Simple spin-reorientation model for rare-earth orthoferrites and orthochromites

A simple theory of spontaneous spin reorientation in rare-earth orthoferrites RFeO$_3$ and orthochromites RCrO$_3$ induced by 4f–3d-interaction is presented. It is shown that both the temperature and the nature of the spin reorientation are the result of the competition between the second- and fourth-order spin anisotropy of the 3d-sublattice, the crystal field for 4f-ions, and also the 4f–3d interaction. A case of the substitution of the rare-earth ions with a non-magnetic impurity is also considered.