Kunio Wakai

Noncollinearity as a size effect of CrO2 small particles

The magnetic structure of small CrO2 particles doped with about 1.2 wt.% 57Fe has been explored by 57Fe Mossbauer spectroscopy as a function of the particle size. Mossbauer spectra, obtained with a longitudinal magnetic field applied, unambiguously establish that a noncollinear structure exists that is most pronounced for the smallest particles. The analysis indicates that a surface effect is the origin of this phenomenon. It follows that a core model proposed earlier for the magnetic structure of CrO2 small particles is warranted.

Magnetic properties of MnBi particles

Platelet‐shaped MnBi particles with diameter smaller than 0.5 μm were prepared by grinding sintered MnBi in a ball mill. The MnBi particles were dispersed and oriented in a polymer binder. The oriented MnBi particles were demagnetized in cooling to 85 °K, at which the coercivity was about 300 Oe. The applied magnetic‐field dependence of coercivity was measured at 300 °K on the demagnetized MnBi particles. The coercivity remarkably increased with increasing strength of magnetic field, and in a magnetic field strength of 16 kOe, the maximum coercivity of 16 kOe was attained. It is suggested that ’’nuclei’’ from which magnetization reversal occurs exist in particles in the demagnetized state, and disappear by applying a strong magnetic field.

Effect of Grinding on the Coercivity of MnBi Particles

MnBi particles were prepared by grinding sintered MnBi in a ball mill. The coercivity of these MnBi particles was measured at 300 K and 85 K. At 300 K, the coercivity increased remarkably by grinding, and showed the highest value of 16000 Oe. At 85 K, the coercivity was very low compared with that at 300 K, and was about 80–400 Oe. For the particles which showed the coercivity of 16000 Oe at 300 K, the temperature dependence of coercivity was measured in the temperature range between 300 K and 85 K. The results were explained by considering the single-domain behavior of these particles.

Improvement of corrosion resistance of CoNi thin films by surface passivation

This paper describes improvement of corrosion resistance of CoNi films by surface passivation. The CoNi films were obliquely evaporated on polyethylene terephthalate (PET) films, and passivated by a newly developed mild oxidation method. The passivated films were subjected to environmental exposure test at a relative humidity of 90% and a temperature of 60 °C. No changes of recording performance were observed even after a one‐month exposure. The chemical structure of the passive layer formed on the CoNi films was studied by x‐ray photoelectron spectroscopy and reflected electron diffraction. It was concluded that their strong corrosion resistance was attributed to the very thin, amorphous cobalt(III) oxyhydroxide formed on the surface, containing a small amount of Co2+ ions.

Magnetic Properties of Cobalt-Contained Iron Oxides (Part 1)

Acicular γ-Fe2O3 particles were treated at 90°C in alkali solution containing CO2+ and Fe2+. The coercivity of resultant particles remarkably increased with increasing CO2+/Fe2+ ratio, and reached a maximum value in the vicinity of 0.4 to 0.5. The increase of coercivity was attributed to the magnetic anisotropy of iron-cobalt ferrite which was considered to crystallize at the surface of γ-Fe2O3. The magnetic properties of γ-Fe2O3 particles coated by iron-cobalt ferrite, such as a variation of coercivity in annealing at 60°C or printing effect, were extremely stable compared with those of cobalt-substituted iron oxides.