Exchange anisotropy of quaternary γ-Mn-Ir-Rh-Ru / Co₇₀Fe₃₀ bilayer has been investigated. Unidirectional anisotropy constant, J_K, exhibited gradual change against the chemical composition of the quaternary system. Contour map of J_K shows a broad peak at the compositional range from Mn₇₅Ir₂₅ to Mn₇₉Ir₁₅Rh₆. The J_K decreases mainly along the direction to which the Ru content increases. The thinnest critical thickness of the antiferromagnetic layer is obtained for Mn-Ir alloy among the respective terminal binary alloy systems. We concluded that Mn-Ir is the most promising antiferromagnetic material for exchange-biased bilayers in ultra-high density hard disk drives among the present γ-Mn-Ir-Rh-Ru quaternary system.

抄録全体を表示

A new granular-type perpendicular magnetic recording media was designed to reduce the intermediate layer thickness. A medium with Ru intermediate layer of only 2 nm was prepared successfully on a highly oriented high-B_s FeCo soft underlayer (SUL). The CoPt-TiO₂ recording layer had a high crystal orientation, a high H_c of 6.5 kOe, and a high squareness ratio of 0.99. The magnetic property of the SUL was also good. The granular structure of the CoPt-TiO₂ recording layer in the new-designed medium is similar to that in the old-designed one.

抄録全体を表示

We investigated the potential of an anodic alumina template for high-density patterned media. Anodic alumina hole density of 1.86 Tdots/in² was achieved by suppressing surface roughness by alloying aluminum with Ti and Hf. However, it was difficult to fabricate the embedded type patterned media with a recording layer of thinner than 25nm in an anodic alumina template of 1.03 Tdots/in². This necessitated a switch from an embedding process to a coating process, so we developed a new anodizing process for patterned media of 1 Tdots/in². The process formed the niobium oxide pillar structure successfully with high density in an anodic alumina template. 1.03 Tdots/in² patterned media fabrication was achieved by depositing the Co₇₀Pt₃₀ recording layer on the pillars.

抄録全体を表示

Thermal stability of magnetization in a nano-magnet is an important issue for the retention of stored data in a magnetic random access memory based on spin-torque switching. Within the thermal activation model, the distribution of the switching current of spin-torque switching (I_c) at finite temperature is roughly proportional to the inverse of the thermal stability parameter (Δ). In this paper, we present a new method for evaluating Δ from the distribution of I_c. This method can give us correct Δ at any given pulse width.

抄録全体を表示

Superior parietal lobule, which relates to spatial attention and shifts of spatial attention, is believed to play a key role in perceptual reversal. As an active approach to investigate brain function, rTMS can make artificial excitatory or inhibitory activation in a local region of brain, and when compared to the passive approach (normal brain activation), rTMS provides a flexible and controllable method to investigate the function of the interested region. In this study, to investigate the function of the right superior parietal lobule during perceptual reversal, a 1 Hz 240-pulse biphasic rTMS at 90% of resting motor threshold was applied over the right superior parietal lobule and right posterior temporal lobe, respectively. As a control, a No-TMS Trial was also conducted. The inter-reversal time of perceptual reversals between these three conditions were compared. It was found that compared to the other two conditions, rTMS applied over right posterior temporal lobe caused longer inter-reversal time. This result suggests that the right superior parietal lobule plays a critical role in perceptual reversal. Moreover, it suggests that 1 Hz 240-pulse rTMS causes a suppression effect on perceptual reversal.

抄録全体を表示

Quantum correction to spin transfer torque is calculated in the adiabatic limit. It is shown that electron spin density and thus spin transfer torque has a reduction factor due to weak localization effect. This reduction factor is exactly the same as the reduction rate of current, and therefore, the torque and force on domain wall is the same as the classical case if renormalization of the current due to localization is taken account of.

抄録全体を表示

The magnetization distribution in the patterned magnetic layer of master media and soft underlayer (SUL) resulting from perpendicular magnetic duplication utilizing the bit printing method was calculated by using micromagnetic simulation. As a result, it was found that the magnetizations of the magnetic layer of the master form a vortex structure. The magnetization distributions of the magnetic layer of the master and the SUL do not depend on the saturation magnetization and thickness of the SUL, but only on the duplication field. The peak-to-peak value of the recording field and the recording field gradient at the edge of the magnetic pattern of the master are almost independent of the saturation magnetization and thickness of the SUL. However, these values decrease slightly when the SUL thickness is reduced by less than 10 nm and the distance between master and SUL is increased.

抄録全体を表示

Exchange-coupled composite (ECC) media are candidates for achieving recording densities of over 1 Tbpsi. In this study, we investigated the SNR and thermal stability of ECC media, using a micromagnetic simulator. We found that the vertical-exchange (A_⊥) interaction between the soft and hard regions greatly influences both the SNR and the thermal stability, but in opposite ways. In other words, a weak A_⊥ results in a high thermal stability, while a strong A_⊥ increases the SNR. Thus, it is difficult to simultaneously obtain a high SNR and thermal stability using ECC media. To solve this problem, we propose a reverse ECC (RECC) medium in which the positions of the soft and hard regions are reversed with respect to those in conventional ECC media. Under the condition of a weak A_⊥ of 5 pJ/m, which is suitable for achieving good thermal stability, RECC media have SNRs that are 7.5 dB higher than those of ECC media at 2000 kfci. This is because the field gradient of the writing field in the hard region of RECC media is considerably improved by closing to the ABS of the writing head. This study demonstrates that it is difficult for conventional ECC media to achieve 1 Tbpsi recording, and shows that RECC media have the possibility of attaining recording densities of over 1 Tbpsi.

抄録全体を表示

The thermally assisted reversal of micro-magnetization was simulated for sub-100-nm dots with perpendicular magnetization by using a Langevin-Landau-Lifshitz-Gilbert equation, where thermal effects are modeled by random fields. Two different thermally assisted reversal methods of 1) a thermal relaxation scheme and 2) a switching scheme were compared. Superior performance was obtained for the thermal relaxation scheme from the view point of reducing the reversal field. An auto correlation function in the time domain was introduced to analyze the stochastic properties of the thermally fluctuating directions of magnetization.

抄録全体を表示

We developed a very sensitive high-frequency carrier-type magnetic field sensor with a pT resolution using a transmission-line-type sensor. Meander-type sensor elements using amorphous CoNbZr films were fabricated. We obtained a magnetic field resolution of 3.0 x 10^-12 T/Hz^1/2 at 1 Hz. The detectable magnetic field was limited by the 1/f noise. We detected the magnetocardiogram (MCG) signal by using the sensor.

抄録全体を表示

Switched reluctance motors (SR motors) have many advantages such as simple and solid construction, low-cost manufacturing, excellent reliability at high temperatures, and large torque density. However, the higher torque ripple from magnetic saliency is a serious problem preventing its applications from being expanded. To reduce torque ripple, we propose a novel instantaneous method of controlling torque using flux-based commutation and phase-torque distribution. It provides efficient driving for SR motors with limited torque ripple. In this paper, we consider the application of this method of controlling torque to the SR motors of electric vehicles (EVs) by simulation.

抄録全体を表示

We propose a method of non-invasively evaluating the permeability of cell membranes in normal rat brain using diffusion magnetic resonance imaging (MRI). Diffusion MRI signals reflect the intra- and extracellular diffusion coefficients of water molecules in permeable biological tissues and cell membranes. The signals were acquired using a 4.7 T MRI system and applying a motion-probing gradient (MPG) in six directions. Numerical simulations using the finite-difference method were conducted for various combinations of intracellular diffusion coefficients and cell-membrane permeabilities. We defined the evaluative function as the difference between the signals from numerical simulations and experimentally obtained signals. We found that the minimum value for the evaluative function enabled us to estimate the intracellular diffusion coefficient and membrane permeability of the rat brain, which were corresponded to (1.3±0.1) × 10^-3 mm²/sec and 66±24 μm/sec. Our method was useful for non-invasively estimating the cell-membrane permeability of biological tissues, and can easily be applied to human tissues and other samples.

抄録全体を表示

The combination of transcranial magnetic stimulation (TMS) and electroencephalogram (EEG) is an effective tool for investigating the cortical reactivity and the functional connectivity in the brain. In our previous study, we reported a method of removing stimulus artifact during TMS with Sample-and-Hold circuit and EEG activity evoked by TMS could be measured successfully. In addition to this method, independent component analysis (ICA) was also applied to recorded EEG data in order to remove the stimulus artifact from for off-line analysis. By using these methods, short latency (< 15 ms) EEG responses to TMS could be obtained. In this paper, we focused on the propagation of EEG activity elicited by TMS. We observed both the EEG topography and the distribution of the current density over the whole head by changing the stimulus site. When motor cortex was stimulated, the propagation of EEG activity to contralateral hemisphere could be clearly observed. However, when posterior parietal cortex was stimulated, no or less propagation of EEG responses could be recognized. These results suggest that the responses evoked by TMS over motor cortex propagate to contralateral hemisphere along the axon through the corpus callosum.

抄録全体を表示