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  • 1.
    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 performance2017Inngår i: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 9, nr 5, s. 1896-1900Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 2.
    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 performance2017Inngår i: 2017 IEEE International Magnetics Conference, INTERMAG 2017, Institute of Electrical and Electronics Engineers (IEEE), 2017, artikkel-id 8007567Konferansepaper (Fagfellevurdert)
    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.

  • 3. Bonetti, Stefano
    et al.
    Kukreja, R
    Chen, Z
    Macià, F
    Hernàndez, J. M.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Backes, D
    Frisch, J
    Katine, J
    Malm, Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Urazhdin, S
    Kent, A. D.
    Stöhr, J.
    Ohldag, H.
    Dürr, H. A.
    Direct observation and imaging of a spin-wave soliton with p−like symmetry2015Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, artikkel-id 8889Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The prediction and realization of magnetic excitations driven by electrical currents via the spin transfer torque effect, enables novel magnetic nano-devices where spin-waves can be used to process and store information. The functional control of such devices relies on understanding the properties of non-linear spin-wave excitations. It has been demonstrated that spin waves can show both an itinerant character, but also appear as localized solitons. So far, it was assumed that localized solitons have essentially cylindrical, s−like symmetry. Using a newly developed high-sensitivity time-resolved magnetic x-ray microscopy, we instead observe the emergence of a novel localized soliton excitation with a nodal line, i.e. with p−like symmetry. Micromagnetic simulations identify the physical mechanism that controls the transition from s− to p−like solitons. Our results suggest a potential new pathway to design artificial atoms with tunable dynamical states using nanoscale magnetic devices.

  • 4.
    Chen, Tingsu
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    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, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Rusu, Ana
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Department of Physics, University of Gothenburg and Nanosc AB.
    Spin-Torque and Spin-Hall Nano-OscillatorsInngår i: Proceedings of the IEEE, ISSN 0018-9219, E-ISSN 1558-2256Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 5.
    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.
    Iacocca, Ezio
    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.
    Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part I: Analytical Model of the MTJ STO2015Inngår i: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 62, nr 3, s. 1037-1044Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 6.
    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.
    Iacocca, Ezio
    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.
    Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part II: Verilog-A Model Implementation2015Inngår i: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 62, nr 3, s. 1045-1051Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 7.
    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 circuitry2015Inngår i: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 111, s. 91-99Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 8.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Iacocca, Ezio
    Mohseni, Seyed Majid
    Sani, Sohrab R.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Bookman, Lake
    Hoefer, Mark A.
    Dumas, Randy K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Univ Gothenburg, Sweden.
    Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators2016Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, artikkel-id 11209Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 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) (Annet vitenskapelig)
    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 informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Majid Mohseni, S.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dürrenfeld, P.
    Ranjbar, M.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Anh Nguyen, T. N.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Vietnam Academy of Science and Technology, Viet Nam.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Magnetic droplet solitons in orthogonal spin valves2015Inngår i: Fizika Nizkih Temperatur, ISSN 0132-6414, E-ISSN 1816-0328, Vol. 41, nr 10, s. 1063-1068Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 11.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. University of Gothenburg, Sweden.
    Mohseni, S. M.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dürrenfeld, P.
    Ranjbar, M.
    Sani, Redjai Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Anh Nguyen, T. N.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. Vietnam Academy of Science and Technology, Viet Nam.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. University of Gothenburg, Sweden.
    Magnetic droplet solitons in orthogonal spin valves2015Inngår i: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 41, nr 10, s. 833-837Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 12.
    Chung, Sunjae
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Sani, Sohrab Redjai
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Iacocca, E.
    Dumas, R. K.
    Nguyen, Thi Ngooc Anh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Pogoryelov, Ye
    Muduli, P. K.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Hoefer, M.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Spin transfer torque generated magnetic droplet solitons (invited)2014Inngår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, nr 17, s. 172612-Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 13. Consolo, G.
    et al.
    Finocchio, G.
    Siracusano, G.
    Bonetti, S.
    Eklund, Anders
    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.
    Azzerboni, B.
    Non-stationary excitation of two localized spin-wave modes in a nano-contact spin torque oscillator2013Inngår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 114, nr 15, s. 153906-Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 14. Dumas, Randy K.
    et al.
    Iacocca, E.
    Bonetti, S.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Persson, J.
    Heinonen, O.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Spin-Wave-Mode Coexistence on the Nanoscale: A Consequence of the Oersted-Field-Induced Asymmetric Energy Landscape2013Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, nr 25, s. 257202-Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 15.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Microwave Frequency Stability and Spin Wave Mode Structure in Nano-Contact Spin Torque Oscillators2016Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The nano-contact spin torque oscillator (NC-STO) is an emerging device for highly tunable microwave frequency generation in the range from 0.1 GHz to above 65 GHz with an on-chip footprint on the scale of a few μm. The frequency is inherent to the magnetic material of the NC-STO and is excited by an electrical DC current by means of the spin torque transfer effect. Although the general operation is well understood, more detailed aspects such as a generally nonlinear frequency versus current relationship, mode-jumping and high device-to-device variability represent open questions. Further application-oriented questions are related to increasing the electrical output power through synchronization of multiple NC-STOs and integration with CMOS integrated circuits.

    This thesis consists of an experimental part and a simulation part. Experimentally, for the frequency stability it is found that the slow but strong 1/f-type frequency fluctuations are related to the degree of nonlinearity and the presence of perturbing, unexcited modes. It is also found that the NC-STO can exhibit up to three propagating spin wave oscillation modes with different frequencies and can randomly jump between them. These findings were made possible through the development of a specialized microwave time-domain measurement circuit. Another instrumental achievement was made with synchrotron X-rays, where we image dynamically the magnetic internals of an operating NC-STO device and reveal a spin wave mode structure with a complexity significantly higher than the one predicted by the present theory.

    In the simulations, we are able to reproduce the nonlinear current dependence by including spin wave-reflecting barriers in the nm-thick metallic, magnetic free layer. A physical model for the barriers is introduced in the form of metal grain boundaries with reduced magnetic exchange coupling. Using the experimentally measured average grain size of 30 nm, the spin wave mode structure resulting from the grain model is able to reproduce the experimentally found device nonlinearity and high device-to-device variability.

    In conclusion, the results point out microscopic material grains in the metallic free layer as the reason behind the nonlinear frequency versus current behavior and multiple propagating spin wave modes and thereby as a source of device-to-device variability and frequency instability.

  • 16.
    Eklund, Anders
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Bonetti, Stefano
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Sani, Sohrab R.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Persson, Johan
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Banuazizi, S. Amir Hossein
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Iacocca, Ezio
    Östling, Mikael
    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.
    Malm, B. Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field2014Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, nr 9, s. 092405-Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 17.
    Eklund, Anders
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Bonetti, Stefano
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Sani, Sohrab R.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Persson, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Malm, Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    1/f and white frequency noise in a synchronized spin torque oscillator pair2011Inngår i: 56th Annual Conference on Magnetism and Magnetic Materials, 2011, s. 504-504Konferansepaper (Fagfellevurdert)
  • 18.
    Eklund, Anders
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dvornik, Mykola
    Göteborgs universitet.
    Qejvanaj, Fatjon
    NanOsc AB.
    Jiang, Sheng
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Göteborgs universitet.
    Dumas, Randy K.
    Göteborgs universitet.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Göteborgs universitet.
    Malm, B. Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Nonlinearity, frequency stability and device-to-device variability in nano-contact spin torque oscillators with grainy thin filmsManuskript (preprint) (Annet vitenskapelig)
    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.

  • 19.
    Eklund, Anders J.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mohseni, Seyed Majid
    NanOsc AB.
    Persson, Johan
    NanOsc AB.
    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.
    Triple mode-jumping in a spin torque oscillator2013Inngår i: 2013 22nd International Conference on Noise and Fluctuations, ICNF 2013, New York: IEEE conference proceedings, 2013, s. 6578965-Konferansepaper (Fagfellevurdert)
    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.

  • 20.
    Le, Quang Tuan
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Banuazizi, Seyed Amir Hossein
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden.
    Fallahi, Vahid Fallahi
    Faculty of Physics, Amirkabir University of Technology.
    Nguyen, Thi Ngoc Anh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Yamanouchi, M.
    Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku University.
    Enobio, Eli C. I.
    Center for Spintronics Integrated Systems, Tohoku University.
    Ikeda, S.
    Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku University.
    Ohno, Hideo
    Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku University.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Sweden; NanOsc AB, Sweden.
    Ultra-high frequency tunability in low-current and low-field spin-torque oscillators based on perpendicular magnetic tunnel junctionsManuskript (preprint) (Annet (populærvitenskap, debatt, mm))
    Abstract [en]

    We demonstrate ultra-high frequency tunability of up to 4.4 GHz/mA, and low threshold currents of about -21 $\mu$A, in spin-torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions, in which both free and fixed layers have perpendicular magnetic anisotropy (PMA). By using different thicknesses of the two CoFeB layers, their individual PMA strengths can be tailored to achieve significant relative misalignment of their respective magnetizations in moderate in-plane fields. We observe a broad maximum in both the device resistance and the generated microwave power around maximum misalignment. Maximum frequency tunability is observed at low-to-moderate fields and decrease rapidly after maximum misalignment.

  • 21.
    Malm, Gunnar
    et al.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektronik.
    Eklund, Anders
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektronik.
    Dvornik, Mykola
    Gothenburg University, Physics.
    Micromagnetic Modeling of Telegraphic Mode Jumping in Microwave Spin Torque Oscillators2019Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    The time domain stability of microwave spin torque oscillators (STOs) has been investigated by systematic micromagnetic simulations. A model based on internal spin wave reflection at grain boundaries with reduced exchange coupling was implemented and used to study the oscillator under quasi-stable operating conditions. Telegraphic mode jumping between two operating frequencies (23.3 and 24.1 GHz) was observed in the time domain with characteristic dwell times in the range of 10-100 ns. The oscillating volume was shown to have a different shape at the distinct operating frequencies. The shape difference is governed by spin wave reflections at the grain boundaries. The resulting non-linear behavior of the oscillator was shown to be a collective effect of spin wave scattering at different locations within a few spin wavelengths from the nano-contact.

  • 22.
    Redjai Sani, Sohrab
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Persson, J.
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Pogoryelov, Ye
    Muduli, P. K.
    Eklund, Anders
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Malm, Gunnar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Dmitriev, A.
    Käll, M.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mutually synchronized bottom-up multi-nanocontact spin-torque oscillators2013Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 4, s. 2731-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Spin-torque oscillators offer a unique combination of nanosize, ultrafast modulation rates and ultrawide band signal generation from 100 MHz to close to 100 GHz. However, their low output power and large phase noise still limit their applicability to fundamental studies of spin-transfer torque and magnetodynamic phenomena. A possible solution to both problems is the spin-wave-mediated mutual synchronization of multiple spin-torque oscillators through a shared excited ferromagnetic layer. To date, synchronization of high-frequency spin-torque oscillators has only been achieved for two nanocontacts. As fabrication using expensive top-down lithography processes is not readily available to many groups, attempts to synchronize a large number of nanocontacts have been all but abandoned. Here we present an alternative, simple and cost-effective bottom-up method to realize large ensembles of synchronized nanocontact spin-torque oscillators. We demonstrate mutual synchronization of three high-frequency nanocontact spin-torque oscillators and pairwise synchronization in devices with four and five nanocontacts.

1 - 22 of 22
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