Magnetically ordered and disordered sublattices in geometrically frustrated Ni chromite

The magnetic order disorder of the Cr moments in the antiferromagnetic state of the ferrimagnet NiCr2O4 is reinvestigated by neutron powder diffraction to clarify the observation of reduced Cr moments located on a pyrochlore like lattice [M. Reehuis et al., Phys. Rev. B 91, 024407 2015 ]. The change of the spin structure in the slightly Cu doped chromite Ni0.98Cu0.02Cr2O4 has been studied as well, where orthorhombic lattice distortions become stronger. For both chromites we observe at 2 K two magnetic phases one with two propagation vectors k 0,0,1 and 0,0, kz , and a second one with k 1 2,1 2,1 2 . The latter phase disappears at 21 K for NiCr2O4 and at 24 K for Ni0.98Cu0.02Cr2O4. Data analysis shows that the two phases reside on two disparate Cr sublattices. These are identified by different chain directions of strongly bonded spin pairs along diagonals in the pseudotetragonal a 1 a 2 plane. The less distorted NiCr2O4 shows the same subdivision of the lattice still with a saturated Cr moment for the 0,0,1 0,0,kz phase, however, with a strongly reduced moment for the 1 2,1 2,1 2 phase, whereas an almost saturated moment shows up in the Cu doped chromite. The reduced moment in NiCr2O4 indicates disorder on one Cr sublattice down to low temperature due to stronger frustration on the less distorted tetragonal pyrochlore lattice. Magnetoelastic effects and further neighbor spin interactions are mainly considered in the discussion of the observed peculiar ordering processes. Notable is the transition temperature of the 1 2,1 2,1 2 phase of NiCr2O4 around 21 K, where anomalies for the specific heat and for the magnetodielectric behavior have recently been reported [T. D. Sparks et al., Phys. Rev. B 89, 024405 2014

Spin reorientation by Ni doping in Cu1 xNixCr2O4 spinels with x 0 and 0.1, and evidence for canted magnetic Cr order above the onset of a ferromagnetic Cu

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 amp; 8722;xNixCr2O4 we identify a spin reorientation for the ferromagnetic modes with a nickel content between x 0 and 0.

HFM EXED The High Magnetic Field Facility for Neutron Scattering at BER II

An overview of the high magnetic field facility for neutron scattering at Helmholtz Zentrum Berlin HZB is given. The facility enables elastic and inelastic neutron scattering experiments in continuous magnetic fields up to 26.3 T combined with temperatures down to 0.6