Interplay between structural and magnetic phase transitions in copper ferrite studied with high-resolution neutron diffraction

A detailed neutron diffraction study of copper ferrite in a broad temperature range has allowed to precisely access the peculiarities of magnetic and structural phase transitions in it. On heating from 2 to 820 K, a fully inverted tetragonal (sp. gr. I41/amd) spinel CuFe2O4 is observed up to a TC≈660 K, where a cubic phase (sp. gr. Fd3m) appears, and up to T≈700 K, both structural phases coexist. The inversion parameter of spinel structure does not change at the transition to the cubic phase. Deformation of the (Cu,Fe)O6 octahedra in the tetragonal phase corresponds to the Jahn-Teller nature of the structural phase transition. Néel ferrimagnetic structure - a ferromagnetic ordering of the magnetic moments of Fe3+ in the tetrahedral (A) and moments of Fe3+ and Cu2+ in the octahedral (B) positions with opposite directions of magnetization of the sublattices - disappears at TN≈750 K. The magnetic moment in the A-positions (Fe3+) and the total one in the B-positions (Fe3++Cu2+) at T<30 K are equal to 4.06(6) and 4.89(8) μB, respectively. The difference between these values corresponds to a spin moment of Cu2+. Qualitative analysis of the magnetic interactions in the inverted mixed spinel showed that the dominant antiferromagnetic interaction between A and B sublattices, which is required to stabilize the collinear Néel order in CuFe2O4, follows naturally from the standard superexchange theory. In the co-existence range of structural phases diffraction peaks are significantly broadened. The size effects providing the main contribution to peak broadening is also superimposed with the microstrain-conditioned peak broadening. In the tetragonal phase, microstrains in the crystallites are highly anisotropic.