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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 Fd (3) over barm, a = 8.3329(1) angstrom at 19 K) to a tetragonal, antiferromagnetically ordered structure (I4(1)/amd, a = 5.8961(1) and c = 8.3211(1) angstrom at 10 K) occurs via a sharp first order transition at 12.4 K. A small hysteresis of 0.15 K 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 (1.63 mu B) of the saturated Cr3+ moment.
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