Siberian Federal University

This work presents a review of the magnetic properties and magnetic phase diagrams of three isostructural orthorhombic antiferromagnets from the Pb2Fe2−xMnxGe2O9 family, with x = 0, 0.16, and 0.43. In pure Pb2Fe2Ge2O9, the magnetic properties are determined solely by the magnetic S-ions Fe3+. The magnetic structure of this crystal is characterized by an antiferromagnetic vector oriented along the orthorhombic c-axis and a weak ferromagnetic moment along the a-axis. The partial substitution of Fe3+ ions with highly anisotropic Mn3+ ions leads to a reorientation of the antiferromagnetic vector toward the orthorhombic b-axis in the crystal with the highest substitution level x = 0.43, accompanied by the disappearance of weak ferromagnetism. At an intermediate substitution level x = 0.16, the competition between the anisotropic contributions of the Fe3+ and Mn3+ ion subsystems results in a spontaneous spin-reorientation transition at Tc = 22 K. The temperature and field dependences of the magnetization for all three crystals were analyzed on the basis of a thermodynamic potential derived from symmetry considerations. Based on this analysis, magnetic field–temperature phase diagrams were constructed for each crystal for various orientations of the magnetic field relative to the orthorhombic axes.