INFLUENCE OF EXTERNAL-FIELD ON SPIN REORIENTATION TRANSITIONS IN UNIAXIAL FERROMAGNETS - I - GENERAL-ANALYSIS FOR BULK AND THIN-FILM SYSTEMS

A general phenomenological discussion of spin reorientation transition s (SRT's) is given which contributes to the understanding of both bulk and thin-film behavior in applied fields. The two principal field con figurations (parallel and perpendicular to the crystallographic axis o f symmetry) are considered. In contrast to spontaneous SRT's, only two phases compete when an external magnetic field is applied, a canted a nd a field-aligned one. The complete stability analysis leads to a bas ic phase diagram which encompasses both configurations at once. The co existence of phases is seen to persist even with the field and is desc ribed quantitatively. The practical implications of the analysis for t emperature-driven and thickness-driven SRT's are carefully considered.