Low temperature neutron diffraction study of MgCr₂O₄ spinel

The low temperature structural and magnetic transition in the normal spinel MgCr₂O₄ has been studied using high resolution neutron powder diffraction. The transition from a cubic, paramagnetic phase (space group , a = 8.3329(1) at 19K) to a tetragonal, antiferromagnetically ordered structure (I4₁/amd, a = 5.8961(1) and c = 8.3211(1) at 10K) occurs via a sharp first order transition at 12.4K. A small hysteresis of 0.15K between cooling and warming experiments is observed. The structural and magnetic transitions are coincident and their order parameters (the tetragonal strain and Cr moment) scale with each other. The structural transition partially relieves magnetic frustration in the Cr network, although remaining frustration is evident from the low value (1.63ν_B) of the saturated Cr ³⁺moment.