Intergrain necking and the reversal of magnetization of fine, highly acicular ferromagnetic skeleton particles

Abstract

In this paper the relationship between the reversal of magnetization and the morphology of very fine, iron-based and highly acicular particles used in high-density magnetic recording media is investigated. The iron-based particle referred here is of a skeleton type, which consists of a very fine granular material; the so-called “grain”. The grain belongs to the bcc phase and its size ranges between about 100 to 300 Å depending on usage. Therefore the grain can be treated as a single domain particle. As the morphology of the skeleton particle prepared for 8 m/m video-recording media is mainly determined by the grain size and the intergrain necking, the effects on the coercivity were studied. The magnetization reversal of a long “chain-of-spheres” which are in contact with each other over a finite area was investigated to determine the quantitative relationship with “intergrain necking” of the skeleton particles when a type of “exchange anisotropy”, which is proportional to the contact area between two adjacent unit spheres, is introduced into the chain. The symmetric fanning mode is preferential in increasing the intergrain necking. The introduction of the exchange anisotropy can result in decrease in the coercivity with increasing intergrain necking, this quantitatively reproduces the experimental behavior observed for very fine, highly acicular skeleton particles of α-Fe. On the other hand, no essential change in the behavior of the angular variation of the coercivity is induced even if exchange anisotropy is introduced into the chain. Finally, under the present scheme, it has been discussed as to how to interpret the experimentally-found dependence of coercivity on grain size, where a possibility to introduce an influence of the unit-sphere size on the characteristic constant of the exchange anisotropy is suggested.