Keiichi Yanagisawa

A new planar microtransformer for use in micro-switching converters

A planar microtransformer for use in microswitching converters of portable equipment is described. This microtransformer, consisting of planar coils and CoZrRe amorphous magnetic layers on a Si substrate, is fabricated by a dry process. It is about 0.3-mm thick, and 3*4 mm/sup 2/. Its coils provide a relatively high inductance of 33 nH/mm/sup 2/. This microtransformer is implemented in a forward converter, which operates well at 32 MHz. >

Real-Space Observation of Skyrmion Lattice in Helimagnet MnSi Thin Samples

Observing and characterizing the spin distributions on a nanometer scale are of vital importance for understanding nanomagnetism and its application to spintronics. The magnetic structure in MnSi thin samples prepared from a bulk, which undergoes a transition from a helix to a skyrmion lattice, was investigated by in situ observation using Lorentz microscopy. Stripe domains were observed at zero applied field below 22.5 K. A skyrmion lattice with 6-fold symmetry in real space appeared when a field of 0.18 T was applied normal to the film plane. The lattice constant was estimated to be 18 nm, almost identical to the helical period. In comparison with the marginally stable skyrmion phase in a bulk sample, the skyrmion phase was stable over a wide range of temperatures and magnetic fields in the thin samples.

Planar microtransformer with monolithically-integrated rectifier diodes for micro-switching converters

A microtransformer that is monolithically integrated with rectifier diodes is developed as the first step towards developing a monolithic microswitching converter that can be integrated with semiconductor devices and magnetic components. This microtransformer consists of planar coils and CoZrRe amorphous magnetic layers on Si substrate. Two Schottky barrier diodes are formed on the Si substrate and directly connected to the secondary coil. This microtransformer chip is shown to perform both conversion and rectifying functions.

A compact buck-converter using a thin-film inductor

We have developed a compact buck-converter using a thin-film inductor that is suitable for portable electronic equipment. The inductor consists of planar Cu coils and CoZrTa magnetic layers on a Si substrate; it is fabricated by a dry process. The converter is 3 mm thick and 10/spl times/6 mm in area; it operates at 1.6 MHz and has an efficiency of up to 85%.

Investigation of magnetic structure and magnetization process of yttrium iron garnet film by Lorentz microscopy and electron holography

The micromagnetic structure and magnetization process of perpendicular Y3Fe5O12 (YIG) films were studied by Lorentz microscopy and electron holography. The closure domain structure inside the thin transmission electron microscopy specimen exhibits the same period as the magnetization pattern observed by magnetic force microscopy indicating the perpendicular anisotropy of the YIG film. Through observation of stray fields, it is concluded that the shapes of domain and domain walls are sensitive to the specimen thickness; moreover, a closure domain configuration observed in thin specimen is the stable energy state as determined by the balance between the crystalline anisotropy and shape anisotropy. Domain wall movement is observed by applying a magnetic field, in situ, inside the microscope in both horizontal and perpendicular directions; the saturation fields observed are qualitatively in agreement with the results of the hysteresis loop.

Reduced-area magnetic bit recording and detection using magnetic force microscopy based on application of bidirectional magnetomotive force

The size of the magnetization region of perpendicularly recorded bits is reduced using a method based on magnetic force microscopy (MFM). A new bit is written over part of the previous bit by using an alternating magnetomotive force. Detected force gradient trajectories and images reveal that this method successfully reduces the bit cell size down to half that of an isolated bit. A comparison of images of MFM-recorded bits and bits recorded using a thin-film head suggests that the recording density is limited mainly by the writing ability of the MFM tip. >

Nanoscale Magnetic Characterization of Tunneling Magnetoresistance Spin Valve Head by Electron Holography

Nanostructured magnetic materials play an important role in increasing miniaturized devices. For the studies of their magnetic properties and behaviors, nanoscale imaging of magnetic field is indispensible. Here, using electron holography, the magnetization distribution of a TMR spin valve head of commercial design is investigated without and with a magnetic field applied. Characterized is the magnetic flux distribution in complex hetero-nanostructures by averaging the phase images and separating their component magnetic vectors and electric potentials. The magnetic flux densities of the NiFe (shield and 5 nm-free layers) and the CoPt (20 nm-bias layer) are estimated to be 1.0 T and 0.9 T, respectively. The changes in the magnetization distribution of the shield, bias, and free layers are visualized in situ for an applied field of 14 kOe. This study demonstrates the promise of electron holography for characterizing the magnetic properties of hetero-interfaces, nanostructures, and catalysts.

CoZrRe amorphous film heads for high-density magnetic recording

CoZrRe amorphous film heads with superior performance have been developed for use in high-density magnetic recording. These films produce near-zero magnetostriction of less than +2.5*10/sup -7/ and saturation magnetization of 1.2 T. It is found that the addition of a small amount of Re to CoZr markedly improves the stability of magnetic properties during head fabrication. However, the process temperature must be less than 200 degrees C. To realize the required temperature, a dry process is developed, using sputtered SiO/sub 2/ films for insulation instead of conventional hard-cured photoresist. A maximum process temperature of about 150 degrees C is obtained As a result, a high linear density of more than 2100 fr/mm (53 kfci) at a spacing of 0.1 mu m is achieved using CoZrRe film heads and metal-sputtered media with a coercivity of 1300 Oe. >

Quantitative evaluation of magnetic flux density in a magnetic recording head and pseudo soft underlayer by electron holography

The magnetic interaction between the pole tip of a single-pole head and a pseudo soft underlayer in perpendicular magnetic recording was observed by electron holography. The magnetic flux density inside the soft underlayer was quantitatively evaluated. The distribution of magnetic flux density was calculated using the finite element method, and the influences of the modulation of the reference wave and stray fields were investigated by comparison with experimental results. The flux density observed was found to be underestimated due to the modulation of the phase shift in reference wave. The magnetic flux measured experimentally was larger than that inside the specimen because of the relatively large stray fields above and below the specimen in the direction of the electron beam.

Observing the Magnetization Distribution in the Pseudo Soft Underlayer of Perpendicular Magnetic Recording Media by Electron Holography

We visualized the magnetic flux density of the cross track direction in the pseudo soft underlayer (SUL) of perpendicular magnetic recording media. We deposited a pseudo-SUL on the fabricated slider of a perpendicular magnetic recording head, and used electron holography to visualize and quantify the magnetic flux magnitude and distribution in the pseudo-SUL. We observed the pseudo-SUL magnetized by the write field from the writer pole tip. The magnetic flux magnitude and full width at half maximum increased with an increasing write field. We observed a correspondence between an actual recording situation and the electron holography observation.