Orientational phase transition into a modulated magnetic state in the weak ferromagnet FeBO3: Mg

The transition from a homogeneous into a modulated magnetic state in the easy-plane weak ferromagnet FeBO3: Mg is studied by a magnetooptic method. At T < 135 K, the application of a magnetic field in the basal plane of the crystal is shown to excite the modulation of its magnetic order parameter, which manifests itself in a periodic deviation of the local ferromagnetism vector from the magnetization direction. The conditions for the existence of a modulated magnetic superstructure in FeBO3: Mg are studied, and its preferred orientation in the basal plane of the crystal is analyzed. A magnetic H-T phase diagram that shows the boundaries between the homogeneous and inhomogeneous magnetic states of this weak ferromagnet is constructed. The modulation period and the azimuthal angle specifying the local ferromagnetism vector direction in the structure are studied as a function of temperature and magnetic field. The results obtained are discussed in terms of the theory of magnetic ripple using the model of anisotropic magnetic centers appearing in a crystal doped by magnesium ions.