Hiroshi Imamura

National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan

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Publications (159)439.84 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Generation of practical random numbers (RNs) by a spintronics-based, scalable truly RN generator called "spin dice" was demonstrated. The generator utilizes the stochastic nature of spin-torque switching in a magnetic tunnel junction (MTJ) to generate RNs. We fabricated eight perpendicularly magnetized MTJs on a single-board circuit and generated eight sequences of RNs simultaneously. The sequences of RNs of different MTJs were not correlated with each other, and performing an exclusive OR (XOR) operation among them improved the quality of the RNs. The RNs obtained by performing a nested XOR operation passed the statistical test of NIST SP-800 with the appropriate pass rate.
    Applied Physics Express 07/2014; 7(8):083001. · 2.73 Impact Factor
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    ABSTRACT: Magnetization switching assisted by high-frequency voltage was demonstrated in magnetic tunnel junctions with perpendicular magnetization. Ferromagnetic resonance was excited by high-frequency voltage instead of by a high-frequency magnetic field. A frequency-dependent reduction in the switching field (coercive force) was clearly observed with a maximum reduction rate of more than 80% under a radio-frequency power application of 3 dBm. The developed technique can provide a novel approach to energy-assisted magnetization switching with low ohmic dissipation.
    Applied Physics Express 06/2014; 7(7):073002. · 2.73 Impact Factor
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    ABSTRACT: Microwave oscillation properties of spin torque vortex oscillators (STVOs) consisting of an FeB vortex free layer were investigated. Because of a high MR ratio and large DC current, a high emission power up to 3.6 µW was attained in the STVO with a thin FeB free layer of 3 nm. In STOs with a thicker FeB layer, e.g., 10 nm thick, we obtained a large Q factor greater than 6400 while maintaining a large integrated emission power of 1.4 µW. Such excellent microwave performance is a breakthrough for the mutual phase locking of STVOs by electrical coupling.
    Applied Physics Express 06/2014; 7(6):063009. · 2.73 Impact Factor
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    ABSTRACT: We report on the oscillation behavior of spin torque oscillators having a perpendicularly magnetized free layer and in-plane magnetized reference layer as a function of bias field angle. The measurement results show that both emission power and oscillation frequency are strongly dependent on the bias field angle. When the bias field was tilted by only a few degrees away from the axis normal to the film toward either parallel or antiparallel configuration, the power increased by about 1.5 times or decreased by two orders of magnitude, and the peak frequency varied by about ±1 GHz.
    Applied Physics Express 05/2014; 7(6):063005. · 2.73 Impact Factor
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    ABSTRACT: We investigated the low-field oscillation in a spin-torque oscillator (STO) consisting of a perpendicularly magnetized free layer and an in-plane magnetized reference layer. A sharp drop in oscillation frequency at a low magnetic field was observed when the free layer of the STO possessed high perpendicular magnetic anisotropy. In contrast to the results obtained in the past, the investigation performed in this study was consistent with the theoretical predictions in the low-field region. The study presented unveils the oscillation mechanism in the entire field range, which is crucial in designing practical STO devices.
    Japanese Journal of Applied Physics 05/2014; 53(6):060307. · 1.07 Impact Factor
  • Yohei Kota, Hiroshi Imamura, Munetaka Sasaki
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    ABSTRACT: We studied lattice deformation effect on exchange interaction in the corundum-type Cr2O3 theoretically. First-principles electronic structure calculations were performed to evaluate the total energy and exchange coupling constants of Cr2O3 under lattice deformation. We found that a few percent elastic deformation is expected via misfit strain and that the first- and second-nearest neighbor exchange coupling constants of Cr2O3 strongly depend on the lattice deformation. These results imply a possibility for improving the thermal stability of Cr2O3 based magnetoelectric devices by lattice deformation.
    Journal of Applied Physics 04/2014; 115(17). · 2.21 Impact Factor
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    ABSTRACT: Measurements of thermally excited ferromagnetic resonance were performed on spin torque oscillators having a perpendicularly magnetized free layer and in-plane magnetized reference layer (abbreviated as PMF-STO in the following) for the purpose of obtaining magnetic properties in the PMF-STO structure. The measured spectra clearly showed a large main peak and multiple smaller peaks on the high frequency side. A Lorentzian fit on the main peak yielded Gilbert damping factor of 0.0041. The observed peaks moved in proportion to the out-of-plane bias field. From the slope of the main peak frequency as a function of the bias field, Lande g factor was estimated to be about 2.13. The mode intervals showed a clear dependence on the diameter of the PMF-STOs, i.e., intervals are larger for a smaller diameter. These results suggest that the observed peaks should correspond to eigenmodes of lateral spin wave resonance in the perpendicularly magnetized free layer.
    Journal of Applied Physics 04/2014; 115(17). · 2.21 Impact Factor
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    ABSTRACT: We systematically investigated the Gilbert damping (α) and interfacial perpendicular magnetic anisotropy (Ki) in magnetic tunnel junctions with a MgO-barrier/FeB/MgO-cap layered structure using the spin-torque diode effect. By increasing the MgO cap thickness, α decreased, whereas Ki increased monotonically. Values down to 0.0054 for α and up to 3.3 erg/cm2 for Ki were obtained for a MgO cap thickness of 0.6 nm. The small α and large Ki suggest that MgO-capped FeB is a suitable free layer for spintronics devices such as spin-torque oscillators and spin-torque magnetic random access memories.
    Applied Physics Express 02/2014; 7(3):033004. · 2.73 Impact Factor
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    ABSTRACT: The self-oscillation of the magnetization in a spin torque oscillator (STO) with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer in the absence of an applied magnetic field was studied by numerically solving the Landau-Lifshitz-Gilbert equation. It was pointed out that field-like torque was necessary to realize stable self-oscillation in this type of STO at zero field. The numerical simulation at finite temperature showed that the presence of the field-like torque led to a high power with a relatively high oscillation frequency.
    Applied Physics Letters 01/2014; 104(15). · 3.79 Impact Factor
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    Tomohiro Taniguchi, Hiroshi Imamura
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    ABSTRACT: The spin torque switching rate of an in-plane magnetized system in the presence of an applied field is derived by solving the Fokker-Planck equation. It is found that three scaling currents are necessary to describe the current dependence of the switching rate in the low-current limit. The dependences of these scaling currents on the applied field strength are also studied.
    Journal of Applied Physics 01/2014; 115(17). · 2.21 Impact Factor
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    ABSTRACT: The current dependence of the spin torque switching rate in a thermally activated region of an in-plane magnetized system was studied. The logarithm of the switching rate depended nonlinearly on current in the high-current region, I_{c} < I < I_{c}^{*}, where I_{c} and I_{c}^{*} are critical currents distinguishing the stability of the magnetization. We also found that the attempt frequency had a minimum around I_{c}, and that the attempt frequency at I_{c} was three orders of magnitude smaller than that at zero current, contrary to the assumption in previous analyses of experiments that it remains constant.
    12/2013;
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    ABSTRACT: The oscillation properties of a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer are studied based on an analysis of the energy balance between spin torque and damping. The critical value of an external magnetic field applied normal to the film plane is found, below which the controllable range of the oscillation frequency by the current is suppressed. The value of the critical field depends on the magnetic anisotropy, the saturation magnetization, and the spin torque parameter.
    12/2013;
  • Nobuhiko Yokoshi, Hiroshi Imamura, Hideo Kosaka
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    ABSTRACT: We theoretically show that an entangled state can be prepared between distant heavy-hole spin qubits by using III–V semiconductor quantum dots (QDs) in cascaded cavities. In this scheme, using quantum state transfer between photons and carrier spins, the local spin-singlet state in a virtually excited trion is stretched into the two separated QDs via a single-photon transmission. We calculate the effective Liouville equation and determine the optimal conditions for entanglement creation. We also discuss the possibility of distributing entangled electron spins in the same manner.
    Physical Review B 10/2013; 88(15). · 3.66 Impact Factor
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    ABSTRACT: Highly sensitive microwave devices that are operational at room temperature are important for high-speed multiplex telecommunications. Quantum devices such as superconducting bolometers possess high performance but work only at low temperature. On the other hand, semiconductor devices, although enabling high-speed operation at room temperature, have poor signal-to-noise ratios. In this regard, the demonstration of a diode based on spin-torque-induced ferromagnetic resonance between nanomagnets represented a promising development, even though the rectification output was too small for applications (1.4 mV mW(-1)). Here we show that by applying d.c. bias currents to nanomagnets while precisely controlling their magnetization-potential profiles, a much greater radiofrequency detection sensitivity of 12,000 mV mW(-1) is achievable at room temperature, exceeding that of semiconductor diode detectors (3,800 mV mW(-1)). Theoretical analysis reveals essential roles for nonlinear ferromagnetic resonance, which enhances the signal-to-noise ratio even at room temperature as the size of the magnets decreases.
    Nature Material 10/2013; · 35.75 Impact Factor
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    Tomohiro Taniguchi, Hiroshi Imamura
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    ABSTRACT: The current dependence of the switching barrier for spin torque switching of an in-plane magnetized ferromagnet was studied. Two scaling currents, I_{c}$ and I_{c}^{*}(>I_{c}), were introduced to distinguish the magnetization stability. In the low-current region I<I_{c}, the switching barrier is linear to the current with another scaling current \tilde{I}_{c}, while such linear scaling does not hold in the high-current region I_{c} < I < I_{c}^{*}. The linear scaling is valid for the high temperature and the long current pulse duration time.
    Applied Physics Express. 09/2013; 6(10).
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    Tomohiro Taniguchi, Hiroshi Imamura
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    ABSTRACT: The optimum condition of an applied field direction to maximize spin torque diode voltage was theoretically derived for a magnetic tunnel junction with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer. We found that the diode voltage for a relatively small applied field is maximized when the projection of the applied field to the film-plane is parallel or anti-parallel to the magnetization of the pinned layer. However, by increasing the applied field magnitude, the optimum applied field direction shifts from the parallel or anti-parallel direction. These analytical predictions were confirmed by numerical simulations.
    Journal of Applied Physics 08/2013; 114(5). · 2.21 Impact Factor
  • Hiroshi Tsukahara, Hiroko Arai, Hiroshi Imamura
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    ABSTRACT: We theoretically analyzed the current-driven vortex--antivortex pair rotation in a magnetic thin film where spatially localized spin-polarized current is injected through two nonmagnetic nanocontacts. We performed the micromagnetic simulations and found that the dynamics of magnetization strongly depends on the distance between the contacts. If the distance between the contacts is large enough to separate the trajectories of vortex--antivortex (VA) pair rotation, the two VA pairs rotate around each contact with an angular frequency identical to that of a single contact system. In contrast, if the distance between the contacts is so short that the trajectory of each VA pair rotation can overlap with each other, two VA pair rotations are merged into one VA pair rotation whose trajectory encloses the two contacts as if a large elliptical contact exists at their midpoint. We showed that the rotation frequency decreases with increasing length of the trajectory. The rotation frequency for the trajectory enclosing the two contacts is much smaller than that for the single contact system. We also found that there is an intermediate distance where VA pair rotation collapses.
    Journal of the Physical Society of Japan 08/2013; 82(8):4701-. · 2.09 Impact Factor
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    ABSTRACT: The magnetization dynamics of spin torque oscillator (STO) consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer was studied by solving the Landau-Lifshitz-Gilbert equation. We derived the analytical formula of the relation between the current and the oscillation frequency of the STO by analyzing the energy balance between the work done by the spin torque and the energy dissipation due to the damping. We also found that the field-like torque breaks the energy balance, and change the oscillation frequency.
    IEEE Transactions on Magnetics 07/2013; 50(1). · 1.42 Impact Factor
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    ABSTRACT: We investigated the Gilbert damping of thin films with a MgO-barrier/Fe80B20/MgO-cap/Ta structure, in which the Fe80B20 layer corresponds to a free layer of a perpendicularly magnetized magnetic tunnel junction, by using ferromagnetic resonance (FMR). We systematically varied the thickness of the MgO cap layer and estimated the damping constant (α) by measuring the dependence of the FMR spectrum on the magnetic field angle. The MgO cap layer was found to reduce α to about 0.005, which is less than half the value without the cap. The significant reduction can be explained by the suppression of spin pumping by the dielectric MgO cap layer.
    Applied Physics Express 07/2013; 6(7):3002-. · 2.73 Impact Factor
  • Tomohiro Taniguchi, Hiroshi Imamura
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    ABSTRACT: The spin torque assisted magnetization switching in the thermally activated region was theoretically studied based on the rate equation. We found the dependence of the switching current on the current pulse duration time to be linear for b = 1 and nonlinear for b = 2. Here, b is the exponent of the current term in the switching rate formula, which has been assumed to be unity while a theory with b = 2 was proposed recently. A comparison of the calculated results with the recent experiments of Heindl et al. [J. Appl. Phys. 109, 073910 (2011)] indicates b ≠ 1.
    Journal- Korean Physical Society 06/2013; 62(12). · 0.51 Impact Factor

Publication Stats

1k Citations
439.84 Total Impact Points

Institutions

  • 2006–2014
    • National Institute of Advanced Industrial Science and Technology
      • • Spintronics Research Center
      • • Human Technology Research Institute
      Tsukuba, Ibaraki, Japan
  • 1998–2013
    • Tohoku University
      • • Institute for Materials Research
      • • Research Institute of Electrical Communication
      • • Graduate School of Engineering
      • • Graduate School of Information Sciences
      Sendai, Kagoshima-ken, Japan
  • 2012
    • National Institute for Materials Science
      • Magnetic Materials Unit
      Tsukuba, Ibaraki-ken, Japan
  • 2007–2010
    • Tsukuba Research Institute
      Edo, Tōkyō, Japan
    • RIKEN
      Вако, Saitama, Japan
  • 2009
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2003
    • Karlsruhe Institute of Technology
      • Institute for Theoretical Solid State Physics
      Karlsruhe, Baden-Wuerttemberg, Germany