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

Abstract

Abstract 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. I 4 1 / amd ) spinel CuFe 2 O 4 is observed up to a T C ≈660 K, where a cubic phase (sp. gr. Fd 3 m ) 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)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 Fe 3+ in the tetrahedral (A) and moments of Fe 3+ and Cu 2+ in the octahedral (B) positions with opposite directions of magnetization of the sublattices – disappears at T N ≈750 K. The magnetic moment in the A-positions (Fe 3+ ) and the total one in the B-positions (Fe 3+ +Cu 2+ ) at T B , respectively. The difference between these values corresponds to a spin moment of Cu 2+ . 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 CuFe 2 O 4 , 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.