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  • 51.
    Sulymenko, Olga
    et al.
    Taras Shevchenko Natl Univ Kyiv, Fac Radiophys Elect & Comp Syst, UA-01601 Kiev, Ukraine..
    Prokopenko, Oleksandr
    Taras Shevchenko Natl Univ Kyiv, Fac Radiophys Elect & Comp Syst, UA-01601 Kiev, Ukraine..
    Lisenkov, Ivan
    Northeastern Univ, Dept Elect & Comp Engn, Boston, MA 02115 USA..
    Åkerman, Johan
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Tyberkevych, Vasyl
    Oakland Univ, Dept Phys, Rochester, MI 48309 USA..
    Slavin, Andrei N.
    Oakland Univ, Dept Phys, Rochester, MI 48309 USA..
    Khymyn, Roman
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Ultra-fast logic devices using artificial "neurons" based on antiferromagnetic pulse generators2018In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 124, no 15, article id 152115Article in journal (Refereed)
    Abstract [en]

    It has been shown previously that spin-Hall oscillators based on current-driven bi-layered film structures containing an antiferromagnet (AFM) and a normal metal can generate ultra-short (similar to 2 ps) "spike-like" pulses in response to an external current stimulus of a sufficient amplitude, thus operating as ultra-fast artificial "neurons." Here, we report the results of numerical simulations demonstrating that a single AFM "neuron" can perform the logic functions of OR, AND, MAJORITY, or Q-gates, while a circuit consisting of a small number n < 5 of AFM "neurons" can function as a FULL-ADDER or as a dynamic memory loop with variable clock frequency. The clock frequencies of such AFM-based logic devices could reach tens of GHz, which make them promising as base elements of future ultra-fast high-efficiency neuromorphic computing. 

  • 52. Tacchi, S.
    et al.
    Troncoso, R. E.
    Ahlberg, M.
    Gubbiotti, G.
    Madami, M.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Landeros, P.
    Interfacial Dzyaloshinskii-Moriya Interaction in Pt/CoFeB Films: Effect of the Heavy-Metal Thickness2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 118, no 14, article id 147201Article in journal (Refereed)
    Abstract [en]

    We report the observation of a Pt layer thickness dependence on the induced interfacial DzyaloshinskiiMoriya interaction in ultrathin Pt(d(Pt))/CoFeB films. Taking advantage of the large spin-orbit coupling of the heavy metal, the interfacial Dzyaloshinskii-Moriya interaction is quantified by Brillouin light scattering measurements of the frequency nonreciprocity of spin waves in the ferromagnet. The magnitude of the induced Dzyaloshinskii-Moriya coupling is found to saturate to a value of 0.45 mJ/m(2) for Pt thicknesses larger than similar to 2 nm. The experimental results are explained by analytical calculations based on the three-site indirect exchange mechanism that predicts a Dzyaloshinskii-Moriya interaction at the interface between a ferromagnetic thin layer and a heavy metal. Our findings open up a way to control and optimize chiral effects in ferromagnetic thin films through the thickness of the heavy-metal layer.

  • 53. Tiwari, D.
    et al.
    Sharma, R.
    Heinonen, O. G.
    Åkerman, Johan
    KTH, School of Engineering Sciences (SCI), Applied Physics. University of Gothenburg, Sweden.
    Muduli, P. K.
    Influence of MgO barrier quality on spin-transfer torque in magnetic tunnel junctions2018In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 2, article id 022406Article in journal (Refereed)
    Abstract [en]

    We studied the bias dependence of spin transfer torque in the MgO-based magnetic tunnel junction using a field-modulated spin torque ferromagnetic resonance measurement technique for three devices with tunneling magnetoresistances (MRs) of 60%, 67%, and 73%, respectively. The devices with a lower MR ratio showed the presence of multiple modes, while the device with higher MR (73%) showed a single resonance mode. We found a lower out-of-plane torkance in our devices compared to the in-plane torkance. The out-of-plane torque is linear with applied bias, while the bias dependence of in-plane torque shows a strong dependence on the MR ratio and hence the barrier quality.

  • 54. Tiwari, Dhananjay
    et al.
    Behera, Nilamani
    Kumar, Akash
    Durrenfeld, Philipp
    Chaudhary, Sujeet
    Pandya, D. K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Muduli, P. K.
    Antidamping spin-orbit torques in epitaxial-Py(100)/beta-Ta2017In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 111, no 23, article id 232407Article in journal (Refereed)
    Abstract [en]

    We perform spin torque ferromagnetic resonance measurements on the Si(100)/TiN(100)/epi-Py(100)/beta-Ta system. We demonstrate current induced modulation of the Gilbert damping constant, which is about 30% for a current density of 6.25 x 10(9) A/m(2). We show that the observed modulation of the Gilbert damping constant cannot be explained by spin transfer torques arising from the spin Hall effect of the beta-Ta layer. An additional mechanism such as antidamping spinorbit torque resulting from the interface or the crystalline structure of Py thin films needs to be considered. Published by AIP Publishing.

  • 55. Wang, C.
    et al.
    Xiao, D.
    Zhou, Y.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Liu, Y.
    Phase-locking of multiple magnetic droplets by a microwave magnetic field2017In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 7, no 5, article id 056019Article in journal (Refereed)
    Abstract [en]

    Manipulating dissipative magnetic droplet is of great interest for both the fundamental and technological reasons due to its potential applications in the high frequency spin-torque nano-oscillators. In this paper, a magnetic droplet pair localized in two identical or non-identical nano-contacts in a magnetic thin film with perpendicular anisotropy can phase-lock into a single resonance state by using an oscillating microwave magnetic field. This resonance state is a little away from the intrinsic precession frequency of the magnetic droplets. We found that the phase-locking frequency range increases with the increase of the microwave field strength. Furthermore, multiple droplets with a random initial phase can also be synchronized by a microwave field.

  • 56. Xiao, D.
    et al.
    Tiberkevich, V.
    Liu, Y. H.
    Liu, Y. W.
    Mohseni, S. M.
    Chung, Sunjae
    KTH, School of Engineering Sciences (SCI), Applied Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Ahlberg, M.
    Slavin, A. N.
    Åkerman, Johan
    KTH, School of Engineering Sciences (SCI), Applied Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Zhou, Yan
    Parametric autoexcitation of magnetic droplet soliton perimeter modes2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 2, article id 024106Article in journal (Refereed)
    Abstract [en]

    Recent experiments performed in current-driven nanocontacts with strong perpendicular anisotropy have shown that spin-transfer torque can drive self-localized spin waves [W. H. Rippard, A. M. Deac, M. R. Pufall, J. M. Shaw, M. W. Keller, S. E. Russek, G. E. W. Bauer, and C. Serpico, Phys. Rev. B 81, 014426 (2010); S. M. Mohseni, S. R. Sani, J. Persson, T. N. A. Nguyen, S. Chung, Y. Pogoryelov, and J. Akerman, Phys. Status Solidi RRL, 5, 432 (2011)], that above a certain intensity threshold can condense into a nanosized and highly nonlinear dynamic state known as a magnetic droplet soliton [S. M. Mohseni, S. R. Sani, J. Persson, T. N. A. Nguyen, S. Chung, Y. Pogoryelov, P. K. Muduli, E. Iacocca, A. Eklund, R. K. Dumas, S. Bonetti, A. Deac, M. A. Hoefer, and J. Akerman, Science 339, 1295 (2013)]. Here we demonstrate analytically, numerically, and experimentally that at sufficiently large driving currents and for a spin polarization direction tilted away from the normal to a nanocontact plane, the circular droplet soliton can become unstable against the excitations in the form of periodic deformations of its perimeter. We also show that these perimeter excitation modes (PEMs) can be excited parametrically when the fundamental droplet soliton precession frequency is close to the double frequency of one of the PEMs. As a consequence, with increasing magnitude of a bias magnetic field the PEMs with progressively higher indices and frequencies can be excited. Full qualitative and partly quantitative agreement with experiment confirm the presented theoretical picture.

  • 57. Xiao, Dun
    et al.
    Liu, Yaowen
    Zhou, Y.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101), Material Physics, MF (Closed 20120101). University of Gothenburg, Sweden; Shahid Beheshti University, Tehran, Iran.
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. University of Gothenburg, Sweden.
    Merging droplets in double nanocontact spin torque oscillators2016In: Physical Review B, ISSN 2469-9950, Vol. 93, no 9, article id 094431Article in journal (Refereed)
    Abstract [en]

    We demonstrate how magnetic droplet soliton pairs, nucleated by two separated nanocontact (NC) spin torque oscillators, can merge into a single droplet soliton. A detailed description of the magnetization dynamics of this merger process is obtained by micromagnetic simulations: A droplet pair with a steady-state in-phase spin precession is generated through the spin-transfer torque effect underneath two separate NCs, followed by a gradual expansion of the droplets' volume and the out-phase of magnetization on the inner side of the two droplets, resulting in the droplets merging into a larger droplet. This merger occurs only when the NC separation is smaller than a critical value. A transient breathing mode is observed before the merged droplet stabilizes into a steady precession state. The precession frequency of the merged droplet is lower than that of the droplet pair, consistent with its larger size. Merged droplets can again break up into droplet pairs at high enough magnetic field with a strong hysteretic response.

  • 58. Xing, Xiangjun
    et al.
    Pong, Philip W. T.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Department of Physics, University of Gothenburg, Fysikgränd 3, 412 96 Gothenburg, Sweden.
    Zhou, Yan
    Paving Spin-Wave Fibers in Magnonic Nanocircuits Using Spin-Orbit Torque2017In: Physical Review Applied, E-ISSN 2331-7019, Vol. 7, no 5, article id 054016Article in journal (Refereed)
    Abstract [en]

    Recent studies reveal that domain walls in magnetic nanostructures can serve as compact, energy-efficient spin-wave waveguides for building magnonic devices that are considered promising candidates for overcoming the challenges and bottlenecks of today's CMOS technologies. However, imprinting long strip-domain walls into magnetic nanowires remains a challenge, especially in bent geometries. Here, through micromagnetic simulations, we present a method for writing strip-domain walls into bent magnetic nanowires using spin-orbit torque. We employ Y-shaped magnetic nanostructures as well as an S-shaped magnetic nanowire to demonstrate the injection process. In addition, we verify that the Y-shaped nanostructures that incorporate strip-domain walls can function as superb spin-wave multiplexers and that spin-wave propagation along each conduit can be controllably manipulated. This spin-wave multiplexer based on strip-domain walls is expected to become a key signal-processing component in magnon spintronics.

  • 59. Yin, Yuli
    et al.
    Ahlberg, Martina
    Durrenfeld, Philipp
    Zhai, Ya
    Dumas, Randy K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Ferromagnetic and Spin-Wave Resonance on Heavy-Metal-Doped Permalloy Films: Temperature Effects2017In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 8, article id 3502604Article in journal (Refereed)
    Abstract [en]

    Broadband ferromagnetic resonance (FMR)spectroscopy is used to study the temperature (T)and dopantconcentration dependence of the magnetodynamic properties of Permalloy (Py = Ni80Fe20)and Py-100- M-x(x) films, where the dopant M = Pt, Au, and Ag. The saturation magnetization (MS)and Gilbert damping constant (a)are determined from the uniform FMR mode, while the spin wave stiffness (D)is extracted using the first perpendicular standing spinwave mode. The temperature dependence of D is best described by a T (2) law, which suggests a noticeable effect of the itinerant character of the electrons. The spin wave stiffness is also estimated using Bloch's law and the two methods are compared. The results strongly imply that not only spin wave and Stoner excitations, but also other mechanisms contribute to the reduction of MS. The damping increases with T for all samples, but the enhancement is most pronounced for Py doped with 30 at.% Au.

  • 60. Zahedinejad, M.
    et al.
    Awad, A. A.
    Dürrenfeld, P.
    Houshang, A.
    Yin, Y.
    Muduli, P. K.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. University of Gothenburg, Sweden.
    Current Modulation of Nanoconstriction Spin-Hall Nano-Oscillators2017In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 8, article id 7858729Article in journal (Refereed)
    Abstract [en]

    A single nanoconstriction spin-Hall nano-oscillator (NC-SHNO) in out-of-plane fields is presented as a nonlinear amplitude and frequency modulator operated by radio-frequency (RF) current modulation. The current modulation was carried out in different NC-SHNO nonlinearity regimes corresponding to negative, zero, and positive values of df/dI in order to investigate the device response to an 80 MHz modulating current. Our study shows that current modulation of SHNOs can be quantitatively predicted by a nonlinear frequency and amplitude modulation (NFAM) model using the values of df/dI and d2f/dI2 extracted from the free-running frequency f versus current I profile. The NFAM model reproduces the asymmetric sideband amplitude as well as the red and blue shift of the frequency in excellent agreement with the experimental results. The ability to predict the modulation process is a necessary benchmark in designing SHNO modulators for future integrated microwave circuits.

  • 61.
    Zahedinejad, M.
    et al.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Mazraati, Hamid
    KTH, School of Engineering Sciences (SCI), Applied Physics. NanOsc AB, S-16440 Kista, Sweden.
    Fulara, H.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Yue, J.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Jiang, Sheng
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Awad, A. A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, School of Engineering Sciences (SCI), Applied Physics. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden.
    CMOS compatible W/CoFeB/MgO spin Hall nano-oscillators with wide frequency tunability2018In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 13, article id 132404Article in journal (Refereed)
    Abstract [en]

    We demonstrate low-operational-current W/Co20Fe60B20/MgO spin Hall nano-oscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates. Thanks to a record high spin Hall angle of the beta-phase W (theta(SH) = -0.53), a very low threshold current density of 3.3 x 10(7) A/cm(2) can be achieved. Together with their very wide frequency tunability (7-28GHz), promoted by a moderate perpendicular magnetic anisotropy, HiR-Si/W/CoFeB based SHNOs are potential candidates for wide-band microwave signal generation. Their CMOS compatibility offers a promising route towards the integration of spintronic microwave devices with other on-chip semiconductor microwave components.

12 51 - 61 of 61
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