H. Yokoyama

Properties of Ba ferrite particles for perpendicular magnetic recording media

Fine Ba ferrite particles, suitable for coated perpendicular magnetic recording media, have been prepared. The particles, about 0.08 μm in average diameter, are thin hexagonal platelets with easy magnetization axes normal to their planes. Coercivity H c is controllable in a wide range, without significant reduction in magnetization, by Co and Ti substitution. The measured temperature dependences of H c and σ s showed stable characteristics. Good squareness ratio was obtained from an orientation capability measurement.

Particle size effects on magnetic properties of BaFe12−2xTixCoxO19 fine particles

Particle size effects on magnetic properties of BaFe12−2xTixCoxO19 fine particles were investigated. Saturation magnetization of these particles decreased with decreasing particle size. This result can be explained with the existence of a nonmagnetic thin layer, several angstroms in thickness, on the surface of the particles. Particle size dependence of coercivity for these particles, below 1000 A in size, follows the superparamagnetism theory. For the decrease in coercivity above 2000 A, multidomain nucleation is inferred as one of the possible mechanisms. Particle shape dependence of coercivity is well explained with the coherent rotation model, considering shape anisotropy.

Surface effect on saturation magnetization of Co and Ti substituted Ba-Ferrite fine particles

The surface effect on saturation magnetization of Co and Ti substituted Ba-ferrite platelet fine particles is reported. The saturation magnetization decreases linearly with increasing hexagonal c-plane surface area of the particles, and the effect of the side plane surface area on the saturation magnetization is negligible. This result indicates that a low saturation magnetization layer exists predominantly on the c-plane surfaces. The saturation magnetization of the c-plane surface layers increases noticeably with decreasing temperature, and approaches the saturation magnetization value for the bulk Ba-ferrite. The low saturation magnetization at the surface layer is explained by a thermal fluctuation effect on the magnetic ions at the surface layer which have a weaker exchange field than the ions inside the particles. Therefore, particles with small diameter to thickness ratio values are beneficial in obtaining higher saturation magnetization under a given particle volume.

Improvement in the temperature coefficient of coercivity for barium ferrite particles

An improvement in the temperature coefficient of coercivity, dHc/dT, for Co-Ti-substituted Ba ferrite particles for high-density magnetic recording media has been sought by substituting various third elements into the particles. The substitution of Sn ions for some of the Ti ions was found to be promising for this purpose, effectively reducing dHc/dT. The substitution was not detrimental to saturation magnetization or to the size distribution of the particles. The coercivity and particle size of the Co-Ti-Sn-substituted particles could be widely controlled. The value of dHc/dT was reduced to below half of that for Co-Ti-substituted particles. Ba ferrite media utilizing these particles provide an improved margin for operating temperature changes and high-performance recording. >

Magnetization reversal for barium ferrite particulate media

A perpendicularly oriented Ba ferrite particulate medium was investigated with regard to the magnetization reversal process. Coercivity and remanence coercivity for the medium were found to be decreasing and increasing functions, respectively, of the angle between the easy axis and the applied field. The angular dependence of remanence coercivity, being supported by the rotational hysteresis integral value, implies that the magnetization reversal process for the medium is incoherent. A fanning model for the two aligned particles with uniaxial crystalline anisotropy, was developed. This model yields variable angular dependencies of coercivity for varying ratios of a crystalline anisotropy field (H k to a dipole-dipole interaction field (H m ). Taking an appropriate H k /H m value, the model can predict the angular dependence of coercivity and remanence coercivity for the medium.

Coercivity for Ba-ferrite superfine particles

Intrinsic coercivity for Co and Ti substituted Ba-ferrite superfine particles free from agglomeration, and the particle agglomeration effect, were investigated by measuring coercivity for Ba-ferrite particles dispersed and attached onto SiO 2 particles. The coercivity for small aspect ratio (diameter to thickness ratio) Ba-ferrite platelet particles increases with decreasing the Ba-ferrite concentration in the samples and attains a constant value, which can be regarded as the intrinsic particle coercivity. Small aspect ratio Ba-ferrite platelet particles showed coercivity decrease with particle agglomeration, while large aspect ratio particles showed coercivity increase. The particles with intermediate aspect ratio showed no coercivity change due to particle agglomeration. The coercivity increase with increasing agglomeration, which seems to be characteristic behavior for Ba-ferrite platelet particles, is caused by a formation of particle stacking agglomeration, which behaves like particles with small aspect ratio.

Barium ferrite fine particles for high density magnetic recording media

The physical subjects which appeared in the mateial design process of realizing the required barium ferrite particle characteristics for high density magnetic recording medium application were discussed. Relatively large temperature dependence of coercivity for Co+2 substituted barium ferrite particles could be reduced by removing spin binding of Co+2 ions in magntoplumbite crystal sites, or reducing amount of Co+2 ions in magntoplumbite crystal sites. Saturation magnetization of barium ferrite particles could be increased by reducing the amount of nonmagnetic charge compensating ions, by reducing Fe ions at siteis with minority spin direction or by incooorating spinel ferrites with large saturation magetization as intergrown crystal structure. Recent experimenal results suggest that the particle homogenuity in size, shape and chemical composition is a important parameter to obtain further advanced recording media characteristics.

Relationship Between Particle Volume and Saturation Magnetization on Ba-Ferrite Fine Powder

I. Purpose Saturation magnetization of hexagonal platelet Ba-ferrite fine particles, for use in particulate perpendicular magnetic recording media, decreases with increasing specific surface area, in particular with increasing c-plane component of specific surface area of particles [1,2]. We reasoned that this is probably caused by the existence of a nonferromagnetic layer on the c-plane surfaces of the platelet particles [2]. However, it is known that as the volume of fine magnetic powders diminishes and the proportion of very fine particles increases, the thermal fluctuation effect

Angular Dependence of Coercivity for Barium Ferrite Coated Perpendicular Media

The authors studied the angular dependences of the coercivity Hc and remanent coercivity rHc in Ba-ferrite perpendicular-oriented film and other media, to investigate magnetization reversal in Ba-ferrite particles in the media. Comparisons with behaviors predicted by the Stoner-Wohlfarth coherent rotation model indicated that while magnetization reversal in such film shows single-domain behavior, it cannot be regarded as necessarily coherent in nature, although reversals within individual particles may be so.

Stability of Ba-Ferrite Magnetic Recording Media

Stress and temperature stabilities of Ba-ferrite perpendicular magnetic recording media have been investigated. The stress-induced rate of decrease of remanent magnetization in the media was found to be as small (1 × 10-5 per kg/cm2) as the values for metal and Co-γ-Fe2O3 media. The remanent magnetization in the perpendicular direction remains almost constant in the temperature range from -10 to 80°C. When signals are reproduced with an 8-mm VCR in the still-frame mode, the output decreases after 120 min, and the drop in output after 10 million passes with a 3.5-in floppy disk drive was less than 1 dB. These results confirm the excellent stability of Ba-ferrite recording media.