Field-Induced Spin Reorientation in YFeO3 and YCrO3

Application of a sufficiently strong magnetic field H_x along the antiferromagnetic easy axis (x) of the isomorphic, distorted perovskites YFeO₃ and YCrO₃, results in the smooth rotation of the antiferromagnetic vector in the x‐z plane, reaching the z axis at the critical fields H_cr=74±1 kOe and H_cr=40±1 kOe at 4.2°K, for YFeO₃ and YCrO₃, respectively. A detailed analysis of the magnetization curve m_x(H_x) provides an unambiguous determination of the quartic and uniaxial‐type anisotropy in the x‐z plane of YFeO₃ (spin= $\frac{5}{2}$ ). In agreement with theory, quartic anisotropy terms are not found in YCrO₃ (spin= $\frac{3}{2}$ ). Only in the latter case are the uniaxial anisotropy and the reorientation field calculable from the initial susceptibilities. Comparison of m_x(H_x) with m_z(H_z) for H>H_cr provides a quantitative separation between the antisymmetric exchange (H_D) and single‐ion anisotropy (H_xz) contributions to the weak ferromagnetic canting. We find |H_xz/H_D| <0.02 for YFeO₃ and |H_xz/H_D| <0.05 for YCrO₃, at 4.2°K.