Masahiro Amemiya

Coercivity of γ‐Fe2O3 particles growing iron‐cobalt ferrite

Acicular γ‐Fe2O3 particles were heated at 90°C in alkali solution containing Co2+ and Fe2+. The coercivity of the resultant particles remarkably increased with the increasing Co2+/Fe2+ ratio, and in the neighborhood of the Co2+/Fe2+ ratio of 0.4, reached a maximum value. When the particles with a Co2+/Fe2+ ratio of 0.5 were dissolved in hydrochloric acid, the cobalt content and the coercivity were rapidly decreased with increasing dissolved weight. Iron‐cobalt ferrite was expected to grow on the surface of γ‐Fe2O3 particles. The increase of coercivity was attributed to the iron‐cobalt ferrite.

On the coercivity of cobalt-ferrite epitaxial iron oxides

Acicular γ-Fe 2 O 3 particles were treated in alkaline solution containing cobaltous and ferrous ions. When cobalt content is increased, the surface area of the oxides decreases, and cobalt-ferrite is considered to be crystallized on the surface of γ-Fe 2 O 3 . The coercivity of the oxides remarkably increases with increasing the cobalt content, and the increase of coercivity is considered to be due to the crystalline magnetic anisotropy of the cobalt-ferrite. The coercivity of the oxides is explained by considering the shape anisotropy of acicular particles and the crystalline magnetic anisotropy of cobalt-ferrite.

Morphology and magnetic properties of the iron oxide layer formed on iron acicular particles

Iron oxide layer was formed on iron acicular particles to protect the particles from further oxidation. The typical thickness of the oxide layer was about 30 A, and the saturation magnetization in the oxide layer was calculated to be about 40 emu/g at 300 K. It was found that the oxide layer consisted of ferromagnetic iron oxides and superparamagnetic region, and the ferromagnetic iron oxides acted as protection layer against further oxidation of particles.

Formation and magnetic properties of γ-Fe 2 O 3 particles surface modified with crystallized cobalt-ferrite

Acicular γ-Fe 2 O 3 particles were heated at 90°C in alkali solution containing Co2+and Fe2+with Co2+/Fe2+ratio of 0.5. The coercivity of resultant particles increased linearly with increasing the Co2+content, and the coercivity of 900 Oe was obtained for the particles with Co2+content of 7 wt%. The shape of the particles is acicular, and an appreciable variation of morphology by the treatment in alkali solution was not observed. Cobalt-ferrite was expected to crystallize epitaxially on the surface of γ-Fe 2 O 3 particles, and the increase of coercivity was attributed to the magnetic anisotropy of the cobalt-ferrite. A variation of coercivity by annealing at 60°C and print-through were small compared with those of the particles in which iron were homogeneously substituted by cobalt ions. Such stability was explained by considering that a very high concentration of cobalt ions exist only on the surface of γ-Fe 2 O 3 particles, and the migration of cobalt ions is extremely difficult.

Morphology of cobalt-ferrite epitaxial iron oxides

Cobalt-ferrite was crystallized on γ-Fe 2 O 3 particles by the heating of γ-Fe 2 O 3 in an alkaline solution containing cobaltous and ferrous ions. The lattice images of the oxides showed that cobalt-ferrite was crystallized epitaxially along the [110] direction parallel to the direction of the needle axis of core γ-Fe 2 O 3 particles. It was considered that the magnetic field of the core γ-Fe 2 O 3 particles caused the uniaxial magnetic anisotropy of the oxides.

Mössbauer study of the oxides layer formed on the surface of acicular iron particles

The structure of the iron oxides layer formed on the surface of acicular iron particles is investigated by means of Mossbauer absorption measurements using a sample of which surface is made of enriched57Fe. The iron oxides layer consists of ferrimagnetic Fe 3 O 4 and superparamagnetic Fe 3 O 4 at 300 K. The superparamagnetic region of Fe 3 O 4 diminishes below about 110 K. The volume ratio of the ferrimagnetic region to the superparamagnetic region of Fe 3 O 4 is calculated to be about 7:3 at 300 K.

Effect of magnetic anisotropy of Ba‐ferrite particles on squareness of perpendicular recording media

Magnetic anisotropy constants of Ba‐ferrite platelet particles are measured by means of a torque magnetometer, and the magnetic easy direction of the particles is calculated from the anisotropy constants. The squareness in the perpendicular direction of tapes depends on the magnetic easy direction of the particles, and decreases as the magnetic easy direction inclines from the direction perpendicular to the platelet plane to the direction parallel to the platelet plane.

Magnetic annealing and relaxation in cobalt‐substituted acicular iron oxide particles

The magnetic anisotropy induced by annealing in a magnetic field and the activation energy were measured for cobalt‐substituted acicular iron oxide particles using a torque meter. The induced magnetic anisotropy linearly increased with the cobalt content in the particles. The induced magnetic anisotropy at 293 K in the particles containing a cobalt of 1.6 wt. % was 1.1×105 erg/cc, and this value nearly agreed with the value reported in iron–cobalt ferrite. The change of coercivity in the particles was attributed to the induced magnetic anisotropy. The activation energy in the particles was 0.8–0.9 eV, and this value was close to the value reported in iron‐cobalt ferrite.

Defect structure in oxygen-excess manganese-magnesium ferrite

Abstract Specific gravity, crystal lattice constant, saturation magnetization and ferrimagnetic Curie temperature were measured to clarify the type of point defects in the oxygen-excess manganese-magnesium ferrite containing a slight amount of copper (Cu0·086Mg0·214Mn0·986Fe1·714O4+δ). Of those properties, crystal lattice constant and saturation magnetization decrease, while ferrimagnetic Curie temperature increases, as the oxygen content in the ferrite increases. Specific gravity is almost independent upon the oxygen content. From the quantitative analyses of these characteristics, it can be concluded that the point defects in oxygen-excess manganese-magnesium ferrite are of cation vacancies on its B-sites (octahedral sites).

Recording characteristics of tapes prepared using iron oxide particles treated with organic titanate

The adsorption behavior of organic titanate on an iron foil having an ideal iron oxide surface is examined to improve read‐write characteristics of particulate magnetic recording media by the surface treatment of iron oxide magnetic particles with organic titanate. Organic titanate significantly reduces the surface hydrophilicity of iron oxide surface of the iron foil. Heat treatment is required to form the chemical bonding between the organic titanate and the iron oxide surface. The method of surface treatment is applied to Co‐modified iron oxide particles. As a result, the magnetic properties and surface roughness of the tape are remarkably improved by the increase of dispersibility of particles due to the reduction of hydrophilicity with the organic titanate treatment. The carrier‐to‐noise ratio (C/N) at the frequency of 6.0 MHz in the tape is increased about 2 dB compared with the tape prepared using untreated particles.