| Popis: |
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$_{1-x}$Cr$_x$O$_3$ 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$_{1-x}$Cr$_x$O$_3$ of two regions of negative magnetization $0.25 \leq x\leq 0.5$ and $x\approx 0.8$ with the corresponding magnetization reversal points reaching room temperature at $x\approx 0.45$. 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 ${\bf G}$ 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. |
