Published papers

磁気異方性エネルギーの異なるCo-Cr-Pt基グラニュラー膜を積層したECC(exchanged coupled composite)媒体は、膜厚方向にスピン波が誘引されると磁化反転磁場が減少する。本研究では、ECC媒体の層間交換結合を中間層の膜厚によって制御した媒体を作製し、そのマイクロ波アシスト磁化反転特性を測定した。特に、磁化反転アシスト効果が最大化する周波数、およびアシスト効果に必要な交流磁場強度の閾値の中間層膜厚依存性を調べた。

酸化シリコン基板上に二つのコプレーナ線路を交差させた交差型コプレーナ線路とCo/Ni多層膜を蒸着した。この交差型コプレーナ線路に二方向からマイクロ波を入力し、位相変調器を用いて相対的な位相差を変えることで、生成される交流磁場の偏向を制御した。生じた交流磁場を用いて強磁性共鳴の測定を行ったところ、磁場の偏向を回転させることで磁化の方向について選択的に共鳴が励起された様子が観測された。

Microwave assisted magnetic reversal behavior for single exchange coupled nano-pillar was analyzed considering exchange coupled composite media using micromagnetic simulator. In the case that the thickness of the soft magnetic layer is substantially thinner than exchange length, the magnetization behaviors for soft and hard layers are almost uniform because of the strong exchange coupling between hard magnetic layer and thin soft magnetic layer. On the other hand, magnetization behaviors dominated by the soft and the hard magnetic layers were found at low and high frequency region for the ECC pillar when the thickness of soft magnetic layer is close to exchange length. Microwave frequency and microwave field strength required for magnetization reversal were also found to be lower and smaller than those for hard magnetic layer only.

We demonstrate a reduction of coercive field in Ni_<81>Fe_<19> films using a spin transfer torque generated by a spin pumping effect. When a microwave field resonates one of the ferromagnetic layer of a pseudo spin valve consisting of Ni_<81>Fe_<19> 100 nm/Cu 10 nm/Ni_<81>Fe_<19> 10 nm trilayer, a spin current is generated by the spin pumping effect. The spin current is injected to another ferromagnetic layer and the subsequent spin transfer torque can modulate the coercive field of the ferromagnetic layer. The coercive fields of two ferromagnetic layers are measured using the giant magneto-resistance (GMR) effect as a function of microwave frequency. From the frequency dependence of the coercive field, it is confirmed that the coercive field of Ni_<81>Fe_<19> 10 nm layer becomes minimum at the frequency where a ferromagnetic resonance is excited in another Ni_<81>Fe_<19> 100 nm layer. This is the clear evidence that the spin transfer torque generated by the spin pumping effect can assist the magnetization reversal of ferromagnetic thin film in the spin valve structure.

Microwave-assisted magnetic recording (MAMR) is one of the promising candidates to realize an areal density of HDDs being larger than 1 Tbpsi. The MAMR is the method to reduce the switching field by exciting the energy of a spin system using a ferromagnetic resonance (FMR) phenomenon. Understanding of the FMR properties of the medium is, therefore, most important to realize the MAMR-HDDs. In this article, we will discuss on the experimental method to measure the FMR spectrum of the patterned ferromagnetic thin film using a micro-fabricated coplanar waveguide (CPW). The CPW is used not only to apply a microwave field inducing the FMR but also to detect the FMR via S-parameter measurement using a vector network analyzer. The magnetization reversal caused by the application of the microwave can be also measured from the change in the FMR frequency.

Thermal stability of submicron scale Co90Pt10 island has been investigated using relief-structured giant magnetoresistive samples. Elongated islands with the lateral dimension of 0.5 mu m x 2.0 mu m and the thickness of 6 nm show nonlinear increase of the coercive field with decreasing temperature. The evaluated energy barrier becomes larger than 80k(B)T for the thickness ranging from 6 to 18 nm. (C) 2003 Elsevier B.V. All rights reserved.

We have investigated the magneto-transport properties of a GMR material consisting of CoPt(10nm)/Cu(10nm)/ NiFe(6nm) in which only the top NiFe layer is patterned by the half-milling technique. The top NiFe film was structured into a two-dimensional particle array with lateral dimensions of w = 0.2 mum and aspect ratio L/w = 1, 2 4 or 8. From MR measurements, the remanent magnetization of the NiFe particle array was found to gradually decrease with L/w, which can be ascribed to the relative increase of an end domain area in the particles, The longitudinal switching field H-swt markedly decreases as L/w < 2. From the micromagnetic simulation, it was found that the decrease of H-swt was caused by the change of switching configuration from an edge reversal to a coherent rotation with decreasing L/w. (C) 2002 Elsevier Science B.V. All rights reserved.

The possibility of selective magnetization switching of a three-layed, stacked magnetic pillar using a micromagnetic simulation that considers three-dimensional magnetic recording is discussed. Selective magnetization switching was estimated to be possible with the assistance of a circularly polarized microwave field having an adequate frequency that corresponds to the effective field. This study demonstrates that the value of the saturation magnetization of the magnetic layers is one of the keys for suppressing selective magnetization switching errors that are caused by magnetostatic fields. However, relatively large saturation magnetization of up to 600 emu/cm^3 in the 2nd layer also allows for successful selective magnetization switching, thus enabling three-dimensional magnetic recording.

Microwave assisted magnetic recording(MAMR) is one of the candidates for realizing ultrahigh density recording over Tbits/inch^2.In this study, signal-to-noise ratio (SNR) and recorded track width have been investigated for achieving high track density in MAMR. Moderate intergrain exchange interaction is know to improve SNR by suppressing the generation of islands reversals in recorded bits. On the other hand, intergrain exchange interaction was found to widen recording track width, which tends to decrease track density. However, track density as high as 1129 ktpi was estimated to be realizable in MAMR on the assumption of no intergrain exchange interaction in this study.