Published papers

Changes in magnetic-domain configurations caused by the application of radio-frequency (rf) fields ranging from 3 to 18 GHz were observed using magnetic force microscope. rf fields barely altered the magnetization of Co particles with scattered large crystalline anisotropy; however, for NiFe particles, magnetization switching occurred when the rf approached the natural ferromagnetic-resonance frequency of 5.92 GHz. However, the reduction of the switching field fluctuated widely among the particles. Scattering of the effective energy barrier to thermal fluctuations caused by an applied rf field can explain the stochastic switching properties.

We have developed a magneto-optic Kerr effect (MOKE) imaging system with a charge-coupled-device (CCD) camera by using the rotating compensator technique. We chose optimal conditions of the rotation frequency of the compensator with stable rotation along with a CCD camera frame rate that allowed precise control of the exposure timing in order to link with the angle of the compensator. Precise timing management of the CCD exposure enables us to carry out repeated experiments, which greatly improves the signal-to-noise ratio of the longitudinal MOKE signal. We applied the technique to the material characterization of the Ni-81 Fe-19 thin film and its microstructure, and succeeded in evaluating the spatial variation of the complex magneto-optic constant Q of the sample. Because of its attractive advantages such as high-speed and compactness, the present method provides a novel platform for investigating the domain structures in various magnetic materials. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Microwave-assisted magnetization switching (MAS) processes for granular films were demonstrated by time integration of the Landau Lifshitz Cilbert equation. At low microwave frequencies, hysteresis loops become narrower, and at higher frequencies, an extra step occurs in the curves. In the case when the extra step occurs, saturation field increases to be comparable to that for non-MAS case, although switching field decreases. The occurrence of the extra step possibly originates from "unstable switching process" predicted by Bertotti et al., and the magnetization contour maps indicated large un-switched agglomerates remains when MATS proceeds through the unstable switching process. The unstable switching processes are also observed even when dc incident angles are large.

Microwave-assisted magnetization reversal in a 0.36 mu m wide Ni80Fe20 wire with a natural ferromagnetic resonance frequency of 8.42 GHz was investigated experimentally. Magnetization switching could be detected from a discontinuous change in the ferromagnetic resonance frequency of the wire. The application of microwaves with a frequency of 7.5 GHz and a power of 19.2 dBm reduced the switching field from 217 to 142 Oe. While high-power microwave-assisted switching reduced the dc switching field, it substantially broadened the switching field distribution. This is mainly attributed to inhomogeneous resonance properties of the nucleation sites in the wire. (c) 2007 American Institute of Physics.

The present article reports a geometrically induced magnetoresistance (MR) behavior observed in magnetic strips consisting of a symmetric layer structure with lateral sizes down to 0.2 mu m. The MR ratio was markedly improved from 2.8% for an as-grown film of Co(6 nm)/Cu(6 nm)/Co(6 nm) to 4.6% by structuring it into a strip with 0.3 mu m width. The parabolic MR behavior observed in the hard-axis direction suggests that the magnetization in the two magnetic layers rotates toward the opposite directions, which can be explained by the magnetostatic interaction between the surface magnetic charges at the pattern edges. The dependence of MR saturation field on the pattern width agrees well with a theoretical prediction based on a coherent rotation model. The MR ratio was markedly increased by the increase of the layer number and the thickness of the magnetic layer, the highest value of 7.8% was obtained for a strip made of [Co(6 nm)/Cu(4 nm)](2) Co(6 nm). A linear- and hysteresis-free MR response was performed with an alternative field modulation under a transverse bias field. (C) 1999 American Institute of Physics. [S0021-8979(99)63008-0].

Hydrogenation characteristics and magnetic properties of Ni fine particles with thin Pd shell were investigated. The Pd shell appeared to turn weakly ferromagnetic below about 20 K, but this magnetic polarization was suppressed by hydrogenation.

A novel ferromagnetic tunnel junction with transition metal oxides has been fabricated with photo-lithographic technique. Well defined tunneling properties were observed in a novel barrier material of TiO_x with the thickness down to 2 nm. Spin dependent transport properties have been confirmed for Fe_3O_4/TiO_x/M (M=Co, Co/NiFe, NiFe) tunnel junctions, where highest magneto-resistence (MR) change of 7% was observed for Co. Drastic improvement of field sensitivity was attained with the exchange coupled Co/NiFe bi-layer.

Understanding the dynamic process of microwave-assisted magnetization reversal (MAMR) in a dipole-dipole coupled granular perpendicular medium is essential for its application to future magnetic storage devices. We experimentally investigated the magnetization reversal process in CoCrPt-SiO2 granular films in terms of the dependence of the switching field on the strength of an applied in-plane radio-frequency magnetic field. The reversal process changed from non-uniform rotation to uniform rotation when the frequency increased toward the ferromagnetic resonance frequency. In the resonant condition, the switching field agreed well with a model assuming uniform rotation of the magnetization in each grain. The agreement suggests that the decoherence of ferromagnetic resonance due to a dynamic dipolar field is weak in adequately exchange-coupled granular films with a saturation magnetization of 415 emu/cm(3). This finding is significant for the development of the practical medium necessary for MAMR writing. (C) 2013 AIP Publishing LLC.

We report structural and magnetic properties of Fe thin films, deposited onto (100)MgO substrates by rf magnetron sputtering. The successful epitaxial growth of Fe<100>(100) parallel to MgO<110>(100) was achieved at a practical growth rate of 0.73 nm/s. The M-H hysteresis measured along Fe<100> directions exhibits a superior squareness with a low coercivity (H-c = 5 Oe). The saturation field along <110> directions, exceeding 500 Oe, demonstrates the crystalline cubic anisotropy comparable to the bulk Fe. The anisotropic properties of the initial susceptibility against an oscillation field of 1 mOe amplitude were also confirmed with inductive measurements.

Cooling velocity of patterned Tb-Fe film with a lateral size of 50 x 50 mu m(2) was experimentally investigated for the application to heat-assisted writing operation in high-density magnetic random access memories. The patterned Tb-Fe film was heated-up by using pulsed YAG laser irradiations with the duration of 1 ns which is realizing a semi-adiabatic heating process. The ratio of magnetization reversal in the patterned Tb-Fe film was estimated by measuring an anomalous Hall voltage. The main results are as follows: (1) Magnetic field required for magnetization reversal of Tb-Fe film was decreased from 1.4 kOe to 200 Oe by applying the heat-assisted magnetization reversal method. (2) Cooling velocity of patterned Tb-Fe film can be estimated by using pulsed magnetic fields with various time delays from the pulsed laser irradiation. (3) With increasing the thermal conductivity coefficient of electrical leads, the velocity to cool the patterned Tb-Fe film increased.

eng=In the present study, high sensitive detection of the reversible susceptibility modulation in the magnetic thin film was performed by an LC circuit resonant technique. A flat pick-up coil with the inductance of 0.80 μH and a capacitance of 100 pF construct the LC resonant circuit. The reversible susceptibility x of the magnetic thin film, embedded in the pick-up coil, was measured with a sinusoidal exciting field (H). The out put signal was modulated by applying an orthogonal bias field (H_b). A multilayer sample of [NiFe(100 nm)/Cu(10 nm)]_20, exhibits the most sensitive modulation effect among the fabricated samples. That is, the resonant frequency f_r was decreased from 11.58 MHz (H_b = 0 Oe) to 8.22 MHz (H_b = 7 Oe) at H_ = 2.5 mOe, suggesting the marked increase of x . The value of H_b, which minimizes f_r, is comparable to the anisotropy field. Thus, the observed drastic increase of x can be associated with the compensation of the anisotropy with H_b, as confirmed from the theoretical prediction. The practical out put voltage change of 55%/Oe was realized for the fabricated resonator.

© 2018 American Physical Society. In this paper, we detect the coupled oscillation of magnetic vortices in Ni81Fe19 gourd-shaped thin films electrically to evaluate the nonadiabatic spin-transfer torque (STT) coefficient β, which is important for understanding a variety of STT-related phenomena. We excite a translational mode in coupled magnetic vortices by applying an alternating electrical current that causes both STT and Oersted field torque. Since the coupled oscillation modes due to the STT and Oersted field torque have totally different eigenfrequencies, the coupled oscillation spectrum can be used to evaluate β, which is generally difficult to distinguish from the Oersted field contributions. Based on the ratio between the amplitudes of the spectrum's Lorentzian and anti-Lorentzian components, the value of β is found to be 0.085±0.016, which is 4.3 times larger than the Gilbert damping constant α.

An alternating current (ac) magnetic field or spin current can reduce the switching field of a ferromagnet through resonance excitation of a large-angle precession of magnetization. The nonlinear magnetization dynamics of this switching scheme completely differ from the general ferromagnetic resonance phenomenon, which is linearly excited by a small ac magnetic field. To understand these dynamics, it is necessary to evaluate the effective potential barrier height for switching, ΔUeff. However, most previous studies have measured the consequent precession angle in the nonlinear dynamics by magneto-optical methods and/or by applying a magneto-resistive effect. Here, we applied the cooperative switching method, which evaluates the ΔUeff of the nonlinear dynamics under a sub-ns-wide magnetic field impulse, and observed a nontrivial reduction of ΔUeff in a submicron-wide NiFe strip. The strong reduction of ΔUeff under a negative magnetic field was caused by a saddle-node bifurcation in the nonlinear dynamics. In a micromagnetics simulation, we also confirmed that the magnetization is nonuniformly excited at the shallowest ΔUeff.

Mechanical rotation of a crystal lattice in ferromagnetic materials can be energetically coupled with its magnetization via magnetoelastic coupling or spin rotation coupling. Surface acoustic wave (SAW) in piezoelectric materials is one of the promising candidates to realize the mechanical excitation of magnetization dynamics. In order to understand the mechanical rotation induced magnetization dynamics quantitatively, we examined the ferromagnetic resonance in an Ni film using a SAW in a LiNbO3 substrate. By decreasing a period of interdigital transducer as short as 4 μm which is one-fifth of the previous work by Weiler [M. Weiler and co-workers, Elastically driven ferromagnetic resonance in Nickel thin films. Phys Rev Lett 106, 117601 (2011)], the fundamental frequency of SAW could be higher than 800 MHz. From the dependence of microwave absorption on the angle between the magnetization and the wave vector of SAW, it was confirmed that the Rayleigh type SAW, which was significant to obtain a large mechanical coupling with the magnetization, was dominantly excited in the 800-MHz-SAW device.