Molecular-field theory of the magnetic configurations and transverse magnetization processes in systems with high-order uniaxial anisotropy and strong antisymmetric exchange, with applications to some Sr-substituted hexagonal ferrites

Abstract

We study in the molecular-field approximation the equilibrium configuration and the phase transitions of a magnetic model system consisting of two sets of moments of equal magnitude, coupled by isotropic, anisotropic, and antisymmetric exchange, in the presence of uniaxial anisotropy, under the effect of a transverse magnetic field. The anisotropy is developed up to the sixth-order terms, with no restricting conditions upon the relative strength of the different interactions. Two possible interpretations of the model are developed: as a two-sublattice system, and as a quasicontinuous distribution of magnetic moments, exhibiting spiral magnetic structures. The theoretical results are then applied to the interpretation of the unusual magnetic structures, magnetization curves, and phase transitions observed in some Sr-substituted hexagonal ferrites of the $Y$ and $Z$ families, allowing an estimation of all the interaction parameters.