Orbital dilution effect in ferrimagnetic Fe1 ? xMnxCr2O4: competition between anharmonic lattice potential and spin?orbit coupling

Magnetic and structural phase diagram in a spinel-type solid solution system Fe1 − xMnxCr2O4 has been investigated. The cubic-to-tetragonal transition temperature Ts 1 is gradually reduced by the substitution of Mn2 + (3d5) for Jahn–Teller-active Fe2 + (3d6) ions, implying the long-range nature of the ferroic interaction between orbitals. In the paramagnetic tetragonal phase for x < 0.5, the c parameter is shorter than a because of the anharmonicity of the elastic energy. The crystal structure further changes to orthorhombic at around the ferrimagnetic transition temperature TN 1. Ts 1 and TN 1 meet at x = 0.5, and Mn substitution of more than 0.5 gives rise to another tetragonal phase with a < c. The systematic change in crystal structure is discussed in terms of competition between the anharmonic lattice potential and the intra-atomic spin–orbit interaction at Fe2 + .