Published papers

The spin-dependent CPP (current perpendicular to plane) transport properties in semiconductive-oxide(MOx)/ ferromagnet multilayers consisting of CoPt/Co/MOx/Co/MOx/Co/CoPt, where M = Ti, Sn, were investigated by magneto-transport measurement. The XRD patterns reveal that the structures of sputtered MOx films are amorphous or nanocrystalline. The MR ratio of a TiOx-based multilayer (0.43%) was much smaller than that of a SnOx- based one (2.0%) at T = 77 K. As inferred from the shape of MR curves with a large saturation field above 4 kOe, suppressed MR change in the TiOx-based multilayer is caused by the interlayer diffusion of Co atoms into the TiOx layer. The same behavior was not observed in the SnOx-based one. The MR ratio was reduced by half on increasing t(SnOx) from 4 to 8 nm. This decrease can be ascribed to the scattering of the spin-polarized electron in the SnOx layer. (C) 2002 Elsevier Science B.V. All rights reserved.

Recent rapid progress in spintronic and magnetic storage nanodevices has required nano-magnets to balance competing goals for high coercive field and low switching field. However, a decisive route for highly efficient magnetization switching has not been established yet. Here we propose a novel switching method using a spin wave of magnetic structures twisted in a nanometre scale. We have experimentally demonstrated extremely low field-magnetization switching in a highly coercive FePt by using a spin wave excited in a soft magnetic permalloy (Ni81Fe19), where permalloy is exchange-coupled to FePt through the interface. We can tune the switching field by varying the magnitude and frequency of the radio frequency magnetic field, and a significant decrease in switching field by one order of magnitude is achieved under the optimum conditions. The spin wave-assisted magnetization switching is a promising technique for ultralow-energy magnetization manipulation.

We present a microscopic theory of spin torque due to diffusive spin currents induced by spin accumulation. The obtained expression is a natural extension of the existing one due to "local" spin currents associated with ordinary electric currents, and is the reciprocal of the spin motive force which induces charge accumulation as studied recently [J. Shibata and H. Kohno, Phys. Rev. B 84, 184408 (2011)]. The result is applied to a domain wall motion in a nonlocal spin injection system, and the torque and force due to diffusive spin current are evaluated. DOI: 10.1103/PhysRevB.87.094404

Thermally assisted magnetization reversal of sub-100 nm dots with perpendicular anisotropy has been investigated using a micromagnetic Langevin model. The performance of the two different reversal modes of (i) a reduced barrier writing scheme and (ii) a Curie point writing scheme are compared. For the reduced barrier writing scheme, the switching field H(swt) decreases with an increase in writing temperature but is still larger than that of the Curie point writing scheme. For the Curie point writing scheme, the required threshold field H(th), evaluated from 50 simulation results, saturates at a value, which is not simply related to the energy barrier height. The value of H(th) increases with a decrease in cooling time owing to the dynamic aspects of the magnetic ordering process. Dependence of H(th) on material parameters and dot sizes has been systematically studied. (C) 2008 Elsevier B.V. All rights reserved.

Temperature dependence of magnetic properties in [Co/Pd]_N multilayer have been systematically studied for lithographically patterned samples with different Co thicknesses t_Co and the bilayer number N by extraordinary Hall effect measurements. The perpendicular coercive field H_c, decreases with the increase of t_Co, excepting a very thin thickness of 1 Å. While a threshold temperature (T_th) of H_c, vanishing linearly increases with the increase of t_Co. The H_c, and T_th, monotonously increases with the increase of N, ranged from 5 to 50. The superior crystalline structure for samples with the larger N was confirmed from enhanced Co/Pd(111) peak height in the XRD pattern. A practical thermal activation coefficient α of 174 was evaluated from a sweep rate dependence of H_c. A marked improvements for the heat assisted MRAM application was attained by air annealing of [Co(1.7Å)Pd(8Å)]_30 at 220℃, that is, the H_c was increased from 1.3 kOe to 2.9 kOe, while T_th reduced from 210℃ to 190℃.

The modulation of a switching field by a spin-transfer torque generated by the spin-pumping effect is demonstrated in spin-valve strips. A spin current pumped into a spin valve prefers an antiparallel configuration of magnetization, so that the hysteresis loop of a soft magnetic layer is shifted by applying an alternating-current magnetic field with a frequency that matched the resonant condition of a hard magnetic layer. Furthermore, we confirmed that an alternating spin current generated by the spin pumping in the hard magnetic layer produces an orthogonal magnetic torque that can reduce the coercive field of the soft magnetic layer.

Modulation of ferromagnetic resonance (FMR) linewidth owing to a spin transfer torque (STT) caused by the absorption of a pure spin current in a permalloy nanowire is demonstrated. To exclude the STT induced by the spin-polarized current, the pure spin current was produced using a lateral spin-valve structure. Self-homodyne FMR detection without applying an external sense current was used for minimizing the influence of Joule heating on the FMR linewidth. The linear variation of the FMR linewidth caused by the STT was clearly observed when the electrical current generating the pure spin current was below 1 mA. (C) 2015 The Japan Society of Applied Physics

© 2019 American Physical Society. Three-magnon scattering can be used to evaluate the lowest frequency of the magnon band in a ferromagnet. Both the ferromagnetic-resonant (FMR) frequency fFMR and the lowest frequency of the magnon band fmin in narrow-shaped NiFe strips were electrically measured using the anisotropic-magnetoresistance effect. The comparison with a micromagnetic simulation shows that fmin of the magnon band can be controlled independent of fFMR by varying the width w and thickness t of the NiFe strip while maintaining a constant t/w ratio. In addition, we found that the frequency difference, fFMR-fmin, can be greatly increased in thicker NiFe strips. Our results show that narrow-shaped ferromagnets allow us to tune the magnon-band structures by varying their w and t. This ability is important for designing magnon circuits in integrated magnonic devices and for improving the quantitative study on the Bose-Einstein condensation of magnons.

We experimentally demonstrate spin-torque generation using a compositional gradient at the interface between titanium and tungsten thin films. The width of the compositional gradient interface (CGI) between films is varied from 1.4 to 2.0 nm via sputtering. Spin-torque ferromagnetic resonance is observed in the ferromagnetic Ni95Cu5 alloy fabricated on a Ti/W bilayer with the CGI. The positive spin torque increases with decreasing CGI width, but a negative spin torque is superimposed owing to the negative spin Hall effect in bulk tungsten. A structural undulation in the CGI eliminates this variation in positive spin torque. The CGI width dependence of the spin torque is associated with the generation of spin and/or orbital angular momentum flow at the CGI. Spin-torque generation using a CGI expands the range of material choice for magnetic nonvolatile memory applications.

The temperature dependence of the switching properties in low sub-mum scale Co/Ni80Fe20 particles with different aspect ratios L/w (= 1-6, w = 0.4 mum) and thicknesses t (= 1-5 nm) are investigated using magnetotransport measurements of relief structured. films with giant magnetoresistance. The shape of each particle is a parallelogram with slope angle theta(=0degrees-60degrees). The switching field of the particles increased with theta due to the variation of the end-domain configuration. Nonlinear temperature dependence of the switching field was observed, which could be explained by the thermal nucleation theory of the magnetization reversal at below 150 K. The amplitude of the energy barrier height DeltaE for the reversal was strongly suppressed by decreasing the thickness of the particles. This degradation of the thermal stability may be caused by the variation of the film morphology by decreasing the thickness. (C) 2003 American Institute of Physics.

The possibility of selective magnetization switching in a three-layered stacked magnetic pillar was discussed using micromagnetic simulation that considers three-dimensional magnetic recording. Selective magnetization switching is possible with the assistance of a circularly polarized microwave field with an adequate corresponding frequency. This study demonstrates that the value of the saturation magnetization of the magnetic layers is critical in suppressing selective magnetization switching errors caused by magnetostatic fields. A relatively large saturation magnetization of up to 600 emu/cm(3) in the second layer also allows successful selective magnetization switching, thus enabling three-dimensional magnetic recording. However, damping constants smaller than 0.1 for the magnetic layers are crucial in the model calculations because of the limited strength of the microwave fields applied to the stacked magnetic pillar. The leakage flux is also estimated considering a multi-bit reproducing process, where eight-digit signals are obtained depending on the magnetization states of the stacked magnetic pillar. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801888]

Microwave-assisted recording on a 500-Gbpsi-class perpendicular medium was experimentally demonstrated at room temperature. Magnetization reversal under a radio-frequency magnetic field was measured by an electrically shorted coplanar waveguide, which enabled us to evaluate the change in the medium's ferromagnetic resonance spectrum. A frequency-dependent reduction in the switching field was clearly observed in response to a microwave impulse 50 ns in duration. A significant reduction of up to 30% in the coercive field was achieved by applying a microwave impulse with an amplitude of 25 dBm and a frequency of 15 GHz. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759169]

We consider the spin current induced from temperature gradient in disordered ferromagnet. Using the diagrammatic calculation, we examine a spin current in the longitudinal direction to the temperature gradient.

Fundamental operations in magneto-static spin wave devices of nucleation, propagation and inductive detection for a spin wave packets (magnetic solitons) are numerically studied by using micromagnetic simulations. The collective spin dynamics and device performances are compared for three different spin wave modes. Simulation results of the group velocity reasonably agree with the theoretical prediction under a thin film approximation. Structural and operational parameters are optimized so as to maximize the inductive output voltage V-out. The practical V-out of 0.052 mV/mu m(2) is obtained at optimized parameters in a spin wave mode of magneto-static backward volume wave.

Microwave-assisted magnetization reversal (MAMR) is numerically investigated for perpendicularly magnetized particles with a natural ferromagnetic resonance frequency of 22.5 GHz. The precession angle of magnetization can be enhanced intensively by increasing the amplitude of the ac hard-axis field with an optimum frequency. However, the precession of magnetization about the easy axis becomes unstable as the precession angle reaches a critical value because the easy-axis component of the effective field exerted on the magnetization becomes zero. The analysis of magnetization dynamics in MAMR showed that the precession of magnetization about an in-plane axis, appearing after the easy-axis component of the effective field becomes zero, plays an important role in the irreversible switching of magnetization. © 2009 American Institute of Physics.

The thermally assisted reversal of micro-magnetization was simulated for sub-100-nm dots with perpendicular magnetization by using a Langevin-Landau-Lifshitz-Gilbert equation, where thermal effects are modeled by random fields. Two different thermally assisted reversal methods of 1) a thermal relaxation scheme and 2) a switching scheme were compared. Superior performance was obtained for the thermal relaxation scheme from the view point of reducing the reversal field. An auto correlation function in the time domain was introduced to analyze the stochastic properties of the thermally fluctuating directions of magnetization.

Time evolution of magnetization during a cooling process in thermally assisted magnetization reversal of rectangular MRAM cell has been numerically investigated by taking account of not only temperature variations of material parameters but also a thermal fluctuation effect. The rectangular MRAM cell with 100×150×20 nm3 in size is assumed, where the shape anisotropy dominates the switching property. The threshold amplitude of magnetic field required for a successful thermally assisted switching is decreased as the exchange stiffness constant is changed from 1×10-6 to 1×10-7 erg/cm. The time for the magnetization to relax into the bias field direction can be reduced by increasing the bias field amplitude. To switch the magnetization within 1 ns, it is found that the bias field should be larger than 165 Oe. An exchange coupling field in a ferromagnetic bilayer system is considered to be available for producing such a large bias field required for a faster switching in thermally assisted MRAMs.

Magnetization reversal via successive ballistic rotation caused by an application of alternative field temporally varying within a relaxation time limit has been numerically investigated. As the Gilbert damping constant is smaller than 0.03, a pronounced beat appears in the temporal evolution of rotating angle of magnetization. The amplitude of the rotating angle at the first peak of the beat is larger than that achieved in the stable ferromagnetic resonance, so that the magnetization reversal occurred at the first peak of the beat. Consequently, subnanosecond switching of magnetization is realized even though a ferromagnetic resonant phenomenon is utilized to enhance the rotating angle of magnetization. In such a switching scheme, it is most important to optimize the frequency of the alternating field to ensure a practical margin of the operation current creating the switching field. (c) 2006 American Institute of Physics.