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. I4(1)/amd) spinet CuFe2O4 is observed up to a T-C approximate to 660 K, where a cubic phase (sp. gr. Fd3m) appears, and up to T approximate to 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)O-6 octahedra in the tetragonal phase corresponds to the Jahn-Teller nature of the structural phase transition. Neel ferrimagnetic structure - a ferromagnetic ordering of the magnetic moments of Fe3+ in the tetrahedral (A) and moments of Fe2+ and Cu2+ in the octahedral (B) positions with opposite directions of magnetization of the sublattices - disappears at T-N approximate to 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) mu(B), 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 Neel 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. (C) 2014 Elsevier B.V. All rights reserved.