Published papers

Signal recording processes were simulated using micromagnetic calculation for microwave assisted magnetic recording in perpendicular magnetic media. Strong magnetostatic field caused by the large saturation magnetization of the media tends to deteriorate recorded bit patterns resulting in low signal to noise ratio for the play back signal. The moderate exchange interaction fields were shown to improve recording characteristics and increase signal to noise ratio even for the large saturation magnetization media. The influence of thermal fluctuation fields on recording characteristics was also discussed and the influence was negligible for the media assumed in this study.

Energy assisted recording technology is considered to be employed in order to realize ultra high density magnetic recording over Tera-bits/inch^2. Microwave assisted recording which is one of the energy assisted recording, was focused in this study, and magnetization reversals for recording media was simulated using micromagnetic simulator. Recorded track width was found to increase with increase in microwave frequency, and region of the magnetization reversal enhanced by microwave field was also found to shift depending on magnetization rotation direction of spin torque oscillator and head field direction during signal recording process.

Microwave-assisted magnetic recording (MAMR) in perpendicularly magnetized Co/Pd multilayer was experimentally investigated using microwave magnetic fields generated by micro-fabricated coplanar waveguide. 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 demonstrate a dual-comb vibrometer where the pulses of one frequency-comb are split into pulse pairs. We introduce a delay between two pulses of each pulse pair in front of the sample, and after the corresponding two consecutive reflections at the vibrating sample surface, the initially introduced delay is canceled by a modified Sagnac geometry. The remaining phase difference between the two pulses corresponds to the change in the axial position of the surface during the two consecutive reflections. The Sagnac geometry reduces the effect of phase jitter since both pulses propagate through nearly the same optical path (in opposite directions), and spurious signals are eliminated by time gating. We determine the amplitude of surface vibration on a surface-acoustic-wave device with an axial precision of 4 pm. This technique enables the highly accurate determination of extremely small displacements.

Y3Fe5O12 is one of the magnetic insulators that can realize high-speed and low-power-consuming spintronics devices. However, it is hard to prepare a high-quality Y3Fe5O12 film via a conventional sputtering process owing to its low crystalline texture, which leads to a crucial increase in the Gilbert damping constant. Moreover, a single crystalline Gd3Ga5O12 substrate, whose lattice constant is well matched with Y3Fe5O12, is indispensable to improve the crystallinity of the Y3Fe5O12 film. In this article, we demonstrated an epitaxial growth of multiple domains for a 30-nm-thick Y3Fe5O12 film by means of magnetron sputtering on a single crystalline 128° Y–X LiNbO3 substrate , which has been widely utilized in surface acoustic wave devices. From the pole figure of x-ray diffraction, an oblique epitaxial growth of Y3Fe5O12(400) is successfully observed on the 128° Y–X LiNbO3 substrate after a high-temperature post-annealing. The saturation magnetization is equivalent to the value of the epitaxial Y3Fe5O12 film on the Gd3Ga5O12 substrate. The relatively low effective Gilbert damping constant of 0.0039 also supports the high crystalline texture of the Y3Fe5O12 film. The developed growth technique will pave the way for the application of the Y3Fe5O12 film on magneto-acoustic devices.

A current perpendicular to plane giant magnetoresistance structure with a cross contact geometry was fabricated by a two-step ion-milling technique. Multilayer pillars with various layer structures and a lateral dimension down to 0.4 mum were embedded between orthogonal top and bottom electrodes. The microfabricated cross point structure minimizes the influence of electrode resistance in measurements of magnetoresistance in current perpendicular to plane geometry. Individual switching of Co layer was observed in a pillar with layer structure of [Co(10 nm)/Cu(4 nm)/Co(2 nm)/Cu(10 nm)](3) and lateral dimension of 0.4x2.0 mum(2). Distinct three-step resistance change was observed for the rising part of the magnetoresistance (MR) curve, corresponding to the switching of Co(2 nm) layers. Somewhat complicated MR behavior in the falling part suggests the existence of a local minimum configuration during the switching of Co(10 nm) layers. The switching behavior of magnetic layers with interlayer dipole coupling was studied by micromagnetic simulations. Markedly different switching field of Co layers can be attributable to the variation of the dipole fields at each layer. (C) 2003 American Institute of Physics.

The effect of fringe-field coupling on the magnetization reversal process in submicrometer scale giant magnetoresistive pillars consisting of [NiFe/Cu](6) has been investigated by means of current perpendicular to plane magnetoresistance measurements and numerical simulations. The parabolic and reversible magnetoresistance curves due to an opposite rotation of magnetization in neighboring NiFe layers were observed. To explain the field sensitivity of the pillars quantitatively, we have proposed a modified single-domain model, where the magnetic energy of the system is reconstructed using the result of the micromagnetic simulation of a magnetostatically coupled bilayer system. The calculated magnetoresistance curves are consistent with the experimental ones. From the calculated results, the amplitude of the saturation field is found to be proportional to the thickness of the ferromagnetic layer and inversely proportional to the square root of the nonmagnetic layer thickness.

GMR-like behavior was observed in magnetic strips consisting of a symmetric layer structure with pattern widths down to 0.2 μm, and was significantly different from that observed in as-grown samples. The dependence of the 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 increasing the number of layers and the thickness of the magnetic layers; 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 obtained with an alternating field modulation under a transverse bias field. A thickness dependence of the GMR property implies that the probable cause of the antiparallel spin orientation in the strip pattern is magnetostatic interaction between the surface magnetic charges at the pattern edges.

面密度1Tbpsi超の記録が可能な次世代HDD実現のための新技術としてマイクロ波アシスト磁気記録(MAMR)が期待されている．MAMRをビットパターンドメディア(BPM)に適用し，BPMの各磁性ピラーを磁気特性の異なる層で多層構造化して，MAMRの周波数選択性を利用することで，任意の層を選択的に磁化反転させる3次元磁気記録が可能になると考えられる．しかし，従来の方法で各磁性層の漏洩磁界を選択的に検出することは困難であると考えられる．そこで本研究では，スピントルク発振素子（STO）を用いた再生ヘッド(STO再生ヘッド)を用いることで，各磁性層の磁化の向きをSTOの周波数の変化として検出可能であるかを計算機解析により検証した．

ある閾値強度よりも大強度のマイクロ波印加の下では、Suhl instabilityという不安定性が現れ、パラメトリックにスピン波が励起される。一方で大振幅歳差運動を励起して磁性体の保磁力を低減するマイクロ波アシスト磁化反転(MAMR)という技術が注目されている。本研究では、ナノスケール金属磁性体中でのSuhl instabilityの性質について調べ、パラメトリックに励起されたスピン波を活用したMAMR実験を行った。その結果、不安定性がMAMRにおいてどのような役割を果たすのかについて明らかにした。

Microwave-assisted magnetic recording (MAMR) in perpendicularly magnetized Co/Pd multilayer was experimentally investigated using microwave magnetic fields generated by micro-fabricated coplanar waveguide. 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.

Energy assisted recording technology is considered to be employed in order to realize ultra high density magnetic recording over Tera-bits/inch^2. Microwave assisted recording which is one of the energy assisted recording, was focused in this study, and magnetization reversals for recording media was simulated using micromagnetic simulator. Recorded track width was found to increase with increase in microwave frequency, and region of the magnetization reversal enhanced by microwave field was also found to shift depending on magnetization rotation direction of spin torque oscillator and head field direction during signal recording process.

Microwave-assisted magnetization reversal characteristics estimated by experiments and micromagnetic simulations are compared through the Sharrock's theory taking a mono-layered CoPt type granular medium. Energy barrier height reduction evaluated by the switching fields under the microwave assistance which is estimated by the micromagnetic simulation were taken account for the magnetization after effects expressed by the Sharrock's equation. In the experiment, switching field reduction due to microwave-assistance is not observed in a lower frequency region. The reason is clarified that the switching fields are mainly affected by magnetization after effects in the frequency region although microwave slightly assists magnetization reversals. Discrepancy in the switching fields estimated by the experiment and micromagnetic simulation combined with Sharrock's equation comes from that the optimum microwave frequency in the experiment corresponds to the unstable switching frequency region for the micromagnetic simulation and assisting energy is excessively estimated in the calculation. In order to account for the switching fields for the experiment by calculation, nonlinear relation between assisting energy and reverse DC fields should be involved in the calculation.

This paper demonstrates high-temperature operation of a sub-micron GMR sensor, biased with a vertically integrated hard magnet. In the present study, fringe fields from vertically integrated CoPt pattern are used for biasing transverse to the short axis of the GMR strip. While, well-defined shape anisotropy in the narrow strip pattern is used for biasing parallel to the short axis. A sputtered Co/Cu multilayer overlaid with CoPt is structured into strip pattern with a width of down to 0.4 mum by electron beam lithography and Ar ion milling. A typical layer structure of the studied samples is [Co(2.0 mn)/Cu(2.0 mn)]/ Ta(8.0 nm)/CoPt(23 nm). The fabricated self-biased GMR strip exhibited superior linear MR response to alternative external fields ranged from 0.4 to 250 Oe at room temperature and 553 K (apparatus limit).

We optically characterized the local changes in the amplitude and phase of a surface vibration that are associated with the magnetoelastic effect. Pulsed laser interferometry was used to directly probe the amplitude and phase of a surface acoustic wave (SAW) propagating on a nickel thin film in a magnetic field, and a conversion from the angular momentum of the SAW to that of the macroscopic spin system was clearly observed. Pulsed laser interferometry opens the door to local probe and spatial mapping of magnon-phonon conversion in various magnetic materials, which can lead to further progress in fundamental and applied research related to magnetism.

We demonstrate a quantitative evaluation of absolute displacement of surface acoustic wave by the time-division dual-comb interferometry with the Sagnac geometry. Due to an ultrahigh stability of the Sagnac geometry, achieved resolution is ~26 pm.

We demonstrate a quantitative evaluation of absolute displacement of surface acoustic wave by the time-division dual-comb interferometry with the Sagnac geometry. Due to an ultrahigh stability of the Sagnac geometry, achieved resolution is -26 pm.