Singularities in the magnetization processes of high anisotropy materials

Singularities of various kind are often observed In the magnetization curve of a ferromagnetic crystal. This is especially true for high anisotropy materials, such as the rare earth-intermetallic compounds. Examples are the First Order Magnetization Processes (FOMP), which are discontinuous rotations of the magnetization vector due to high order anisotropy terms. However, the hard direction anisotropy field of any ferromagnetic crystal is always a critical field at which it reaches saturation. The Singular Point Detection (SPD) technique allows detecting such singularities using polycrystalline samples by the observation of the successive derivatives d¹¹/dH¹¹ The shape of the singularity and the order of differentiation “n” at which it becomes apparent depends on the symmetry of the hard axis. The SPD theory has been recently extended to multidomain crystallites on the basis of the Neel phase theory, and utilized for texture studies of permanent magnets. Moreover it has been proved to be valid also in the presence of canting between the sublattice moments caused by strong competition between anisotropy and exchange; in principle, in the case of ferrimagnetic compounds it is possible to detect spin flop transitions both of second and first order (high POMP). The application of the SPD approach to the general problem of the complex susceptibility tenser and its covariant derivatives can lead to further developments. As an example, the singularity observed in the transverse susceptibility of uniaxial materials is strongly sensitive to the presence of single domain crystallites