M. Takezawa

Domain observation technique for Nd–Fe–B magnet in high magnetic field by image processing using liquid crystal modulator

A different domain observation technique by modulation of a polarizing plane has been developed for Nd–Fe–B sintered magnets. A liquid crystal element inserted in a longitudinal Kerr microscope is used as an optical modulator to acquire a reference image needed to enhance Kerr contrast by image processing. Domain images of the Nd–Fe–B magnet can be clearly observed by this technique in a high magnetic field up to 4.4kOe. It was found that polishing of the sintered magnet reduced its coercive force.

Magnetic domain observation of hydrogenation disproportionation desorption recombination processed Nd–Fe–B powder with a high-resolution Kerr microscope using ultraviolet light

A Kerr microscope that uses ultraviolet (UV) light for high-resolution domain observation was built, and the domain structure and magnetization process of hydrogenation disproportionation desorption recombination (HDDR) powder were examined. The UV Kerr microscope could observe nanometer-sized domain patterns. Applying a dc field of 1.0 kOe to HDDR powder at a desorption recombination (DR) time of 12 min produced abrupt wall motion. The pinning force exerted by the grain boundaries is inadequate for producing high coercivity because the Nd-rich phase layers along these boundaries are absent at a DR time of 12 min. For HDDR powder at a DR time greater than 14 min, changing the magnetic field by up to 1.0 kOe produced no observable wall motion. It follows that the high coercivity of HDDR powder is due to domain wall pinning at the grain boundaries.

Effect of Strain by Mechanical Punching on Nonoriented Si–Fe Electrical Sheets for a Nine-Slot Motor Core

Domain observation has been done to investigate the effect of strain due to a punching process on the magnetization process of a nine-slot core for 12-pole small brushless dc motor by using the Kerr microscopy technique. Stripe domain structures oriented to a transverse direction are observed at the edge of a tooth of the core without stress relief annealing. Due to the domain configuration, permeability near the punched edge of the motor core is reduced. A stress relief annealing at 700degC for 2 h can relive the punching stress

Domain structure of chemically thinned non-oriented electrical sheet

Abstract The anomaly factor tends to increase steeply when the thickness of non-oriented Si–Fe electrical steel sheets is decreased. To investigate the behavior of the anomaly factor in relation to the sheet thickness, magnetic domain observation was performed, using the Kerr effect. Grains of a thinned sheet exhibit a stripe domain pattern. The grains exhibiting the stripe domains on the surface had closure domain structure in a cross-section of the sheet. It was found that a field applied in the sheet plane induced changes of the magnetization component normal to the surface, which may increase the anomaly factor of thin sheets.

Surface Domain Configuration of Nd-Fe-B Sintered Magnets Influenced by Underneath Magnetization

The influence of underneath magnetization of Nd-Fe-B sintered magnets on surface domain configurations was investigated using a Kerr microscope by cutting the magnets at an angle to the c-axis. When the angle formed by the surface and the c-axis is larger than 30 degrees, the observed domain configurations change to maze patterns because of an increase in the demagnetizing field at the surface. In contrast, when the cutting angle is 10 degrees, residual magnetization increases with the applied field. It was found that at a cutting angle of 10 degrees, the underneath domain configurations can be indirectly observed through the surface by propagation of magnetic flux from the underneath grains.

Miniaturization of high-frequency carrier-type thin-film magnetic field sensor using laminated film

We examined a laminated high-frequency carrier-type thin-film magnetic field sensor that consists of CoNbZr soft magnetic films with Nb nonmagnetic conductive interlayer. The lamination can change domain structure of the sensor and obtain high sensitivity. An impedance change of 6 /spl Omega/ and a gain of 43 k/spl Omega//T was achieved when the length of the laminated sensor was 1 mm. The gain is four times larger than that of a monolayer sensor.

Magnetic domain observation of Nd-Fe-B magnets with submicron-sized grains by high-resolution Kerr microscopy

A Kerr microscope that uses UV light for high-resolution observation of magnetic domains was developed, and the domain structure and magnetization process of hot-pressed magnets prepared from hydrogenation-disproportionation-desorption-recombination (HDDR) processed powders were examined. This microscope is capable of distinguishing nanometer-sized domain patterns. The coercivity of the hot-pressed HDDR magnet depends on the area of magnetization reversal during a reversal process. Magnetization reversal occurs simultaneously in a few grains in a low-coercivity magnet because the Nd-rich phase along the grain boundaries is absent. In contrast, in a high-coercivity magnet, magnetization reversal in a grain independently occurs because of an adequate Nd-rich phase along the grain boundaries. It follows that the high coercivity of an HDDR powder compacted by hot pressing is due to domain wall pinning at the grain boundaries.

Dynamic domain observation in narrow thin films

We examined the domain structure of high-frequency carrier-type thin-film magnetic field sensors that consist of narrow rectangular CoNbZr thin film to clarify the magnetization process of the sensor head. Moreover, the cause of disagreement between measured and calculated sensitivities was investigated. The measured impedance change of the sensor head can be explained qualitatively by consideration of the wall displacement owing to the growth of the closure domain.

Micromagnetic study of domain-wall pinning characteristics with grooves in thin films

The pinning characteristics of a 180° domain wall with grooves are investigated using the micromagnetic simulation. The depinning fields required to pull the wall out of the grooved region were strongly related to the pinning characteristics at each step edge. The depinning field difference between the wall movement directions was improved by the increase of the lower depinning field compared to that with the steplike thickness change. It was also found that the depinning fields for various groove widths were almost constant and the wall displacement was further suppressed by the narrower groove having the vertical edge.

Effect of grain size on domain structure of thin non-oriented Si-Fe electrical sheets

In this article, we explored the effect of grain size on the domain structure of thin non-oriented Si-Fe sheets with different grain size.