Nature of magnetocrystalline anisotropy in the basal plane of iron borate

Basal (hexagonal) magnetocrystalline anisotropy of iron borate FeBO3, including crystal field and dipole-dipole contributions, is discussed in detail. Previously, the latter contribution had been ruled out on the basis of symmetry; indeed, considering the magnetic dipole-dipole interaction in the approximation of point dipoles, only uniaxial magnetocrystalline anisotropy is accounted for. In the present work we consider the dipole-dipole interaction energy for extended dipoles and calculate the dipole-dipole contribution to basal magnetocrystalline anisotropy for two different models of an extended dipole: a pair of fictitious magnetic charges and a circular current loop. A comparison between theoretical expressions developed and experimental results obtained by antiferromagnetic resonance allows estimating the effective size of magnetic dipoles and calculating the dipole-dipole contribution to basal magnetocrystalline anisotropy constants of iron borate.