Anisotropy fields of Ba-ferrite and other particulate media

The anisotropy field (H_k) in Co- and Ti-doped platelet-shaped particulate assemblies and in Co-modified iron oxide and very-high-coercivity metal-particle media was determined by measuring the rotational hysteresis losses as a function of the applied field and extrapolating the high-field part of the distribution to obtain the field at which the losses diminish to zero. For Ba-ferrite media with H_c=550-1300 Oe, H_k values ranging from 2.2-6 kOe and not necessarily correlated with the coercivities were obtained. Co-gamma media had H_k values toward the lower part of this range, while the very-high-H _c metal particle media had H_k values in the upper part of this range, in both cases correlated with their coercivities. A high value of H_k is found to be a necessary, but not a sufficient, condition for large angular dependence of the remanence coercivity (H_r). Additional requirements include single-domain particles with predominant crystalline anisotropy and a high degree of morphological perfection of the particles. A high H_k, however, provides for more rectangular remanence loops (and thus reduces recording demagnetization/losses) not only when crystalline anisotropy predominates, but also, to some extent, when shape anisotropy is dominant