Evolution of magnetic phase and cation distribution in Cu1-xZnxFe2O4 studied by neutron powder diffraction

Abstract

CuFe2O4 is a highly interesting material as it is a ferrimagnet with an unusual high magnetic ordering temperature of 780 K. ZnFe2O4, on the other hand, is a frustrated spin system with antiferromagnetic order below 10 K. By doping nonmagnetic Zn ions in CuFe2O4, frustration can be introduced and interesting properties might emerge. Given that, high resolution and high intensity neuron powder diffraction techniques have been applied to study the structural and magnetic phase transition in Cu1-xZnxFe2O4 from 4 K to 750 K. Coexistence of cubic and tetragonal structure in CuFe2O4 was observed in a wide temperature range, which indicates a second order phase transition nature. This transition is caused by Jahn-Teller distortion of the CuO6 octahedra. Although CuFe2O4 and ZnFe2O4 are inverse and normal spinels, respectively, mixed cation distribution was found in doped samples, with Cu and Zn ions sitting both either on the tetrahedral or the octahedral sites. All the doped Cu1-xZnxFe2O4 (x = 0.2 - 1) samples crystallise in the cubic structure and order in the ferrimagnetic spin configuration. Upon doping, the value of oxygen position parameter μ increases, indicating the compression of the octahedra with increasing Zn-composition. Short-range antiferromagnetic order was observed below 10 K in cubic ZnFe2O4. The spin frustration, which leads to the antiferromagnetic order in Cu0.04Zn0.96Fe2O4 and ZnFe2O4 is induced by the competing interaction between the first nearest neighbor and the third nearest neighbour tetrahedra formed by Fe ions on B sites.