Dept Phys, SE-41296 Gothenburg, Sweden..
    Auto-oscillating Spin-Wave Modes of Constriction-Based Spin Hall Nano-oscillators in Weak In-Plane Fields2018Ingår i: Physical Review Applied, E-ISSN 2331-7019, Vol. 10, nr 5, artikel-id 054017Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We experimentally study the auto-oscillating spin-wave modes in Ni(80)Fc(20)/beta-W constriction-based spin Hall nano-oscillators as a function of bias current, strength of the in-plane applied field, and azimuthal field angle in the low-field range of 40-80 mT. We observe two different spin-wave modes: (i) a linearlike mode confined to the internal field minima near the edges of the nanoconstriction, and only weakly dependent on the bias current and the applied-field angle, and (ii) a second, lower-frequency mode with significantly higher threshold current and stronger dependence on both the bias current and the externalfield angle. Micromagnetic modeling qualitatively reproduces the experimental data and reveals that the second mode is a spin-wave bullet and that the spin Hall nano-oscillator mode hops between the two modes, resulting in a substantial increase in linewidths. In contrast to the linearlike mode, the bullet is localized in the middle of the constriction and shrinks with increasing bias current. Using intrinsic frequency doubling at zero field angle, we can reach frequencies above 9 GHz in fields as low as 40 mT, which is important for the development of low-field spintronic oscillators with applications in microwave-signal generation and neuromorphic computing.

  • 34.
    Mazraati, Hamid
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Etesami, Seyyed Ruhollah
    University of Gothenburg.
    Houshang, Afshin
    University of Gothenburg.
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Dvornik, Mykola
    University of Gothenburg.
    Åkerman, Johan
    University of Gothenburg.
    Contracting vs. expanding spin wave bullets in spin Hall nano-oscillatorsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We employ electrical measurements complemented by systematic micromagnetic simulations to reveal the complex dynamics of nanoconstriction-based spin Hall nano-oscillators. In particular, depending on the strength and out-of-plane angle of the applied magnetic field, we observe three distinct types of magnetization auto-oscillation: (a) a linear-like mode localized in the vicinity of the nanoconstriction by the demagnetizing field, (b) a further localized “regular” spin wave bullet, and(c) a “large” bullet that fills the entire area of the nanoconstriction. Although it has been assumed for some time that bullets only emerge if the nonlinearity of the system is negative (corresponding to the attraction of magnons), our results demonstrate that, in patterned films, they could be sustained even if the nonlinearity of the system is positive (corresponding to the repulsion of magnons). So, in contrast to the regular spin wave bullet, the auto-oscillation volume of its novel large counterpart enlarges, with the amplitude enhancing their drift stability and, correspondingly, reducing their linewidth. We demonstrate that tuning can be achieved between the observed modes at a fixed external field by changing only the drive current, thanks to the amplitude-dependent nonlinearity of the auto-oscillations. This flexibility of nanopatterned spin Hall nano-oscillators is desirable to achieve synaptic functionality in oscillator-based neuromorphic computing devices.

  • 35.
    Mazraati, Hamid
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. NanOsc AB, Sweden.
    Le, Tuan Q.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Awad, Ahmad A.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Hirayama, Eriko
    Ikeda, Shoji
    Matsukura, Fumihiro
    Ohno, Hideo
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. NanOsc AB, Sweden; University of Gothenburg, Sweden.
    Free- and reference-layer magnetization modes versus in-plane magnetic field in a magnetic tunnel junction with perpendicular magnetic easy axis2016Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, nr 10, artikel-id 104428Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the magnetodynamic modes of a magnetic tunnel junction with perpendicular magnetic easy axis (p-MTJ) in in-plane magnetic fields using device-level ferromagnetic resonance spectroscopy. We compare our experimental results to those of micromagnetic simulations of the entire p-MTJ. Using an iterative approach to determine the material parameters that best fit our experiment, we find excellent agreement between experiments and simulations in both the static magnetoresistance and magnetodynamics in the free and reference layers. From the micromagnetic simulations, we determine the spatial mode profiles, the localization of the modes and, as a consequence, their distribution in the frequency domain due to the inhomogeneous internal field distribution inside the p-MTJ under different applied field regimes. We also conclude that the excitation mechanism is a combination of the microwave voltage modulated perpendicular magnetic anisotropy, the microwave Oersted field, and the spin-transfer torque generated by the microwave current.

  • 36.
    Mazraati, Hamid
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
    Zahedinejad, Mohammad
    University of Gothenburg.
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. University of Gothenburg.
    Improving the magnetodynamical properties of NiFe/Pt bilayers through Hf dusting2018Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 113, nr 9, s. 092401-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the effect of hafnium (Hf) dusting on the magnetodynamical properties of NiFe/Pt bilayers using spin-torque-induced ferromagnetic resonance measurements on 6 μm wide microstrips on high-resistive Si substrates. Based on two series of NiFe(tNiFe)/Hf(tHf)/Pt(5) stacks, we first demonstrate that the zero-current magnetodynamic properties of the devices benefit from Hf dusting: (i) the effective magnetization of the NiFe layer increases by 4%–8% with Hf present and (ii) the damping α decreases linearly with tHf by up to 40%. The weaker anisotropic magnetoresistance (AMR≈0.3%–0.4%) of the 3 nm NiFe series is largely unaffected by the Hf, while the stronger AMR of the 5 nm NiFe series drops from 0.7% to 0.43% with increasing tHf. We find that the spin Hall efficiency ξSH is independent of the NiFe thickness, remaining unaffected (ξSH = 0.115) up to tHf = 0.4 nm and then decreasing linearly for higher tHf. The different trends of α and ξSH suggest that there is an optimum Hf thickness (≈0.4 nm) for which the threshold current for auto-oscillation should have a minimum, while the much lower damping should improve mutual synchronization. Our results also indicate that the spin-orbit torque is entirely damping-like with no field-like torque component. Finally, the internal spin Hall angle of Pt is estimated to be θSH = 0.22 by calculating the transparency of the interface.

  • 37. Mohseni, S. M.
    et al.
    Hamdi, M.
    Chung, S.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg.
    Sani, S. R.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg.
    Magnetostatically driven domain replication in Ni/Co based perpendicular pseudo-spin-valves2016Ingår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 49, nr 41, artikel-id 415004Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of ferromagnetic layer thickness on the temperature-dependent stray-field-induced coupling mechanism is investigated in perpendicular pseudo-spin-valves based on [Ni/Co](5)/ Cu/Co-[Ni/Co](n) (n = 2, 3, 4, and 5). Experimental observations show that as n increases from 2 to 4, the difference in coercivity and anisotropy between the two ([Ni/Co](5) or bottom-layer, and [Ni/Co](n) or top-layer) layers increases and the room temperature coupling strength decreases. The coupling then increases for n = 5, as the coercivity difference shrinks and anisotropy decreases. At reduced temperature, the layers start to decouple at a temperature, which increases with n from 2 to 4 and decreases for n = 5 via a stray-field domain-replication mechanism. Our results are useful to control the coupling in pseudo-spin-valves for practical applications in magnetoresistive devices.

  • 38. Mohseni, Seyed Morteza
    et al.
    Hamdi, M.
    Yazdi, H. F.
    Banuazizi, S. Amir Hossein
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    Magnetic droplet soliton nucleation in oblique fields2018Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 97, nr 184402Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the auto-oscillating magnetodynamics in orthogonal spin-torque nano-oscillators (STNOs) as a function of the out-of-plane (OOP) magnetic-field angle. In perpendicular fields and at OOP field angles down to approximately 50°, we observe the nucleation of a droplet. However, for field angles below 50°, experiments indicate that the droplet gives way to propagating spin waves, in agreement with our micromagnetic simulations. Theoretical calculations show that the physical mechanism behind these observations is the sign changing of spin-wave nonlinearity (SWN) by angle. In addition, we show that the presence of a strong perpendicular magnetic anisotropy free layer in the system reverses the angular dependence of the SWN and dynamics in STNOs with respect to the known behavior determined for the in-plane magnetic anisotropy free layer. Our results are of fundamental interest in understanding the rich dynamics of nanoscale solitons and spin-wave dynamics in STNOs.

  • 39. Morrison, C.
    et al.
    Miles, J. J.
    Nguyen, T. N. Anh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Fang, Y.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Thomson, T.
    Exchange coupling in hybrid anisotropy magnetic multilayers quantified by vector magnetometry2015Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, nr 17, artikel-id 17B526Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hybrid anisotropy thin film heterostructures, where layers with perpendicular and in-plane anisotropy are separated by a thin spacer, are novel materials for zero/low field spin torque oscillators and bit patterned media. Here, we report on magnetization reversal and exchange coupling in a archetypal Co/Pd (perpendicular)-NiFe (in-plane) hybrid anisotropy system studied using vector vibrating sample magnetometry. This technique allows us to quantify the magnetization reversal in each individual magnetic layer, and measure of the interlayer exchange as a function of non-magnetic spacer thickness. At large (>1 nm) spacer thicknesses Ruderman-Kittel-Kasuya-Yosida-like exchange dominates, with orange-peel coupling providing a significant contribution only for sub-nm spacer thickness.

  • 40.
    Nguyen, T. N. A.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Materialfysik, MF. Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Viet Nam Department of Physics, University of Gothenburg, Gothenburg, 41296, Sweden.
    Fedotova, J.
    Kasiuk, J.
    Bayev, V.
    Kupreeva, O.
    Lazarouk, S.
    Manh, D. H.
    Vu, D. L.
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Altynov, V.
    Maximenko, A.
    Effect of flattened surface morphology of anodized aluminum oxide templates on the magnetic properties of nanoporous Co/Pt and Co/Pd thin multilayered films2018Ingår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 427, s. 649-655Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    For the first time, nanoporous Al2O3 templates with smoothed surface relief characterized by flattened interpore areas were used in the fabrication of Co/Pd and Co/Pt multilayers (MLs) with strong perpendicular magnetic anisotropy (PMA). Alternating gradient magnetometry (AGM) revealed perfectly conserved PMA in the Co/Pd and Co/Pt porous MLs (antidot arrays) with a ratio of remanent magnetization (Mr) to saturation magnetization (MS) of about 0.99, anisotropy fields (Ha) of up to 2.6 kOe, and a small deviation angle of 8° between the easy magnetization axis and the normal to the film surface. The sufficient magnetic hardening of the porous MLs with enhanced coercive field HC of up to ∼1.9 kOe for Co/Pd and ∼1.5 kOe for Co/Pt MLs, as compared to the continuous reference samples (∼1.5–2 times), is associated with the pinning of the magnetic moments on the nanopore edges. Application of the Stoner–Wohlfarth model for fitting the experimental M/MS(H) curves yielded clear evidence of the predominantly coherent rotation mechanism of magnetization reversal in the porous films.

  • 41.
    Nguyen, T. N. Anh
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Fedotova, J.
    Belarusian State Univ, Inst Nucl Problems, Minsk 220030, BELARUS.;AGH Univ Sci & Technol, Fac Phys & Appl Comp Sci, PL-30059 Krakow, Poland..
    Kasiuk, J.
    Belarusian State Univ, Inst Nucl Problems, Minsk 220030, BELARUS..
    Wu, W. -B
    Przewoznik, J.
    AGH Univ Sci & Technol, Fac Phys & Appl Comp Sci, PL-30059 Krakow, Poland..
    Kapusta, C.
    AGH Univ Sci & Technol, Fac Phys & Appl Comp Sci, PL-30059 Krakow, Poland..
    Kupreeva, O.
    Belarusian State Univ Informat & Radioelect, Minsk 220013, BELARUS..
    Lazarouk, S.
    Belarusian State Univ Informat & Radioelect, Minsk 220013, BELARUS..
    Trinh, T. H. Thuy
    Vietnam Acad Sci & Technol, Inst Mat Sci, Hanoi, Vietnam..
    Do, K. Tung
    Vietnam Acad Sci & Technol, Inst Mat Sci, Hanoi, Vietnam..
    Do, H. Manh
    Vietnam Acad Sci & Technol, Inst Mat Sci, Hanoi, Vietnam..
    Vu, D. Lam
    Vietnam Acad Sci & Technol, Inst Mat Sci, Hanoi, Vietnam..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Royal Inst Technol, Mat Phys Dept, S-16440 Kista, Sweden..
    Enhanced Perpendicular Exchange Bias in Co/Pd Antidot Arrays2019Ingår i: Journal of Electronic Materials, ISSN 0361-5235, E-ISSN 1543-186X, Vol. 48, nr 3, s. 1492-1497Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Magnetic nanostructures revealing the exchange bias (EB) effect have attracted much interest in recent years due to their promising applications in spintronics, magneticsensing and recording devices with various functionalities. In this paper, we report on the perpendicular exchange bias effect in a multilayered thin film composed of [Co/Pd] ferromagnetic multilayers exchange-coupled to an antiferromagnetic IrMn. The film was deposited on a porous anodized titania template. Influences of the films' surface morphology as well as the order of layers deposited on the EB effect were studied. The enhancements of the EB field H-EB (up to 30%) and the coercive field H-C (two times) were achieved in the nanoporous films relative to their continuous film counterparts, which could be attributed to the specific morphology of the porous surfaces.

  • 42. Qejvanaj, Fatjon
    et al.
    Mazraati, Hamid
    Jiang, S.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Persson, A.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Magnusson, F.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. NanOsc AB, Sweden.
    Planar Hall Effect Bridge sensor with NiFeX (X = Cu, Ag and Au) sensing layer.2015Konferensbidrag (Övrigt vetenskapligt)
  • 43. Sbiaa, R.
    et al.
    Ranjbar, M.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Domain structures and magnetization reversal in Co/Pd and CoFeB/Pd multilayers2015Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, nr 17, artikel-id 17C102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Domain structures and magnetization reversal of (Co/Pd) and (CoFeB/Pd) multilayers with 7 and 14 repeats were investigated. The Co-based multilayers show much larger coercivities, a better squareness, and a sharper magnetization switching than CoFeB-based multilayers. From magnetic force microscopy observations, both structures show strong reduction in domains size as the number of repeats increases but the magnetic domains for Co-based multilayers are more than one order of magnitude larger than for CoFeB-based multilayers. By imaging domains at different times, breaks in the (CoFeB/Pd) multilayer stripes were observed within only few hours, while no change could be seen for (Co/Pd) multilayers. Although CoFeB single layers are suitable for magnetoresistive devices due to their large spin polarization and low damping constants, their lamination with Pd suffers mainly from thermal instability.

  • 44. Sbiaa, R.
    et al.
    Shaw, J. M.
    Nembach, H. T.
    Al Bahri, M.
    Ranjbar, M.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. University of Gothenburg, Sweden.
    Piramanayagam, S. N.
    Ferromagnetic resonance measurements of (Co/Ni/Co/Pt) multilayers with perpendicular magnetic anisotropy2016Ingår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 49, nr 42, artikel-id 425002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Multilayers of [Co/Ni(t)/Co/Pt]×8 with varying Ni thickness were investigated for possible use as a free layer in magnetic tunnel junctions and spintronics devices. The thickness t of the Ni sub-layer was varied from 0.3 nm to 0.9 nm and the resulting magnetic properties were compared with (Co/Ni) and (Co/Pt) multilayers. As determined from magnetic force microscopy, magnetometry and ferromagnetic resonance measurements, all multilayers exhibited perpendicular magnetic anisotropy. Compared with (Co/Pt) multilayers, the sample with t of 0.9 nm showed almost the same anisotropy field of μ 0 H k = 1.15 T but the damping constant was 40% lower. These characteristics make these multilayers attractive for spin torque based magnetoresistive devices with perpendicular anisotropy.

  • 45. Sharma, R.
    et al.
    Durrenfeld, P.
    Ranjbar, M.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Gothenburg, Sweden.
    Muduli, P. K.
    Modulation rate study in spin torque oscillator based wireless communication system2015Ingår i: 2015 IEEE MAGNETICS CONFERENCE (INTERMAG), ISSN 2150-4598Artikel i tidskrift (Övrigt vetenskapligt)
  • 46. Sharma, Raghav
    et al.
    Sisodia, Naveen
    Durrenfeld, Philipp
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg, Sweden.
    Muduli, Pranaba Kishor
    Time-domain stability of parametric synchronization in a spin-torque nano-oscillator based on a magnetic tunnel junction2017Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, nr 2, artikel-id 024427Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on time-domain stability of the parametric synchronization in a spin-torque nano-oscillator (STNO) based on a magnetic tunnel junction. Time-domain measurements of the instantaneous frequency (f(i)) of a parametrically synchronized STNO showrandom short-term unlocking of the STNO signal for low injected radio-frequency (RF) power, which cannot be revealed in time-averaged frequency domain measurements. Macrospin simulations reproduce the experimental results and reveal that the random unlocking during synchronization is driven by thermal fluctuations. We show that by using a high injected RF power, random unlocking of the STNO can be avoided. However, a perfect synchronization characterized by complete suppression of phase noise, so-called phase noise squeezing, can be obtained only at a significantly higher RF power. Our macrospin simulations suggest that a lower temperature and a higher positive ratio of the fieldlike torque to the spin transfer torque reduce the threshold RF power required for phase noise squeezing under parametric synchronization.

  • 47. Sharma, Raghav
    et al.
    Sisodia, Naveen
    Iacocca, Ezio
    Awad, Ahmad A.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. University of Gothenburg, Sweden.
    Muduli, P. K.
    A high-speed single sideband generator using a magnetic tunnel junction spin torque nano-oscillator2017Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, artikel-id 13422Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An important property of spin-torque nano-oscillators (STNOs) is their ability to produce a frequency modulated (FM) signal, which is very critical for communication applications. We here demonstrate a novel single sideband (SSB) modulation phenomenon using a magnetic tunnel junction (MTJ)-based STNO, which saves transmission bandwidth and in principle should minimize attenuation for wireless communication. Experimentally, lower single sidebands (LSSBs) have been successfully demonstrated over a wide range of modulation frequency, f(m) = 150 MHz-1 GHz. The observed LSSBs are determined by the intrinsic properties of the device, which can be modeled well by a nonlinear frequency and amplitude modulation formulation and reproduced in macrospin simulations. Moreover, our macrospin simulation results show that the range of modulation current and modulation frequency for generating SSBs can be controlled by the field-like torque and biasing conditions.

  • 48. Sheykhifard, Z.
    et al.
    Mohseni, S. Majid
    Tork, B.
    Hajiali, M. R.
    Jamilpanah, L.
    Rahmati, B.
    Haddadi, F.
    Hamdi, M.
    Mohseni, S. Morteza
    Mohammadbeigi, M.
    Ghaderi, A.
    Erfanifam, S.
    Dashtdar, M.
    Feghhi, F.
    Ansari, N.
    Pakdel, S.
    Pourfath, M.
    Hosseinzadegan, A.
    Bahreini, M.
    Tavassoli, S. H.
    Ranjbar, M.
    Banuazizi, Seyed Amir Hossein
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Göteborgs University, Sweden.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Göteborgs University, Sweden.
    Nikkam, N.
    Sohrabi, A.
    Roozmeh, S. E.
    Magnetic graphene/Ni-nano-crystal hybrid for small field magnetoresistive effect synthesized via electrochemical exfoliation/deposition technique2018Ingår i: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 29, nr 5, s. 4171-4178Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two-dimensional heterostructures of graphene (Gr) and metal/semiconducting elements convey new direction in electronic devices. They can be useful for spintronics because of small spin orbit interaction of Gr as a non-magnetic metal host with promising electrochemical stability. In this paper, we demonstrate one-step fabrication of magnetic Ni-particles entrapped within Gr-flakes based on simultaneous electrochemical exfoliation/deposition procedure by two-electrode system using platinum as the cathode electrode and a graphite foil as the anode electrode. The final product is an air stable hybrid element including Gr flakes hosting magnetic Ni-nano-crystals showing superparamagnetic-like response and room temperature giant magnetoresistance (GMR) effect at small magnetic field range. The GMR effect is originated from spin scattering through ferromagnetic/non-magnetic nature of Ni/Gr heterostructure and interpreted based on a phenomenological spin transport model. Our work benefits from XRD, XPS, Raman, TEM, FTIR and VSM measurements We addressed that how our results can be used for rapid manufacturing of magnetic Gr for low field magneto resistive elements and potential printed spintronic devices.

  • 49.
    Spicer, T. M.
    et al.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Keatley, P. S.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Dvornik, M.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Loughran, T. H. J.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Awad, A. A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Durrenfeld, P.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Houshang, A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Ranjbar, M.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Kruglyak, V. V.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Hicken, R. J.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Time resolved imaging of the non-linear bullet mode within an injection-locked nano-contact spin Hall nano-oscillator2018Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 113, nr 19, artikel-id 192405Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nanooscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a nonlinear "bullet" mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal..

  • 50.
    Spicer, T. M.
    et al.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Keatley, P. S.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Loughran, T. H. J.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Dvornik, M.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Awad, A. A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Durrenfeld, P.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Houshang, A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Ranjbar, M.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Kruglyak, V. V.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Hicken, R. J.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Spatial mapping of torques within a spin Hall nano-oscillator2018Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, nr 21, artikel-id 214438Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Time-resolved scanning Kerr microscopy (TRSKM) was used to study precessional magnetization dynamics induced by a radio frequency (RF) current within a Al2O3/Py(5 nm)/Pt(6 nm)/Au(150 nm) spin Hall nano-oscillator structure. The Au layer was formed into two needle-shaped electrical contacts that concentrated the current in the center of a Py/Pt mesa of 4 mu m diameter. Due to the spin Hall effect, current within the Pt layer drives a spin current into the Py layer, exerting a spin transfer torque (STT). By injecting RF current and exploiting the phase sensitivity of TRSKM and the symmetry of the device structure, the STT and Oersted field torques have been separated and spatially mapped. The STT and torque due to the in-plane Oersted field are observed to exhibit minima at the device center that is ascribed to spreading of RF current that is not observed for DC current. Torques associated with the RF current may destabilize the position of the self-localized bullet mode excited by the DC current and inhibit injection locking. The present study demonstrates the need to characterize both DC and RF current distributions carefully.

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