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  • 1. Balinsky, Michael
    et al.
    Haidar, Mohammad
    Ranjbar, Mojtaba
    Durrenfeld, Philipp
    Houshang, Afshin
    Slavin, Andrei
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, 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 Film2016Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 7, artikel-id 3101704Artikel i tidskrift (Refereegranskat)
    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.

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

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

  • 3. Escobar, Roberto A.
    et al.
    Tryputen, Larysa
    Castillo-Sepulveda, Sebastian
    Altbir, Dora
    Chung, Sunjae
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Dept Phys, Sweden.
    Nguyen, Anh Tuan
    Vietnam Acad Sci & Technol, Inst Mat Sci, Hanoi, Vietnam.
    Mohseni, M.
    Akerman, Johan
    Ross, Caroline A.
    Monte Carlo Modeling of Mixed-Anisotropy [Co/Ni](2)/NiFe Multilayers2016Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 7, artikel-id UNSP 4101205Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The magnetic properties of a thin film consisting of an exchange-coupled [Co/Ni](2)/NiFe multilayer have been studied as a function of the NiFe thickness by using Monte Carlo modeling and compared with experimental results of [Co/Ni](4)/Co/NiFe multilayers. Both modeling and experiment showed that the NiFe thickness controls the effective anisotropy. The direction of the easy axis is determined by a competition between the perpendicular crystalline anisotropy of the Co/Ni and the shape anisotropy of the multilayer. As the thickness of the NiFe layer increases, the reversal mechanism of the thin film changes from the nucleation of reverse domains to vortex propagation. Therefore, our results reveal the magnetic configurations and the easy axis reorientation of mixed-anisotropy multilayers.

  • 4.
    Fang, Yeyu
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg, Sweden.
    Zha, Chaolin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    An In Situ anneal study of graded anisotropy FePtCu films2011Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 2Artikel i tidskrift (Refereegranskat)
    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.

  • 5. Houshang, A.
    et al.
    Fazlali, M.
    Redjai Sani, Sohrab
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. NanOsc AB, S-16440 Kista, Sweden.
    Durrenfeld, P.
    Iacocca, E.
    Akerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ Gothenburg, Dept Phys, Gothenburg, Sweden.
    Dumas, R. K.
    Effect of Excitation Fatigue on the Synchronization of Multiple Nanocontact Spin-Torque Oscillators2014Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 5, s. 3000404-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Nanocontact spin-torque oscillators (NC-STOs) act as intrinsically nanoscale and highly current and magnetic field tunable, ultrawide band microwave signal generators. However, their low output power and high phase noise remain critical obstacles toward actual applications. Mutual synchronization of multiple NCs is one possibility to overcome these shortcomings. This letter presents a detailed study of the mutual synchronization in a NC-STO with two NCs. In particular, the effect of repeated measurements on the synchronization behavior is explored. Repeated measurements at high drive currents are shown to significantly degrade the performance of the devices with the most striking consequence being that the devices can no longer be synchronized. Ferromagnetic resonance measurements reveal a decrease in the saturation magnetization and an increase in the damping coefficient in annealed NiFe films, consistent with Cu diffusion into the NiFe from the adjacent Cu layers. This increase in damping will act to sever the spin wave-mediated communication channel between the NCs necessary for synchronization. These results highlight an important consideration when studying the synchronization behavior of multi-NC devices where Joule heating is expected to scale unfavorably with the number of NCs.

  • 6.
    Jiang, Sheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik. NanOsc AB, S-16440 Kista, Sweden..
    Etesami, Seyyed Ruhollah
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Chung, Sunjae
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Uppsala Univ, Dept Phys & Astron, S-75120 Uppsala, Sweden..
    Le, Quang Tuan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Houshang, Afshin
    NanOsc AB, S-16440 Kista, Sweden.;Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Impact of the Oersted Field on Droplet Nucleation Boundaries2018Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 9, artikel-id 3104304Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 7. Ranjbar, M.
    et al.
    Durrenfeld, P.
    Haidar, M.
    Iacocca, E.
    Balinskiy, M.
    Le, T. Q.
    Fazlali, M.
    Houshang, A.
    Awad, A. A.
    Dumas, R. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Goteborgs Universitet, Department of Physics, Goteborg, Sweden .
    CoFeB-Based Spin Hall Nano-Oscillators2014Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 5, s. 3000504-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate magnetization auto-oscillations driven by pure spin currents in spin Hall nano-oscillators based on CoFeB/Pt bilayers. Despite the very low anisotropic magnetoresistance of CoFeB, a substantial microwave signal power can be detected, even at room temperature, indicating that a sizable spin wave amplitude is generated. Spin torque ferromagnetic resonance measurements reveal that the generated auto-oscillation frequency lies below the ferromagnetic resonance frequency of CoFeB and is therefore well described by a self-localized spin wave bullet mode.

  • 8. Yin, Yuli
    et al.
    Ahlberg, Martina
    Durrenfeld, Philipp
    Zhai, Ya
    Dumas, Randy K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Ferromagnetic and Spin-Wave Resonance on Heavy-Metal-Doped Permalloy Films: Temperature Effects2017Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 8, artikel-id 3502604Artikel i tidskrift (Refereegranskat)
    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.

  • 9. Zahedinejad, M.
    et al.
    Awad, A. A.
    Dürrenfeld, P.
    Houshang, A.
    Yin, Y.
    Muduli, P. K.
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. University of Gothenburg, Sweden.
    Current Modulation of Nanoconstriction Spin-Hall Nano-Oscillators2017Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 8, artikel-id 7858729Artikel i tidskrift (Refereegranskat)
    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.

  • 10.
    Zha, Chaolin
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101).
    Dumas, Randy K.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101).
    Persson, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101).
    Mohseni, Seyed Majid
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101).
    Nogues, Josep
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101). Universitat Autònoma de Barcelona, Spain .
    Åkerman, Johan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101). University of Gothenburg, Sweden .
    Pseudo spin valves using a (112)-Textured D022 Mn2.3-2.4Ga fixed layer2010Ingår i: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 1, s. 2500104-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate pseudo spin valves (PSVs) with a (112)-textured D022 Mn2.3-2.4Ga (MnGa) tilted magnetization fixed layer and an in-plane CoFe free layer. Single D022 MnGa films exhibit a small magnetoresistance (MR) typically observed in metals. In MnGa/Cu/CoFe PSVs, a transition from a negative (-0.08%) to positive (3.88%) MR is realized by introducing a thin spin polarizing CoFe insertion layer at the MnGa/Cu interface and tailoring the MnGa thickness. Finally, the exchange coupling between the MnGa and CoFe insertion layer is studied using a first-order reversal curve technique.

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