Magnetic vortex dynamics has been intensively studied because of its potential application to non-volatile and high-density magnetic random access memory. Here, we demonstrate that the magnetic vortex dynamics can be detected by using a magnetic tunnel junction (MTJ). The huge output of the MTJ allows us to detect the magnetic vortex dynamics not only in the frequency domain, but also in the real-time regime. In the high current-density regime, changes of the phase of the output signal have been detected, which suggest switching events of the polarity of the magnetic vortex core.

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The effect of elevated temperature and magnetic field on recorded magnetization structure of a perpendicular magnetic recording HDD medium has been studied by in-situ magnetic force microscopy (MFM). The recorded bit shape started to change in the track and the cross-track directions when heated at 200°C. When a magnetic field was applied at elevated temperatures, variation of recorded magnetization was enhanced compared with the case with only heating. A quantitative analysis of magnetization structure variation under an influence of heat and magnetic field is carried out by taking the difference of MFM images observed for a same area under different environmental conditions.

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Slit-patterned Mn-Ir/Fe-Si exchange-coupled film was annealed under an applied dc magnetic field in vacuum, and its magnetic properties were then evaluated. When a film without slits was annealed, the saturation field H_s in the hard axis direction decreased with increasing annealing temperature, and the ferromagnetic resonance frequency after annealing was lower than that before. On the other hand, when the slit-patterned film was annealed, the saturation field H_s after annealing was larger than that of the film without slits, due to the effect of a shape anisotropy field. In addition, the ferromagnetic resonance frequency changed very little with the annealing temperature. Hence, on account of the shape anisotropy resulting from the introduction of slits, Mn-Ir/Fe-Si exchange-coupled film has a higher FMR frequency. It will be applicable in device fabrication that includes thermal processes because of its uniaxial anisotropy, which includes shape anisotropy that is not influenced by thermal processes.

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Requirements for the use of IC tags on metallic objects have increased recently as systems that use IC tags have become widely adopted. We have proposed and designed a structure for an IC tag that operates on metallic objects. The structure of the IC tag consists of a conductive metal sheet at the bottom and a thin magnetic core made of a high-permeability composite sheet, which is inserted between the receiver coil and the metal sheet. With this structure, magnetic coupling between the antenna coil and the receiver coil is enhanced; furthermore, the influence of the metallic body to which an IC tag is attached is well suppressed. The method used to fabricate magnetic powders for the composite sheet is investigated together with the effect of the rolling process used to make a sheet on the magnetic anisotropy induced in the sheet. The permeability and quality factor are characterized for five samples in the frequency range from 10 MHz to 1000 MHz. The best fabrication method for magnetic composite sheets is to use 0.8∼1 μm size Co powders as magnetic powder material to be mixed with a resin at the weight ratio between 70 w% and 86 w%, and to apply rolling after extrusion or pulling out after extrusion for a making sheet.

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In the design of dc-dc converters, there is an ongoing effort to develop devices with higher power density and efficiency. A forward method is widely used as a circuit topology for applications with low output voltage and high output current. However, the fact that an output choke is necessary inhibits miniaturization and cost reduction. To resolve this problem, attempts have been made to integrate the output choke with the transformer. For example, a forward active clamp topology using a four-legged core has been proposed. This circuit operates as a hybrid of forward and flyback converters. The magnetic flux distribution in a transformer core used in this method has been clarified, but a design method has not been established. By means of an analysis, it was found that the magnetic flux in the leg, which acts as a output choke, has both dc and ac components, which are in a trade-off relation. This paper proposes a detailed transformer design methodology that takes account of this relation. When the core of a transformer was designed using the proposed method, it was possible to reduce the footprint of the magnetic device in comparison with that of a conventional converter.

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