Published papers

High-quality Fe3O4 thin films have been fabricated onto metallic underlayers of Cr/Cu and Al by rf-magnetron sputtering at low substrate temperatures (< 573 K). The measured saturation magnetizations M-s are 462 emu/cm(3) for Al (50 nm)/Fe3O4 (200 nm) and 422 emu/cm(3) for Cr (45nm)/Cu (300nm)/Fe3O4 (200nm), which are markedly enhanced compared with that for the reference sample deposited directly on a glass substrate, and practically comparable to the bulk value of 477 emu/cm(3). Highly conductive transport with an order-disorder change of the Verwey transition was observed in the current-perpendicular-to-plane geometry. The order of decrease in coercive field was achieved by exchange coupling with an overlaid NiFe layer.

In the present study, magnetic nanowires with various cross-sectional dimensions have been fabricated by evaporating a magnetic material on the side edges of the patterned nonmagnetic thin film. The method allows the lengths of the two short axes to be defined precisely by controlling the thickness of the oxide (W-x) and magnetic material (W-y). The MFM images of the produced Co wires (2 mu m length) with dimensions of W-x=W-y=200 nm and W-x=W-y=50 nm present the single domain configuration. The MFM and magnetoresistance (MR) measurement demonstrate that the longitudinal magnetization switching field increases with the decrease of W-x and W-y, which becomes 1.3 kOe for wires with W-x=W-y=50 nm. The profile of the MR hysteresis suggests that considerable coherent rotation takes place before the irreversible longitudinal switching. Transverse saturation fields larger than 6 kOe were measured in the orthogonal two short axis directions, which confirms the successful fabrication of three-dimensional wires with comparable cross-sectional dimensions. (C) 2000 American Institute of Physics. [S0021-8979(00)60608-4].

Linear magntoresistance (MR) response of Co/Cu multilayer strips with sub-mu m width has been performed with a field sensitivity of sub-Oe. The orthogonal spin orientation for neighboring Co layers is initiated with an additional external field applied along the short axis of the strip. The thickness of the Cu layer was optimized to be 2.1 nm, at which the MR ratio of the as grown sample took a maximum value. The MR ratio of the as-grown film of [Co(2 nm)/Cu(2.1 nm)](3)/Co(2 nm)/Cu(2 nm) was 12.4%. The multilayer was patterned into strips with the pattern width w down to 0.2 mu m by means of electron beam lithography and Ar ion milling. The measured maximum MR ratio of the strips with w=0.4 and 0.2 mu m were 9.3% and 8.2%, respectively. The observed parabolic MR profile shows good agreement with the micromagnetic simulation assuming the symmetric scissors motion of the magnetization in the multilayer. A linear MR response was observed in a giant MR strip with 0.4 mu m width by applying alternating external fields, ranged from 0.1 to 10 Oe, under a transverse bias field of 300 Oe. (C) 2000 American Institute of Physics. [S0021-8979(00)54908-1].

Microwave-assisted magnetization reversal (MAMR) characteristics of a single-exchange-coupled composite nanopillar, which consists of ferromagnetically coupled soft and hard magnetic sections, have been discussed in terms of Gilbert damping constant using micromagnetic simulation. The results indicate the occurrence of significant MAMR even when the damping constant of the hard magnetic section is large. The results obtained in this paper greatly contrast those obtained with single-phase hard magnetic pillars. We also found that the magnetization switching time tends to be shorter when the damping constant of the hard magnetic section is large.

Microwave-assisted magnetization reversals of exchange-coupled composite nanopillars were studied by micromagnetic simulation. Magnetization reversal occurred in one or two frequency regions associated with magnetic resonance arising from the soft magnetic section. Significant reductions in ac field strength and frequency, which are required for magnetization switching, were obtained in cases where there was a large interface exchange constant between the soft and hard sections. It was also found that the large saturation magnetization of the soft magnetic section tends to enhance magnetization switching.

The influence of intergrain exchange coupling on the properties of microwave-assisted magnetization reversal has been investigated. The results of micromagnetic simulation suggest that the microwave frequency realizing the magnetization reversal is increased as the exchange stiffness constant among the magnetic grains becomes larger than 1 x 10(-7) erg/cm if an inhomogeneous magnetization reversal occurred. The numerically expected increase in the microwave frequency was observed experimentally in perpendicularly magnetized Co/Pd multilayer with an exchange stiffness constant of similar to 1 x 10(-6) erg/cm. The simulations of signal recording and reproducing processes in a granular medium with assistance of a microwave field also suggest that an adequate intergrain exchange coupling is required to inhibit the nucleation of small island domains that will suppress the amplitude of the signal-noise ratio of the readback signal. (c) 2011 American Institute of Physics. [doi:10.1063/1.3596805]

We present the diagrammatic calculation of the spin current in the presence of the spin-orbit interaction due to impurity and electromagnetic field. By considering the vertex corrections for the normal impurity and the spin-orbit interaction, we derive both the field-driven and diffusion part of the spin current with side-jump and skew-scattering contributions. We found an alternative source of the spin-density generation, given by del x (A) over dot, where A is an electromagnetic vector potential. This generation mechanism is distinct from the conventional spin accumulation one, given by del . (A) over dot.

Microwave-assisted magnetization reversal in a rectangle of a Co/Pd multilayer with a perpendicular magnetic anisotropy is examined using vector network analyzer ferromagnetic resonance (FMR) spectroscopy. A microwave field is applied along the in-plane direction of the rectangle together with a negative dc easy-axis field smaller than the coercive field. Broadening or splitting of the peak profile in the FMR spectrum suggesting the formation of multidomain structure appears after the microwave field is applied. The dominance of microwave-assisted nucleation of magnetization is supported by the frequency dependence of the probability with which the multidomain structure appears.

We study magnetic properties of epitaxial Fe3 Si layers grown on Ge(111) with atomically flat interfaces. An unexpected uniaxial magnetic anisotropy is observed in the film plane for all as-grown samples, and the direction of the uniaxial easy axis is different for each of these samples. By postgrowth annealing, surprisingly, the random orientation of the uniaxial easy axis is aligned to a direction along about [0 1- 1], together with a reduction in the saturation magnetization. We discuss a possible mechanism of the variation in the magnetic properties after the annealing. © 2009 American Institute of Physics.

The influence of magnetostatic coupling in a Ni(81)Fe(19)/Cu/Ni(81)Fe(19) elliptical element on the ferromagnetic resonance (FMR) properties was experimentally and numerically investigated. Discontinuous variations in the FMR frequencies were observed when the relative orientation of magnetization was switched between parallel and antiparallel. Furthermore, the trilayer element exhibits field-insensitive FMR properties for magnetization of antiparallel orientation. This is attributable to the conflicted magnetic field dependence of the FMR frequency in the top and bottom Ni(81)Fe(19) layers with antiparallel magnetization. (c) 2008 American Institute of Physics.

Microwave-assisted magnetization reversal utilizing a transient precession of magnetization was demonstrated in a permalloy hexagon by applying a 25-ns-wide microwave field and a 500-ps-wide pulsed field with a tunable delay to the microwave field. The switching field in a combination of these two fields becomes smaller than that in only the microwave field, and this additional reduction in switching field oscillates relative to the delay time. From the comparison with the results of micromagnetic simulations, we found that the oscillatory behavior is attributed to the beats in transient precession that occurs in the early stage of microwave-field-induced magnetization excitation.

We have experimentally demonstrated a microwave-assisted magnetization reversal (MAMR) in exchange-coupled composite (ECC) media using the linearly polarized microwave fields generated by an electrically shorted coplanar waveguide. The MAMR frequency exhibiting a maximum reduction of coercive field can be decreased using 16 nm-thick ECC media without the suppression of the thermal stability of magnetization. The threshold amplitude of microwave field required for the successful MAMR is only 3% of the anisotropy field of the medium. A splitting of ferromagnetic resonance spectrum, which suggests an appearance of stochastic MAMR, can be observed after applying microwave fields. The stochastic MAMR gives rise to the threshold behavior in the condition of microwave field amplitude needed for MAMR.

Using a laterally configured structure, we demonstrate modulation of the magnetization dynamics of an epitaxial Heusler ferromagnet, Fe3Si, by nonlocally injecting pure spin currents. From the ferromagnetic resonance signals, the effective Gilbert damping constant () of the epitaxial Fe3Si nanomagnet is estimated to be ∼0.003 at room temperature. By using the lateral and nonlocal spin injection, the value of the epitaxial Fe3Si nanomagnet can be modulated from 0.0034 to 0.0024. This study will open a way for control of the magnetization dynamics of high-performance ferromagnetic Heusler alloys due to pure spin currents even in laterally configured structures.

We experimentally demonstrated anisotropic damping of the magnetization dynamics in a highly ordered Heusler compound Co3-xFexSi (x = 1.0, 2.0, and 3.0). The field-swept ferromagnetic resonance linewidth as a function of the resonant frequency was measured to exclude extrinsic contributions from two-magnon scattering and inhomogeneous line-broadening. In L2(1)-ordered Co2FeSi film, a wide variation in the intrinsic Gilbert damping constant from 0.0023 to 0.0061 was clearly observed in the (111) plane, where a weak uniaxial magnetic anisotropy was formed via low-temperature epitaxial growth. The large anisotropy of the Gilbert damping constant was suppressed by increasing x, which suppresses the half-metallicity in Co3-xFexSi films. (C) 2014 The Japan Society of Applied Physics.

Discussed is a spin dependent current perpendicular to plane (CPP) transport observed in a novel semiconductive-oxide/ferromagnet multilayers of [SnOx(6 nm)/M(t nm)×10 (M=Fe, Ni, Co), deposited with a multi-target magnetron sputtering system. This transport property is dominated by the local region of the current path, where the magnetostatic coupling due to the interlayer roughness causes the antiparallel spin configuration in the remanence state.

The magnetostatic effect of a ferromagnetic particle array on an underlying two-dimensional superconducting Nb thin film was investigated in terms of flux penetration processes by means of high sensitive magnetometry. For bare Nb film, a large drop in magnetization due to a flux rearrangement appeared in the low field region below 2.5 mT. However, once magnetic particles were fabricated on the surface of the Nb film, the drop was not observed. In this configuration, the ferromagnetic particles seem to behave as flux guides for the external magnetic field, whereby the flux distribution inside the superconductor is well regulated. Furthermore, a periodical oscillation was observed in the hysteresis loops. This behavior is associated with a flux quantization in the regions where the superconductivity is suppressed by the stray field of the ferromagnetic particles. The local flux density distribution around the ferromagnetic particles was evaluated by using the London equation, and the magnetization curve of the Nb film with periodically modulated superconducting order parameter was calculated. (C) 1996 American Institute of Physics.