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  • 1. Awad, A. A.
    et al.
    Durrenfeld, P.
    Houshang, A.
    Dvornik, M.
    Iacocca, E.
    Dumas, R. K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Long-range mutual synchronization of spin Hall nano-oscillators2017In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 13, no 3, 292-+ p.Article in journal (Refereed)
    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.

  • 2. Balinsky, Michael
    et al.
    Haidar, Mohammad
    Ranjbar, Mojtaba
    Durrenfeld, Philipp
    Houshang, Afshin
    Slavin, Andrei
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Modulation of the Spectral Characteristics of a Nano-Contact Spin-Torque Oscillator via Spin Waves in an Adjacent Yttrium-Iron Garnet Film2016In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 7, 3101704Article in journal (Refereed)
    Abstract [en]

    NiFe-Cu-Co trilayer nano-contact spin-torque oscillators (NC-STOs) fabricated on an yttrium-iron garnet (YIG) film were studied in two different modes. In passive mode, i.e. without any NC-STO auto-oscillations, a microwave current through the nano-contact can excite spin waves (SW) in the YIG film, and, vice versa, antenna generated SWs in the YIG film can be detected by the nano-contact. In active mode, i.e., in the presence of auto-oscillations, significant changes appear in the NC-STO spectrum when its frequency approaches that of the SWs excited in the YIG. These results demonstrate strong coupling between NC-STOs and SWs in YIG and open new possibilities of 1) pure spin-current generation in YIG by NC-STOs; 2) mutual locking of a number of NC-STOs through SWs in YIG; and 3) improvement of NC-STO spectra through SW feedback in YIG.

  • 3. Balinsky, Michael
    et al.
    Ranjbar, Mojtaba
    Haidar, Mohammad
    Durrenfeld, Philipp
    Khartsev, Sergiy
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Slavin, Andrei
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, 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 Bilayers2015In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 6, no 3000604Article in journal (Refereed)
    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. Banerjee, Chandrima
    et al.
    Pal, Semanti
    Ahlberg, Martina
    Nguyen, T. N. Anh
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Vietnam Acad Sci & Technol, Vietnam.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Univ Gothenburg, Sweden.
    Barman, Anjan
    All-optical study of tunable ultrafast spin dynamics in [Co/Pd]/NiFe systems: the role of spin-twist structure on Gilbert damping2016In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 6, no 83, 80168-80173 p.Article in journal (Refereed)
    Abstract [en]

    We investigate optically induced ultrafast magnetization dynamics in [Co(0.5 nm)/Pd(1 nm)](5)/NiFe(t) exchange-spring samples with tilted perpendicular magnetic anisotropy using a time-resolved magneto-optical Kerr effect magnetometer. The competition between the out-of-plane anisotropy of the hard layer, the in-plane anisotropy of the soft layer and the applied bias field reorganizes the spins in the soft layer, which are modified further with the variation in t. The spin-wave spectrum, the ultrafast demagnetization time, and the extracted damping coefficients - all depend on the spin distribution in the soft layer, while the latter two also depend on the spin-orbit coupling between the Co and Pd layers. The spin-wave spectra change from multimode to single-mode as t decreases. At the maximum field reached in this study, H = 2.5 kOe, the damping shows a nonmonotonic dependence on t with a minimum at t = 7.5 nm. For t < 7.5 nm, intrinsic effects dominate, whereas for t > 7.5 nm, extrinsic effects govern the damping mechanisms.

  • 5.
    Banuazizi, Seyed Amir Hossein
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Sani, Sohrab R.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Electronics, Integrated devices and circuits.
    Naiini, Maziar M.
    KTH, School of Information and Communication Technology (ICT), Electronics, Integrated devices and circuits.
    Mohseni, Seyed Majid
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Univ Gothenburg, Sweden.
    Durrenfeld, Philipp
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Electronics, Integrated devices and circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Univ Gothenburg, Sweden.
    Order of magnitude improvement of nano-contact spin torque nano-oscillator performance2017In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 9, no 5, 1896-1900 p.Article in journal (Refereed)
    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. Batlle, X.
    et al.
    Hattink, B. J.
    Labarta, A.
    Åkerman, Johan
    Escudero, R.
    Schuller, I. K.
    Quantitative x-ray photoelectron spectroscopy study of Al/AlOx bilayers2002In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 91, no 12, 10163-10168 p.Article in journal (Refereed)
    Abstract [en]

    An x-ray photoelectron spectroscopy (XPS) analysis of Nb/Al wedge bilayers, oxidized by both plasma and natural oxidation, is reported. The main goal is to show that the oxidation state-i.e., O:(oxidize)Al ratio-, structure and thickness of the surface oxide layer, as well as the thickness of the metallic Al leftover, as functions of the oxidation procedure, can be quantitatively evaluated from the XPS spectra. This is relevant to the detailed characterization of the insulating barriers in (magnetic) tunnel junctions.

  • 7. Batlle, X.
    et al.
    Hattink, B. J.
    Labarta, A.
    Åkerman, Johan
    Escudero, R.
    Schuller, I. K.
    The oxidation state at tunnel junction interfaces2003In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, Vol. 260, no 02-jan, 78-83 p.Article in journal (Refereed)
    Abstract [en]

    The oxidation state at the interfaces of Nb/Al-AlOx/Pb junctions is discussed. Conductance-voltage curves below and above the superconducting temperature suggest tunneling conduction, while X-ray photoelectron spectroscopy shows the existence of a thin AlOx layer at the Nb/Al interface. We demonstrate that at the usual 10(-7) Torr range of base pressures in the sputtering chamber, this is due to the time elapsed between the deposition of the Nb and Al bottom layers, in both Nb/Al-AlOx/Pb tunnel junctions and Nb/Al bilayers. We also give some direct evidence of the oxidation of the top Pb electrode on the Nb electrode surface. Such oxidation probably occurs at the pinholes of the intermediate Al-AlOx layer of the tunnel junctions, as a consequence of the oxidation state at the Nb/Al interface, which helps to avoid barrier shorts and enhances the quality of the tunnel barrier. We therefore suggest that there is oxide formation in other places besides where there is an actual oxide deposited. This is relevant for the performance of magnetic tunnel junctions since in most tunneling studies it is assumed that once the oxide is deposited, that is the only place where there is an oxide. This is also a very general statement applicable to thin film systems.

  • 8.
    Bonanni, Valentina
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Pakizeh, Tavakol
    Pirzadeh, Zhaleh
    Chen, Jianing
    Nogues, Josef
    Vavassori, Paolo
    Hillenbrand, Rainer
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Dmitriev, Alexandre
    Designer Magnetoplasmonics with Nickel Nanoferromagnets2011In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, no 12, 5333-5338 p.Article in journal (Refereed)
    Abstract [en]

    We introduce a new perspective on magnetoplasmonics in nickel nanoferromagnets by exploiting the phase tunability of the optical polarizability due to localized surface plasmons and simultaneous magneto-optical activity. We demonstrate how the concerted action of nanoplasmonics and magnetization can manipulate the sign of rotation of the reflected light's polarization (i.e., to produce Kerr rotation reversal) in ferromagnetic nanomaterials and, further, how this effect can be dynamically controlled and employed to devise conceptually new schemes for biochemosensing.

  • 9.
    Bonanni, Valentina
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg, Sweden.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    First-order reversal curve analysis of graded anisotropy FePtCu films2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 20, 202501- p.Article in journal (Refereed)
    Abstract [en]

    The reversal mechanisms of graded anisotropy FePtCu films have been investigated by alternating gradient magnetometer (AGM) and magneto-optical Kerr effect (MOKE) measurements with first-order reversal curve (FORC) techniques. The AGM-FORC analysis, which clearly shows the presence of soft and hard components, is unable to resolve how these phases are distributed throughout the film thickness. MOKE-FORC measurements, which preferentially probe the surface of the film, reveal that the soft components are indeed located toward the top surface. Combining AGM-FORC with the inherent surface sensitivity of MOKE-FORC analysis allows for a comprehensive analysis of heterogeneous systems such as graded materials.

  • 10.
    Bonetti, Stefano
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Consolo, G.
    Finocchio, G.
    Eklund, A.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Mancoff, F.
    Azzerboni, B.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Time-space analysis of spin wave  dynamics in nano-contact spin torque oscillatorsManuscript (preprint) (Other academic)
  • 11.
    Bonetti, Stefano
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    de Vreede, Niels
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Conte, Gianluca
    Finocchio, Giovanni
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Phase locking of nanocontact spin torque osillators as a function of applied field angleManuscript (preprint) (Other academic)
  • 12.
    Bonetti, Stefano
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Muduli, Pranaba
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Spin torque oscillator frequency versus magnetic field angle: The prospect of operation beyond 65 GHz2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 10Article in journal (Refereed)
    Abstract [en]

    We study the impact of the magnetic field angle on the oscillation frequency of a nanocontact spin torque oscillator (STO) in magnetic fields up to 2.1 T. A model based on a single nonlinear, nonpropagating spin wave mode is found to explain the experimental data. We observe oscillation frequencies as high as 46 GHz in high magnetic fields applied normal to the film plane, and we are able to extrapolate the maximum expected operating frequency to beyond 65 GHz for in-plane magnetic fields. The STO signal remains surprisingly strong at these conditions, which opens up for possible millimeter-wave applications.

  • 13.
    Bonetti, Stefano
    et al.
    Stanford University.
    Pulisgiyo, Vito
    Consolo, Giancarlo
    Mancoff, Fred
    Tiberkevich, Vasyl S.
    Slavin, Andrei N.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Power and linewidth of propagating and localized modes in nanocontact spin-torque oscillators2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 17, 174427- p.Article in journal (Refereed)
    Abstract [en]

    The integrated power and linewidth of a propagating and a self-localized spin-wave mode excited by spin-polarized current in an obliquely magnetized magnetic nanocontact are studied experimentally as functions of the angle theta(e) between the external bias magnetic field and the nanocontact plane. It is found that the power of the propagating mode increases monotonically with theta(e), while the power of the self-localized mode has a broad maximum near theta(e) = 40 degrees and exponentially vanishes near the critical angle theta(e) = 58 degrees, at which the localized mode disappears. The linewidth of the propagating mode in the interval of angles 58 degrees < theta(e) < 90 degrees, where only this mode is excited, is adequately described by the existing theory, while in the angular interval where both modes can exist the observed linewidth of both modes is substantially broadened due to the telegraph switching between the modes. Numerical simulations and an approximate analytical model give a good semiquantitative description of the observed results.

  • 14.
    Bonetti, Stefano
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Tiberkevich, Vasil
    Consolo, Giancarlo
    Finocchio, Giovanni
    Muduli, Pranaba
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven magnetic nanocontacts2010In: Physical Review Letters, ISSN 0031-9007, Vol. 105, no 21, 217204- p.Article in journal (Refereed)
    Abstract [en]

    Through detailed experimental studies of the angular dependence of spin wave excitations in nanocontact-based spin-torque oscillators, we demonstrate that two distinct spin wave modes can be excited, with different frequency, threshold currents, and frequency tunability. Using analytical theory and micromagnetic simulations we identify one mode as an exchange-dominated propagating spin wave, and the other as a self-localized nonlinear spin wave bullet. Wavelet-based analysis of the simulations indicates that the apparent simultaneous excitation of both modes results from rapid mode hopping induced by the Oersted field.

  • 15.
    Bonetti, Stefano
    et al.
    Stanford University, USA.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Nano-Contact Spin-Torque Oscillators as Magnonic Building Blocks2013In: Magnonics: From Fundamentals to Applications, Springer Berlin/Heidelberg, 2013, 177-187 p.Chapter in book (Refereed)
    Abstract [en]

    We describe the possibility of using nano-contact spin-torque oscillators (NC-STOs) as fundamental magnonic building blocks. NC-STOs can act as spin wave generators, manipulators, and detectors, and can hence realize all the fundamental functions necessary for fully integrated magnonic devices, which can be fabricated using available CMOS compatible large-scale spin-torque device production processes. We show in particular how a 200 nm sized nano-contact located on an out-of-plane magnetized permalloy "free" magnetic layer can generate spin waves at f approximate to 15 GHz that propagate up to 4 mu m away from the nano-contact with wavelength lambda = 200-300 nm, decay length lambda(r) approximate to 2 mu m and group velocities v(g) approximate to 3 mu m/ns. We propose that the same type of NC-STOs can be used as spin wave manipulators, via control of the local Gilbert damping, and as spin wave detector using the spin torque diode effect.

  • 16. Chen, Jianing
    et al.
    Albella, Pablo
    Pirzadeh, Zhaleh
    Alonso-Gonzalez, Pablo
    Huth, Florian
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Nogues, Josep
    Vavassori, Paolo
    Dmitriev, Alexandre
    Aizpurua, Javier
    Hillenbrand, Rainer
    Plasmonic Nickel Nanoantennas2011In: SMALL, ISSN 1613-6810, Vol. 7, no 16, 2341-2347 p.Article in journal (Refereed)
    Abstract [en]

    The fundamental optical properties of pure nickel nanostructures are studied by far-field extinction spectroscopy and optical near-field microscopy, providing direct experimental evidence of the existence of particle plasmon resonances predicted by theory. Experimental and calculated near-field maps allow for unambiguous identification of dipolar plasmon modes. By comparing calculated near-field and far-field spectra, dramatic shifts are found between the near-field and far-field plasmon resonances, which are much stronger than in gold nanoantennas. Based on a simple damped harmonic oscillator model to describe plasmonic resonances, it is possible to explain these shifts as due to plasmon damping.

  • 17.
    Chen, Tingsu
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dumas, R. K.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Muduli, P. K.
    Houshang, A.
    Awad, A. A.
    Dürrenfeld, P.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Rusu, Ana
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Univ Gothenburg, Sweden.
    Spin-Torque and Spin-Hall Nano-Oscillators2016In: Proceedings of the IEEE, ISSN 0018-9219, E-ISSN 1558-2256, Vol. 104, no 10, 1919-1945 p., 7505988Article in journal (Refereed)
    Abstract [en]

    This paper reviews the state of the art in spin-torque and spin-Hall-effect-driven nano-oscillators. After a brief introduction to the underlying physics, the authors discuss different implementations of these oscillators, their functional properties in terms of frequency range, output power, phase noise, and modulation rates, and their inherent propensity for mutual synchronization. Finally, the potential for these oscillators in a wide range of applications, from microwave signal sources and detectors to neuromorphic computation elements, is discussed together with the specific electronic circuitry that has so far been designed to harness this potential.

  • 18.
    Chen, Tingsu
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Muduli, Pranaba K.
    Department of Physics, University of Gothenburg and Department of Physics, Indian Institute of Technology.
    Houshang, Afshin
    Department of Physics, University of Gothenburg.
    Awad, Ahmad A.
    Department of Physics, University of Gothenburg.
    Dürrenfeld, Philip
    Department of Physics, University of Gothenburg.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Rusu, Ana
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Department of Physics, University of Gothenburg and Nanosc AB.
    Spin-Torque and Spin-Hall Nano-OscillatorsIn: Proceedings of the IEEE, ISSN 0018-9219, E-ISSN 1558-2256Article in journal (Refereed)
    Abstract [en]

    This paper reviews the state of the art in spin-torque and spin Hall effect driven nano-oscillators. After a brief introduction to the underlying physics, the authors discuss different implementations of these oscillators, their functional properties in terms of frequency range, output power, phase noise, and modulation rates, and their inherent propensity for mutual synchronization. Finally, the potential for these oscillators in a wide range of applications, from microwave signal sources and detectors to neuromorphic computation elements, is discussed together with the specific electronic circuitry that has so far been designed to harness this potential.

  • 19.
    Chen, Tingsu
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dürrenfeld, P.
    Rodriguez, Saul
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Rusu, Ana
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits. University of Gothenburg, Sweden.
    A highly tunable microwave oscillator based on MTJ STO technology2014In: Microwave and optical technology letters (Print), ISSN 0895-2477, E-ISSN 1098-2760, Vol. 56, no 9, 2092-2095 p.Article in journal (Refereed)
    Abstract [en]

    This article presents a fully ESD-protected, highly tunable microwave oscillator based on magnetic tunnel junction (MTJ) spin torque oscillator (STO) technology. The oscillator consists of a compact MTJ STO and a 65 nm CMOS wideband amplifier, which amplifies the RF signal of the MTJ STO to a level that can be used to drive a PLL. The (MTJ STO+amplifier IC) pair shows a measured quality factor (Q) of 170 and a wide tunability range from 3 to 7 GHz, which demonstrate its potential to be used as a microwave oscillator in multiband, multistandard radios.

  • 20.
    Chen, Tingsu
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Iacocca, Ezio
    Rodriguez, Saul
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Rusu, Ana
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part I: Analytical Model of the MTJ STO2015In: IEEE Transactions on Electron Devices, ISSN 0018-9383, Vol. 62, no 3, 1037-1044 p.Article in journal (Refereed)
    Abstract [en]

    Magnetic tunnel junction (MTJ) spin torque oscillators (STOs) have shown the potential to be used in a wide range of microwave and sensing applications. To evaluate the potential uses of MTJ STO technology in various applications, an analytical model that can capture MTJ STO's characteristics, while enabling system-and circuit-level designs, is of great importance. An analytical model based on macrospin approximation is necessary for these designs since it allows implementation in hardware description languages. This paper presents a new macrospin-based, comprehensive, and compact MTJ STO model, which can be used for various MTJ STOs to estimate the performance of MTJ STOs together with their application-specific integrated circuits. To adequately present the complete model, this paper is divided into two parts. In Part I, the analytical model is introduced and verified by comparing it against measured data of three different MTJ STOs, varying the angle and magnitude of the magnetic field, as well as the DC biasing current. The proposed analytical model is suitable for being implemented in Verilog-A and used for efficient simulations at device, circuit, and system levels. In Part II, the full Verilog-A implementation of the analytical model with accurate phase noise generation is presented and verified by simulations.

  • 21.
    Chen, Tingsu
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Iacocca, Ezio
    Rodriguez, Saul
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Rusu, Ana
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part II: Verilog-A Model Implementation2015In: IEEE Transactions on Electron Devices, ISSN 0018-9383, Vol. 62, no 3, 1045-1051 p.Article in journal (Refereed)
    Abstract [en]

    The rapid development of the magnetic tunnel junction (MTJ) spin torque oscillator (STO) technology demands an analytical model to enable building MTJ STO-based circuits and systems so as to evaluate and utilize MTJ STOs in various applications. In Part I of this paper, an analytical model based on the macrospin approximation has been introduced and verified by comparing it with the measurements of three different MTJ STOs. In Part II, the full Verilog-A implementation of the proposed model is presented. To achieve a reliable model, an approach to reproducing the phase noise generated by the MTJ STO has been proposed and successfully employed. The implemented model yields a time domain signal, which retains the characteristics of operating frequency, linewidth, oscillation amplitude, and DC operating point, with respect to the magnetic field and applied DC current. The Verilog-A implementation is verified against the analytical model, providing equivalent device characteristics for the full range of biasing conditions. Furthermore, a system that includes an MTJ STO and CMOS RF circuits is simulated to validate the proposed model for system-and circuit-level designs. The simulation results demonstrate that the proposed model opens the possibility to explore STO technology in a wide range of applications.

  • 22.
    Chen, Tingsu
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Redjai Sani, Sohrab
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Rodriguez, Saul
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Rusu, Ana
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Integration of GMR-based spin torque oscillators and CMOS circuitry2015In: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 111, 91-99 p.Article in journal (Refereed)
    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.

  • 23.
    Chen, Tingsu
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Rodriguez, Saul
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Rusu, Ana
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    An Inductorless Wideband Balun-LNA for Spin Torque Oscillator-based Field Sensing2014In: Electronics, Circuits and Systems (ICECS), 2014 21st IEEE International Conference on, IEEE conference proceedings, 2014, 36-39 p.Conference paper (Refereed)
    Abstract [en]

    This paper presents a wideband inductorless Balun-LNA targeting spin torque oscillator-based magnetic field sensing applications. The LNA consistsof a CS stage combined with a cross-coupled CG stage, which offers wideband matching, noise/distortion cancellation and gain boosting, simultaneously. The Balun-LNA is implemented in a 65 nm CMOS technology, and it is fully ESD-protected and packaged. Measurement results show a bandwidth of 2 GHz - 7 GHz, a voltage gain of 20 dB, an IIP3 of +2 dBm, and a maximum NF of 5 dB. The LNA consumes 3.84 mW from a 1.2 V power supply and occupies a total silicon area of 0.0044 mm2. The measurement results demonstrate that the proposed Balun-LNA is highly suitable for the STO-based field sensing applications.

  • 24.
    Chung, Sunjae
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Sweden.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Iacocca, Ezio
    Mohseni, Seyed Majid
    Sani, Sohrab R.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Bookman, Lake
    Hoefer, Mark A.
    Dumas, Randy K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Univ Gothenburg, Sweden.
    Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators2016In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, 11209Article in journal (Refereed)
    Abstract [en]

    Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current-field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

  • 25.
    Chung, Sunjae
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Sweden.
    Le, Quang Tuan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Ahlberg, Martina
    Department of Physics, University of Gothenburg.
    Awad, Ahmad A.
    Department of Physics, University of Gothenburg.
    Mazraati, Hamid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. NanOsc AB.
    Houshang, Afshin
    Department of Physics, University of Gothenburg.
    Jiang, Sheng
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Nguyen, Thi Ngoc Anh
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Institute of Materials Science, Vietnam Academy of Science and Technology.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Sweden; NanOsc AB, Sweden.
    Magnetic droplet solitons in all-perpendicular nano-contact spin torque oscillatorsManuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    Spin-torque-generated magnetic droplets are nontopological solitons that have both dynamic and static characteristics. Although   theoretical studies of these droplets originally dealt with  an all-perpendicular magnetic system, all experimental demonstrations have so far relied on orthogonal spin valve structures that require a rather strong magnetic field to nucleate the droplet. Here, for the first time, we show the nucleation and sustained operation of magnetic droplets under a low magnetic field using nanocontact spin-torque oscillators (NC-STO),  both the free Co/Ni and fixed Co/Pd multilayers of which have strong perpendicular magnetic anisotropy. Droplet nucleation is observed as a change in the NC-STO resistance and the appearance of significant broadband microwave signal generation below 1 GHz. We also observe another important phenomenon in which the nucleated magnetic droplet can transform into a skyrmionic nanobubble in the low-field regime. Both magnetic droplet solitons and skyrmionic nanobubbles have been studied in detail using micromagnetic simulation.

  • 26.
    Chung, Sunjae
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Majid Mohseni, S.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dürrenfeld, P.
    Ranjbar, M.
    Redjai Sani, Sohrab
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Anh Nguyen, T. N.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Vietnam Academy of Science and Technology, Viet Nam.
    Dumas, R. K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Magnetic droplet solitons in orthogonal spin valves2015In: Fizika Nizkih Temperatur, ISSN 0132-6414, E-ISSN 1816-0328, Vol. 41, no 10, 1063-1068 p.Article in journal (Refereed)
    Abstract [en]

    We review the recent experimental advancements in the realization and understanding of magnetic droplet solitons generated by spin transfer torque in orthogonal nanocontact based spin torque nanooscillators (STNOs) fabricated on extended spin valves and spin valve nanowires. The magnetic droplets are detected and studied using the STNO microwave signal and its resistance, the latter both quasistatically and time-resolved. The droplet nucleation current is found to have a minimum at intermediate magnetic field strengths and the nature of the nucleation changes gradually from a single sharp step well above this field, mode-hopping around the minimum, and continuous at low fields. The mode-hopping and continuous transitions are ascribed to droplet drift instability and re-nucleation at different time scales, which is corroborated by time-resolved measurements. We argue that the use of tilted anisotropy fixed layers could reduce the nucleation current further, move the nucleation current minimum to lower fields, and potentially remove the need for an applied magnetic field altogether. Finally, evidence of an edge mode droplet in a nanowire is presented.

  • 27.
    Chung, Sunjae
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Mohseni, S. M.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dürrenfeld, P.
    Ranjbar, M.
    Sani, Redjai Sohrab
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Anh Nguyen, T. N.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Vietnam Academy of Science and Technology, Viet Nam.
    Dumas, R. K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. University of Gothenburg, Sweden.
    Magnetic droplet solitons in orthogonal spin valves2015In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 41, no 10, 833-837 p.Article in journal (Refereed)
    Abstract [en]

    We review the recent experimental advancements in the realization and understanding of magnetic droplet solitons generated by spin transfer torque in orthogonal nanocontact based spin torque nanooscillators (STNOs) fabricated on extended spin valves and spin valve nanowires. The magnetic droplets are detected and studied using the STNO microwave signal and its resistance, the latter both quasistatically and time-resolved. The droplet nucleation current is found to have a minimum at intermediate magnetic field strengths and the nature of the nucleation changes gradually from a single sharp step well above this field, mode-hopping around the minimum, and continuous at low fields. The mode-hopping and continuous transitions are ascribed to droplet drift instability and re-nucleation at different time scales, which is corroborated by time-resolved measurements. We argue that the use of tilted anisotropy fixed layers could reduce the nucleation current further, move the nucleation current minimum to lower fields, and potentially remove the need for an applied magnetic field altogether. Finally, evidence of an edge mode droplet in a nanowire is presented.

  • 28.
    Chung, Sunjae
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Fallahi, Vahid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Nguyen, T. N. Anh
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Benatmane, Nadjib
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Dumas, R. K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Tunable spin configuration in [Co/Ni]-NiFe spring magnets2013In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 46, no 12, 125004- p.Article in journal (Refereed)
    Abstract [en]

    We investigate a tunable spin configuration in [Co/Ni](4)/Co-NiFe exchange spring magnets. The strong perpendicular magnetic anisotropy of the Co/Ni multilayer, which competes with the in-plane shape anisotropy of the Ni81Fe19 (Py) layer, allows for unique magnetic configurations. By varying the NiFe thickness (t(NiFe)) from 2.6 to 3.0 nm, we show that the magnetization tilt angle can be easily tuned from completely out-of-plane (0 degrees) to in-plane (90 degrees). This transition, which occurs for a small range of t(NiFe), can be estimated from the major loop remanence and one-dimensional micromagnetic calculations. These tunable magnetization tilt spring magnets are highly promising for future applications in spin-transfer torque-based devices.

  • 29.
    Chung, Sunjae
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Sani, Sohrab Redjai
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Iacocca, E.
    Dumas, R. K.
    Nguyen, Thi Ngooc Anh
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Pogoryelov, Ye
    Muduli, P. K.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Hoefer, M.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Spin transfer torque generated magnetic droplet solitons (invited)2014In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, no 17, 172612- p.Article in journal (Refereed)
    Abstract [en]

    We present recent experimental and numerical advancements in the understanding of spin transfer torque generated magnetic droplet solitons. The experimental work focuses on nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has an easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The NC-STO resistance and microwave signal generation are measured simultaneously as a function of drive current and applied perpendicular magnetic field. Both exhibit dramatic transitions at a certain current dependent critical field value, where the microwave frequency drops 10 GHz, modulation sidebands appear, and the resistance exhibits a jump, while the magnetoresistance changes sign. We interpret these observations as the nucleation of a magnetic droplet soliton with a large fraction of its magnetization processing with an angle greater than 90 degrees, i.e., around a direction opposite that of the applied field. This interpretation is corroborated by numerical simulations. When the field is further increased, we find that the droplet eventually collapses under the pressure from the Zeeman energy.

  • 30. Consolo, G.
    et al.
    Finocchio, G.
    Siracusano, G.
    Bonetti, S.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Azzerboni, B.
    Non-stationary excitation of two localized spin-wave modes in a nano-contact spin torque oscillator2013In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 114, no 15, 153906- p.Article in journal (Refereed)
    Abstract [en]

    We measure and simulate micromagnetically a framework based upon a nano-contact spin torque oscillator where two distinct localized evanescent spin-wave modes can be detected. The resulting frequency spectrum is composed by two peaks, corresponding to the excited modes, which lie below the ferromagnetic resonance frequency, and a low-frequency tail, which we attribute to the non-stationary switching between these modes. By using Fourier, wavelet, and Hilbert-Huang transforms, we investigate the properties of these modes in time and spatial domains, together with their spatial distribution. The existence of an additional localized mode (which was neither predicted by theory nor by previous numerical and experimental findings) has to be attributed to the large influence of the current-induced Oersted field strength which, in the present setup, is of the same order of magnitude as the external field. As a further consequence, the excited spin-waves, contrarily to what usually assumed, do not possess cylindrical symmetry: the Oersted field induces these modes to be excited at the two opposite sides of the region beneath the nano-contact.

  • 31. Dave, Renu W.
    et al.
    Steiner, G.
    Slaughter, J. M.
    Sun, J. J.
    Craigo, B.
    Pietambaram, S.
    Smith, K.
    Grynkewich, G.
    DeHerrera, M.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Tehrani, S.
    MgO-based tunnel junction material for high-speed toggle magnetic random access memory2006In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 42, no 8, 1935-1939 p.Article in journal (Refereed)
    Abstract [en]

    We report the first demonstration of a magnetoresistive random access memory (MRAM) circuit incorporating MgO-based magnetic tunnel junction (MTJ) material for higher performance. We compare our results to those of AlOx-based devices, and we discuss the MTJ process optimization and material changes that made the demonstration possible. We present data on key MTJ material attributes for different oxidation processes and free-layer alloys, including resistance distributions, bias dependence, free-layer magnetic properties, interlayer coupling, breakdown voltage, and thermal endurance. A tunneling magnetoresistance (TMR) greater than 230% was achieved with CoFeB free layers and greater than 85% with NiFe free layers. Although the TMR with NiFe is at the low end of our MgO comparison, even this MTJ material enables faster access times, since its TMR is almost double that of a similar structure with an AlOx barrier. Bit-to-bit resistance distributions are somewhat wider for MgO barriers, with sigma about 1.5% compared to about 0.9% for AlOx. The read access time of our 4 Mb toggle MRAM circuit was reduced from 21 ns with AlOx to a circuit-limited 17 ns with MgO.

  • 32. Davies, J. E.
    et al.
    Gilbert, D. A.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Dumas, R. K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Liu, Kai
    Reversal mode instability and magnetoresistance in perpendicular (Co/Pd)/Cu/(Co/Ni) pseudo-spin-valves2013In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 2, 022409- p.Article in journal (Refereed)
    Abstract [en]

    We have observed distinct temperature-dependent magnetization reversal modes in a perpendicular (Co/Pd)(4)/Co/Cu/(Co/Ni)(4)/Co pseudo-spin-valve, which are correlated with spin-transport properties. At 300 K, magnetization reversal occurs by vertically correlated domains. Below 200 K the hysteresis loop becomes bifurcated due to laterally correlated reversal of the individual stacks. The magnetic configuration change also leads to higher spin disorders and a significant increase in the giant magnetoresistance effect. First order reversal curve measurements reveal that the coupled state can be re-established through field cycling and allow direct determination of the interlayer coupling strength as a function of temperature.

  • 33. Dumas, Randy K.
    et al.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Kirby, B. J.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Nogues, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Probing vertically graded anisotropy in FePtCu films2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 5, 054434- p.Article in journal (Refereed)
    Abstract [en]

    Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (T(A)). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300 degrees C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low T(A). A fundamentally different mechanism is found after annealing at 400 degrees C, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500 degrees C, a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated T(A).

  • 34. Dumas, Randy K.
    et al.
    Greene, Peter K.
    Gilbert, Dustin A.
    Ye, Li
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Dept Phys, Sweden.
    Liu, Kai
    Accessing different spin-disordered states using first-order reversal curves2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 10, 104410- p.Article in journal (Refereed)
    Abstract [en]

    Combined first-order reversal curve (FORC) analyses of the magnetization (M-FORC) and magnetoresistance (MR-FORC) have been employed to provide a comprehensive study of the M-MR correlation in two canonical systems: a NiFe/Cu/FePt pseudo spin valve (PSV) and a [Co/Cu](8) multilayer. In the PSV, due to the large difference in switching fields and minimal interactions between the NiFe and the FePt layers, the M and MR show a simple one-to-one relationship during reversal. In the [Co/Cu](8) multilayer, the correlation between the magnetization reversal and the MR evolution is more complex. This is primarily due to the similar switching fields of, and interactions between, the constituent Co layers. The FORC protocol accesses states with much higher spin disorders and larger MRs than those found along the conventional major loop field cycle. Unlike the M-FORC measurements, which only probe changes in the macroscopic magnetization, the MR-FORCs are more sensitive to the microscopic domain configurations as those are most important in determining the resultant MR effect size. This approach is generally applicable to spintronic systems to realize the maximum spin disorder and the largest MR.

  • 35. Dumas, Randy K.
    et al.
    Iacocca, E.
    Bonetti, S.
    Redjai Sani, Sohrab
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Eklund, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Persson, J.
    Heinonen, O.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Spin-Wave-Mode Coexistence on the Nanoscale: A Consequence of the Oersted-Field-Induced Asymmetric Energy Landscape2013In: Physical Review Letters, ISSN 0031-9007, Vol. 110, no 25, 257202- p.Article in journal (Refereed)
    Abstract [en]

    It has been argued that if multiple spin wave modes are competing for the same centrally located energy source, as in a nanocontact spin torque oscillator, that only one mode should survive in the steady state. Here, the experimental conditions necessary for mode coexistence are explored. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field, which leads to a physical separation of the modes, and is further promoted by spin wave localization at reduced applied field angles. Finally, both simulation and experiment reveal a low frequency signal consistent with the intermodulation of two coexistent modes.

  • 36. Dumas, Randy K.
    et al.
    Redjai Sani, Sohrab
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, S. Majid
    Iacocca, Ezio
    Pogoryelov, Yevgen
    Muduli, Pranaba K.
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Durrenfeld, Philipp
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Univ Gothenburg, Sweden.
    Recent Advances in Nanocontact Spin-Torque Oscillators2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 6, 4100107- p.Article in journal (Refereed)
    Abstract [en]

    We present a comprehensive review of the most recent advances in nanocontact spin torque oscillators (NC-STOs). NC-STOs are highly tunable, with both applied magnetic field and dc, broadband microwave signal generators. As opposed to the nanopillar geometry, where the lateral cross section of the entire device has been confined to a typically <100 nm diameter, in NC-STOs, it is only the current injection site that has been laterally confined on top of an extended magnetic film stack. Three distinct material combinations will be discussed: 1) a Co/Cu/NiFe pseudospin valve (PSV) where both the Co and NiFe have a dominant in-plane anisotropy; 2) a Co/Cu/[Co/Ni](4) orthogonal PSV where the Co/Ni multilayer has a strong perpendicular anisotropy; and 3) a single NiFe layer with asymmetric non-magnetic Cu leads. We explore the rich and diverse magnetodynamic modes that can be generated in these three distinct sample geometries.

  • 37. Dumas, Randy K.
    et al.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Lau, June W.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Graded Anisotropy FePtCu Films2011In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 47, no 6, 1580-1586 p.Article in journal (Refereed)
    Abstract [en]

    The fabrication and subsequent analysis of continuously graded anisotropy films are discussed. During deposition, a compositional gradient is first achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after thermal post-annealing, and by utilizing the strong composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The magnetic properties are investigated by surface sensitive magneto-optical Kerr effect and alternating gradient magnetometer (AGM) measurements. AGM first-order reversal curve (FORC) measurements are employed in order to provide a detailed analysis of the reversal mechanisms, and therefore the induced anisotropy gradient. At low annealing temperatures, the FORC measurements clearly indicate the highly coupled reversal of soft and hard phases. However, significant interdiffusion results in virtually uniform films at elevated annealing temperatures. Additionally, the A1 to L1(0) ordering process is found to depend on the film thickness.

  • 38. Durrenfeld, P.
    et al.
    Gerhard, F.
    Chico, J.
    Dumas, R. K.
    Ranjbar, M.
    Bergman, A.
    Bergqvist, Lars
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Gould, C.
    Molenkamp, L. W.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden; NanOsc AB, Sweden.
    Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 21, 214424Article in journal (Refereed)
    Abstract [en]

    The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization M-S, Gilbert damping alpha, and exchange stiffness A. M-S and A are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agreement between theory and experiment for M-S and alpha. The calculated A shows the same trend as the experimental data but has a larger magnitude. In addition to the unique in-plane anisotropy of the material, these tunabilities of the magnetodynamic properties can be taken advantage of when employing NiMnSb films in magnonic devices.

  • 39. Durrenfeld, P.
    et al.
    Gerhard, F.
    Mohseni, S. M.
    Ranjbar, M.
    Sani, S. R.
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Gould, C.
    Molenkamp, L. W.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Low-current, narrow-linewidth microwave signal generation in NiMnSb based single-layer nanocontact spin-torque oscillators2016In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 109, no 22, 222403Article in journal (Refereed)
    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.

  • 40. Durrenfeld, P.
    et al.
    Iacocca, E.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Muduli, P. K.
    Parametric excitation in a magnetic tunnel junction-based spin torque oscillator2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, no 5, 052410- p.Article in journal (Refereed)
    Abstract [en]

    Using microwave current injection at room temperature, we demonstrate parametric excitation of a magnetic tunnel junction (MTJ)-based spin-torque oscillator (STO). Parametric excitation is observed for currents below the auto-oscillation threshold, when the microwave current frequency f(e) is twice the STO free-running frequency f(0). Above threshold, the MTJ becomes parametrically synchronized. In the synchronized state, the STO exhibits an integrated power up to 5 times higher and a linewidth reduction of two orders of magnitude, compared to free-running conditions. We also show that the parametric synchronization favors single mode oscillations in the case of multimode excitation.

  • 41. Durrenfeld, Philipp
    et al.
    Xu, Yongbing
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Department of Physics, University of Gothenburg, .
    Zhou, Yan
    Controlled skyrmion nucleation in extended magnetic layers using a nanocontact geometry2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 5, 054430Article in journal (Refereed)
    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.

  • 42. Dürrenfeld, P.
    et al.
    Gerhard, F.
    Ranjbar, M.
    Gould, C.
    Molenkamp, L. W.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Spin Hall effect-controlled magnetization dynamics in NiMnSb2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 17, 17E103Article in journal (Refereed)
    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.

  • 43. Dürrenfeld, P.
    et al.
    Iacocca, E.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Muduli, P. K.
    Modulation-mediated unlocking of a parametrically phase-locked spin torque oscillator2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 25, 252404- p.Article in journal (Refereed)
    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.

  • 44.
    Eklund, Anders
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Sani, Sohrab R.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Persson, Johan
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Banuazizi, S. Amir Hossein
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Iacocca, Ezio
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, no 9, 092405- p.Article in journal (Refereed)
    Abstract [en]

    The nano-scale spin torque oscillator (STO) is a compelling device for on-chip, highly tunable microwave frequency signal generation. Currently, one of the most important challenges for the STO is to increase its longer-time frequency stability by decreasing the 1/f frequency noise, but its high level makes even its measurement impossible using the phase noise mode of spectrum analyzers. Here, we present a custom made time-domain measurement system with 150MHz measurement bandwidth making possible the investigation of the variation of the 1/f as well as the white frequency noise in a STO over a large set of operating points covering 18-25GHz. The 1/f level is found to be highly dependent on the oscillation amplitude-frequency non-linearity and the vicinity of unexcited oscillation modes. These findings elucidate the need for a quantitative theoretical treatment of the low-frequency, colored frequency noise in STOs. Based on the results, we suggest that the 1/f frequency noise possibly can be decreased by improving the microstructural quality of the metallic thin films.

  • 45.
    Eklund, Anders
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Sani, Sohrab R.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Malm, Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    1/f and white frequency noise in a synchronized spin torque oscillator pair2011In: 56th Annual Conference on Magnetism and Magnetic Materials, 2011, 504-504 p.Conference paper (Refereed)
  • 46.
    Eklund, Anders
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dvornik, Mykola
    Göteborgs universitet.
    Qejvanaj, Fatjon
    NanOsc AB.
    Jiang, Sheng
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Göteborgs universitet.
    Dumas, Randy K.
    Göteborgs universitet.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Göteborgs universitet.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Nonlinearity, frequency stability and device-to-device variability in nano-contact spin torque oscillators with grainy thin filmsManuscript (preprint) (Other academic)
    Abstract [en]

    In nano-contact spin torque oscillators with a frequency range of 10-65 GHz, the propagating spin wave mode attracts interest due both to its high frequency stability and prospective use in magnonic devices. Its dependence of the frequency on the bias current however displays device-to-device variability on the order of several hundred MHz, with device specific nonlinearities that can be either continuous or discontinuous and have negative impact on the frequency stability. A model for this behavior is however still lacking. By using micromagnetic simulations, we investigate the impact of imperfections in the spin wave-carrying free magnetic layer and find that nonlinearities can be created when the propagating spin wave is reflected back to the active region. The oscillation then self-locks at the frequency of the resonant wavelength, resulting in a standing spin wave pattern. Simulations including nine randomly generated film structures with 30 nm-sized grains and exchange-reduced inter-grain boundaries give qualitative and partially quantitative agreement with experimental measurements. The results point out the spin wave-reflecting grain boundaries as a source of device nonlinearity, manufacturing variability and frequency destabilization.

  • 47.
    Eklund, Anders J.
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Redjai Sani, Sohrab
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    NanOsc AB.
    Persson, Johan
    NanOsc AB.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Triple mode-jumping in a spin torque oscillator2013In: 2013 22nd International Conference on Noise and Fluctuations, ICNF 2013, New York: IEEE conference proceedings, 2013, 6578965- p.Conference paper (Refereed)
    Abstract [en]

    In a nano-contact Co/Cu/NiFe spin torque oscillator, mode-jumping between up to three frequencies within 22.5-24.0 GHz is electrically observed in the time domain. The measurements reveal toggling between two states with differing oscillation amplitude, of which the low-amplitude state is further divided into two rapidly alternating modes. Analysis of the mode dwell time statistics and the total time spent in each mode is carried out, and it is found that in both aspects the balance between the modes is greatly altered with the DC drive current.

  • 48. Engel, B. N.
    et al.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Butcher, B.
    Dave, R. W.
    DeHerrera, M.
    Durlam, M.
    Grynkewich, G.
    Janesky, J.
    Pietambaram, S. V.
    Rizzo, N. D.
    Slaughter, J. M.
    Smith, K.
    Sun, J. J.
    Tehrani, S.
    A 4-mb toggle MRAM based on a novel bit and switching method2005In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 41, no 1, 132-136 p.Article in journal (Refereed)
    Abstract [en]

    4-Mb magnetoresistive random access memory (MRAM) with a novel magnetic bit cell and toggle switching mode is presented. The circuit was designed in a five level metal, 0.18-mum complementary metal-oxide-semiconductor process with a bit cell size of 1.55 mum(2). The new bit cell uses a balanced synthetic antiferromagnetic free layer and a phased write pulse sequence to provide robust switching performance with immunity from half-select disturbs. This switching mode greatly improves the operational performance of the MRAM as compared to conventional MRAM. A detailed description of this 4-Mb toggle MRAM is presented.

  • 49. Fang, Yeyu
    et al.
    Dumas, R. K.
    Nguyen Thi Ngooc, Ahn
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101), Materials- and Nano Physics, Material Physics, MF.
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101), Materials- and Nano Physics, Material Physics, MF.
    Miller, C. W.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101), Materials- and Nano Physics, Material Physics, MF.
    A Nonvolatile Spintronic Memory Element with a Continuum of Resistance States2013In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 23, no 15, 1919-1922 p.Article in journal (Refereed)
    Abstract [en]

    A continuum of stable remanent resistance states is reported in perpendicularly magnetized pseudo spin valves with a graded anisotropy free layer. The resistance states can be systematically set by an externally applied magnetic field. The gradual reversal of the free layer with applied field and the field-independent fixed layer leads to a range of stable and reproducible remanent resistance values, as determined by the giant magnetoresistance of the device. An analysis of first-order reversal curves combined with magnetic force microscopy shows that the origin of the effect is the field-dependent population of up and down domains in the free layer.

  • 50.
    Fang, Yeyu
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg, Sweden.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    An In Situ anneal study of graded anisotropy FePtCu films2011In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 2Article in journal (Refereed)
    Abstract [en]

    We provide a detailed study of how the anisotropy (Ku) gradient in a compositionally graded FePtCu film gradually develops as a function of the postannealing temperature (TA). By utilizing the in situ annealing and magnetic characterization capabilities of a physical property measurement system, the evolution of the induced Ku gradient is elucidated. For low TA, the sample primarily remains in the low-Ku A1 phase. At intermediate TA, the gradual development of an A1 to L10 anisotropy gradient occurs. As TA is further increased, a well-developed L10 gradient is realized. Finally, annealing temperatures greater than 475 ◦C reduce the gradient until the film is effectively uniform for TA ≥ 525 ◦C and higher, presumably due to interdiffusion of the Cu through the film thickness. The resulting coercivity shows a nonmonotonic dependence on TA with an initial steep increase as the L10 fraction of the sample increases, a local minimum at TA = 525 ◦C where the gradient vanishes, and a final increase as the uniform L10 film orders completely.

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