Published papers

The broadband ferromagnetic resonance (FMR) of a single-crystalline Fe microwire was measured using a rectifying effect. The experimental result indicates the uniform mode dominates the system. The effective Gilbert damping factor of the Fe wire was estimated by the rectifying effect. A rectifying spectrum is broadened because of the spin wave excitation and the thermal fluctuation as the input power increases.

A magnetic vortex core confined in a micron-scale magnetic disk is resonantly excited by spin-polarized radio-frequency (rf) current and rf field. We show that rectifying voltage spectra caused by the vortex core resonance is dependent on the core polarity. Rectifying voltage spectra are given by the superposition of the polarity-dependent term and the polarity-independent term. The sign of the polarity-dependent rectifying voltage reverses when the sign of polarity P or external field H is reversed. This experimental result can be explained by the anisotropic magnetoresistance effect caused by the vortex core motion.

Magnetic resonance properties for ECC nano-pillar were numerically analyzed. The analysis showed there are three resonance modes in the ECC nano-pillar that ferromagnetic resonances (FMR) at the low and high frequency regions associating with the soft and hard material, and magnetic spin wave resonance at middle frequency region. Microwave assisted magnetization reversals were also simulated assuming ECC nano-pillar with extremely high anisotropy field hard magnetic material. Essential ac field strength and microwave frequency for magnetization reversal were found to be less dependent on anisotropy field for the hard magnetic material, when magnetization reversal is induced by magnetic resonance for the soft magnetic material.

The ferromagnetic resonance (FMR) properties of submicron-scale Ni(81)Fe(19)/Cu/Ni(81)Fe(19) trilayered dots were investigated for application in programable microwave absorbers for broadband filter devices. The trilayered dot shows binary FMR frequencies, depending on the relative orientation of magnetization in the top and the bottom magnetic layers. A parallel configuration of magnetization, which exhibits a lower FMR frequency than that for an antiparallel configuration, can be metastable as the lateral aspect ratio of the dot is larger than the threshold value governed by the Cu layer thickness. The difference in the FMR frequencies between the parallel and antiparallel configurations is gradually decreased with elongation of the dot. This is associated with the suppression of the magnetostatic coupling energy that stabilizes the antiparallel configuration. It was also found that the magnetic configurations can be programed by the selective application of easy-and hard-axis magnetic fields used to saturate the magnetization. This is an appropriate manner to realize the antiparallel configuration of magnetization in the case of a dot with a plural stack of magnetic layers with the same coercive field. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3042231]

We generate a circularly polarized (CP) magnetic field using two crossed coplanar waveguides (TCCWs). By using the CP field, we selectively excite a ferromagnetic resonance between positively and negatively magnetized Co/Ni multilayers. The CP field arises from a superposition of Ampere fields generated by microwaves orthogonally propagating along the crossed arms of the TCCWs. The polarity of the magnetic field can be tuned continuously from circular to linear by exploiting a phase delay between the orthogonal microwaves. From a local measurement of the magneto-optical Kerr effect, we also find that the ellipticity of the CP field is distributed over the intersection of the TCCWs. These results indicate that TCCWs have great potential as CP-field generators that can be integrated into micron-scale electrical devices. Published by AIP Publishing.

We experimentally investigated the crystallographic anisotropy of the intrinsic Gilbert damping constant in single crystalline Fe(001) film deposited on MgO(001) substrate. The frequency-domain ferromagnetic resonance (FMR) spectrum was measured at given external magnetic fields via a vector network analyzer. From the field-domain FMR linewidth obtained as a function of resonant frequency, the intrinsic Gilbert damping constants along the crystallographic axes in the (001) plane were successfully obtained by evaluating the extrinsic influence of two magnon scattering and inhomogeneity. A four-fold crystalline anisotropy of the intrinsic Gilbert damping constant, similar to the magnetic crystalline anisotropy in the (001) plane, was clearly observed. Furthermore, we found that the intrinsic Gilbert damping constant along the easy axis was smaller than that along the hard axis.

Antivortex creation in a cross-shaped wire was demonstrated by using magnetoresistance and magnetic force microscopy measurements. The magnetization process was stochastic and exhibited a wide variety of magnetoresistance curves. By developing a conventional method, we have achieved electrical determination criteria to distinguish antivortex creation from many other magnetization states. The electrical criteria provide a simple method without large-scale apparatus and promote further experiments such as spin-current driven antivortex dynamics. (C) 2015 The Japan Society of Applied Physics

Abstract We report a large modulation of spin pumping using temperature-induced magnetic phase transitions in c-axis oriented single crystalline dysprosium (Dy). From the temperature variation of the magnetic susceptibility, transitions from paramagnetic (PM) to ferromagnetic (FM) phases via antiferromagnetic (AFM) phase are clearly observed in the Dy. Unlike polycrystalline Dy, the spin pumping of both PM- and AFM-Dy are strongly suppressed owing to the increased non-dissipative backflow of spin current by the long-range spin transport, although two orders of magnitude difference exist between FM- and AFM-phases.

The demonstration of the charge-to-spin conversion, especially with enhanced spin Hall conductivity, is crucial for the development of energy-efficient spintronic devices such as spin-orbit torque (SOT) based magnetoresistive random access memories. In this work, fully epitaxial Ru/Cu heterostructures were fabricated with interface engineering and nanolayer insertions consisting of Cu (1 nm)/Ru (1 nm) structures with different numbers of periods. The atomically controlled interface was confirmed by the high-resolution high-angle annular dark-field scanning transmission electron microscopy, and the epitaxial relationship persists even in the hybrid nanolayer insertion structures. The spin current generation was detected by the measurement of unidirectional spin Hall magnetoresistance, and the effective damping-like spin Hall efficiency (ξ DL) was further quantitatively evaluated by the spin-torque ferromagnetic resonance with thickness dependence of the ferromagnetic layer. It is found that the sharp interface Ru/Cu film has a sizeable ξ DL of −2.2% and the insertion of Cu/Ru nanolayers at the interface can increase the ξ DL value to −3.7%. The former could be attributed to the interface spin-orbit filtering effect and the latter may be further understood by the intrinsic contribution from the local electronic structure tuning due to the lattice distortion near the interface. A large effective spin Hall conductivity is achieved to be (3∼5) × 105 ℏ 2 e Ω−1 m−1 in the epitaxial Ru/Cu hybrid nanolayers, which is in the same range as that of platinum. This work indicates that the interfacial control with hybrid nanolayer structures can extend the SOT-based materials to highly conductive metals, even with weak spin-orbit interactions, toward high stability, low cost, and low energy consumption for spintronic applications.

We demonstrate the phase shift of the Rayleigh-type surface acoustic wave (RSAW) in a ferromagnetic Ni81Fe19 film based on the gyromagnetic Einstein-de Haas (EdH) torque that can be attributed to the back action of spin-wave excitation. The EdH torque modulates the transverse velocity of the bulk acoustic wave that causes the bipolar phase shift of RSAW. When compared with a magnetoelastic torque, the phase shift can be more prominently enhanced by increasing the frequency. The gyromagnetic torque in the RSAW paves the way for controlling spin-mechatronics devices without magnetoelastic torque which is specific to a certain material.

The broadband ferromagnetic resonance study on both the single crystalline and polycrystalline Fe wires were performed using the rectifying effect. The effective Gilbert damping in the polycrystalline Fe wire was about three times larger than that in the single crystalline wire. This is attributed to the enhancement of the energy dissipation due to the incoherent rotation of the magnetization at the grains and grain boundaries in the polycrystalline wire. The difference between the experimental data and analytical calculation can be explained by the strong magnetic shape anisotropy that overcomes the external static magnetic field and forces the magnetization to be directed along the wire axis.

We microscopically derive the spin-transfer torque due to diffusive spin current arising from nonequilibrium spin accumulation as realized in systems with nonlocal spin valve structure. Spin relaxation effects in spin transport is taken into account by introducing magnetic impurities.

Nucleation and erasing of sub-mu m(2) domains in spin valve strips have been performed with current pulses applied through conductors, A spin valve material of NiFe/Co/Cu/Co was deposited by magnetron sputtering and patterned into sense lines of 40 parallel strips with 0.4 mu m width and 2 mu m pitch. Selective domain nucleation under parallel conductors with 1 mu m width and 2.6 mu m pitch has been confirmed by the current induced MR change, Pulses with opposite polarity performed erasing of nucleated domains. Reproducible MR change with a variation of +/-10 % relative to the mid value was realized in 10(4) operations of the domain nucleation process.

A new Heusler alloy, Fe2CrSi, which has high spin polarization (P), low saturation magnetization (M-s), and a low Curie temperature (T-C), was investigated in order to fabricate high-performance magnetic tunnel junctions (MTJs) with a high tunnel magnetoresistance ratio and with low critical current for the spin-transfer switching method, or a low switching field for the thermally assisted magnetization reversal technique. The main results are as follows: (1) P and the magnetic moment of Fe2CrSi with an L2(1) structure were 0.98 and 1.98 mu(B)/f.u., respectively, according to density of states calculations. (2) Fe2CrSi films show the (100) orientation with a B2 structure on a MgO substrate upon a thermal treatment with optimum temperature and duration. (3) Fe2CrSi films have M-s and T-C values of 385 emu/cm(3) and 630 K, respectively. (4) The (100) oriented epitaxial MTJs are produced with Fe2CrSi films fabricated with the optimized thermal treatment condition. It is found that the Fe2CrSi Heusler alloy films are a suitable ferromagnetic material for high-performance magnetic random access memory. (C) 2008 American Institute of Physics.

In many interacting-electron systems, the microscopic spin angular momentum of electrons is conserved along with the macroscopic angular momentum in electrical current flows, i.e., vorticity. Such spin-vorticity coupling expands the choice of materials for spintronics devices. In this paper, we evaluate the magnitude of an alternating spin current generated by the spin-vorticity coupling with a gigahertz-order surface acoustic wave in a Cu thin film. We measure the gigahertz alternating spin current by an electrical method based on the inverse spin-Hall effect. From the amplitude of the spin current, we can determine the conversion efficiency of the angular momentum between local lattice rotation and electron spin in the Cu film. The conversion efficiency is four orders of magnitude larger than the case of spin current generation via kilohertz-order vorticity in turbulent flow of liquid mercury [M. Matsuo et al., Phys. Rev. B 96, 020401(R) (2017)]. Such a huge conversion efficiency is attributable to a smaller inconsistency of energy scale between lattice rotation and electron spin than the case of liquid vorticity.