Spin And Orbital Order In The Vanadium Spinel Mgv2o4

We present a study of the frustrated spinel MgV2O4, which possesses highly coupled spin, lattice, and orbital degrees of freedom. Using large single crystal and powder samples, we find a distortion from spinel at room temperature (space group F (4) over bar 3m) which allows for a greater trigonal distortion of the VO6 octahedra and a low-temperature space group (I (4) over bar m2) that maintains the mirror plane symmetry. The magnetic structure that develops below 42 K consists of antiferromagnetic chains with a strongly reduced moment while inelastic neutron scattering reveals one-dimensional behavior and a single band of excitations. The implications of these results are discussed in terms of various orbital-ordering scenarios. We conclude that although spin-orbit coupling must be significant to maintain the mirror plane symmetry, the trigonal distortion is large enough to mix the 3d levels leading to a wave function of mixed real and complex orbitals.