Magnetization Process of a Screw Spin System. II

Abstract

Magnetization processes and structure changes in helical and modified helical spin arrangements due to application of a magnetic field are studied for finite temperatures and absolute zero in a number of cases of different anisotropy energy. In the case of a strong anisotropy giving rise to a propser helical structure and of no anisotropy within the plane, a field applied within the plane causes structure changes helix to fan and then to parallel alignment as the field is increased, as known already for absolute zero, but it is shown that the first transition is of the second kind when the temperature is close to the Néel temperature, whereas it is of the first kind below a certain critical temperature; the second transition is always of the second kind. The case of p-fold symmetry within the plane is also studied; in particular, the case of twofold symmetry with a field applied either along the easy direction or hard direction is studied in detail for arbitrary temperatures, the results being summarized in Figs. 3 and 4; the case of sixfold symmetry is also studied in some detail for absolute zero and it is shown that, with an applied field along one of the easy directions, the transition from fan to ferromagnetism becomes discontinuous (i.e., of the first kind) when the anisotropy constant increases and that the fan structure disappears with a further increase in the anisotropy constant (Fig. 6). With a field applied along one of the six hard directions, the fan structure does not disappear, and there appears an oblique ferromagnetism at higher values of the anisotropy constant (Fig. 7). Finally, structure changes for longitudinal sinusoidal structure, oblique helix, and conical structure are discussed. Comparison with experiment is made on a number of materials, particularly on Ho, and most, if not all, observed features are accounted for.