Magnetic and structure transition of Mn3-xFexO4 solid solutions under high-pressure and high-temperature conditions

Abstract

Magnetic and structure transitions of Mn_3–xFe_xO₄ solid solutions under extreme conditions are clarified by neutron time-of-flight scattering diffraction and X-ray Mössbauer measurement. The ferrimagnetic-to-paramagnetic transition temperature (100 °C) of Mn₂FeO₄ spinel is different from the tetragonal-to-cubic structure transition temperature (180 °C). The structure transition temperature decreases with increasing pressure. The transition is not coupled with the magnetic transition. Synchrotron X-ray Mössbauer experiments have revealed the pressure effects on the distribution of Fe²⁺ and Fe³⁺ at the tetrahedral and octahedral sites in the spinel structure. Ferrimagnetic MnFe₂O₄ and Mn₂FeO₄ spinels show sextet spectral features with hyperfine structure elicited by internal magnetic fields. Cubic MnFe₂O₄ spinel and tetragonal Mn₂FeO₄ transform to high-pressure orthorhombic postspinel phase above pressures of 18.4 GPa and 14.0 GPa, respectively. The transition pressure decreases with increasing Mn content. The postspinel phase has a paramagnetic property. Mn₂O₁₀ dimers of two octahedra are linked via common edge in three dimentional direction. The occupancy of Fe²⁺ in the tatrahedral site is decreased with increasig pressure, indicating more oredered structure. Consequently, the inverse parameter of the spinel structure is increased with increasing pressure. The magnetic structure refinements clarify the paramagnetic and ferrimagnetic structure of MnFe₂O₄ and Mn₂FeO₄ spinel as a function of pressure. The magnetic moment is ordered between A and B sites with the anti-parallel distribution along the b axis. The nuclear tetragonal structure (a_N, a_N, c_N) has the ferrimagnetic structure but the orthorhombic magnetic structure has the ferrimagnetic structure with the lattice constants (a_M, b_M, c_M). The magnetic moment is ordered between A and B sites with the anti-parallel distribution along the b_M axis.