Structural transformation and magnetic competition in Yb(Mn_1−xFe_x)O₃

Structural and magnetic properties of the Yb(Mn_1−xFe_x)O₃ (0⩽x⩽1) system have been systematically investigated. Initial samples were prepared via a sol-gel method. A pure hexagonal phase was only obtained for samples with x⩽0.5. With high-pressure annealing, a pure orthorhombic perovskite phase was achieved for all the compositions. The ⁵⁷Fe Mössbauer spectrum for x=0.5 shows that only Fe³⁺ ions exist in the system; there was no evidence of chemical inhomogeneities. With increasing x, the Néel temperature T_N increases for both hexagonal and orthorhombic phases. The orthorhombic Yb(Mn_0.5Fe_0.5)O₃ shows an interesting weak ferromagnetic state in the temperature range of 239–298 K, the ferromagnetism disappearing abruptly on cooling below T_t=239 K. The transition at T_t appears to be a reorientation of the spin axis of a type-G antiferromagnetic order from the orthorhombic a axis to the b axis in the (010) plane.