Thermal expansion of Sm-Tb orthoferrites in the region of the spin reorientation transition

A new model of spin reorientation is discussed on the basis of the analysis of experimental data on thermal expansion of samarium-terbium orthoferrite and the earlier studies of its magnetic and crystallooptical properties. During the orientational phase transitions, the zero values of the thermal and optical parameters of rare earth orthoferrites correspond to the alignment of the magnetization vector along the 110] crystallographic directions. For orthorhombic Sm-Tb orthoferrite crystals, these directions coincide with the twofold axes in the plane.