Features of magnetization and spin reorientation in weak ferrimagnets of the YFe_1-xCr_xO_3 type

A brief critical review is given of the 50-year history of experimental and theoretical studies of the magnetic properties of a new promising class of weak ferrimagnets such as RFe_1-xCr_xO_3 with a non-magnetic R-ion (R = La , Y, Lu), i.e. systems with competing signs of the Dzyaloshinsky vectors Fe-Fe, Cr-Cr and Fe-Cr. The spin Hamiltonian of the system is considered taking into account isotropic exchange, antisymmetric Dzyaloshinsky-Moriya exchange, and second- and fourth-order single-ion anisotropy. Within the framework of the molecular field approximation, calculations were made of the Neel temperatures, the average magnetic moments of 3d ions, total and partial magnetizations, and effective anisotropy constants. The existence in the model system YFe_1-xCr_xO_3 of two regions of negative magnetization 0.25 ≤ x≤ 0.5 and x≈ 0.8 with the corresponding magnetization reversal points reaching room temperature at x≈ 0.45. The phenomenon of spin reorientation observed for single-crystal samples in a wide range of concentrations is explained by a sharp decrease in the contribution of antisymmetric exchange to magnetic anisotropy with increasing deviation from the parent compositions and competition between the contributions of single-ion anisotropy of Fe and Cr ions. It has been suggested that the spatial orientation of the Neel G vector and the G_xyz configuration are the reason for the small value of saturation magnetization observed experimentally for compositions inside or near the region of negative magnetization.