Spin reorientation by Ni doping in Cu1−Ni Cr2O4 spinels with x = 0 and 0.1, and evidence for canted magnetic Cr order above the onset of a ferromagnetic Cu

Abstract In ferrimagnetic spinels AB2O4 the magnetic structure is strongly influenced by lattice distortions, geometric frustration and the electronic properties of the cations at the A and B sites. Here, we report a comprehensive study on the temperature dependence of the magnetic structure of CuCr2O4 and Cu0.9Ni0.1Cr2O4 using neutron diffraction. CuCr2O4 undergoes a first continuous magnetic transition around 155 K into a canted long-range spin order on the Cr sublattice established by an antiferromagnetic and a ferromagnetic mode. Below 130 K a second transition occurs into a ferromagnetic order on the Cu sublattice resulting in a ferrimagnetic spin arrangement. Correlations between the appearance of magnetic modes and changes in the lattice geometry at different temperatures are discussed giving insight to magnetoelastic coupling. The occurrence of a ferromagnetic Cr mode above 130 K questions the common interpretation that a strong antiferromagnetic coupling between A and B spins in ferrimagnetic AB2O4 spinels is responsible for spin canting. From our neutron diffraction measurements of Cu1−xNixCr2O4 we identify a spin reorientation for the ferromagnetic modes with a nickel content between x = 0 and 0.1.