The Sensitivity of the Magnetic Properties of the ZnCr2O4 and MgCr2O4   Spinels to Non-stoichiometry
  We report that small amounts of metal atom non-stoichiometry are possible in the ZnCr2O4 and MgCr2O4 spinels. The non-stoichiometry, though less than 2%, significantly impacts TN and the nature of the magnetic correlations above TN. The Zn1+xCr2-xO4 spinel is particularly sensitive. While stoichiometric ZnCr2O4 displays antiferromagnetic short range correlations in the susceptibility above TN, ferromagnetic correlations are observed in non-stoichiometric, hole doped Zn1+xCr2-xO4. The Mg1+xCr2-xO4 spinels are less profoundly affected by non-stoichiometry, though significant changes are also observed. We contrast the magnetic properties of Zn1+xCr2-xO4 and Mg1+xCr2-xO4 (x=0, 0.02, 0.04) with those of materials with the equivalent amounts of isovalent non-magnetic Ga3+ substituted on the Cr3+ site to separate the effects of static site disorder and hole doping. 
S. E. Dutton, Q. Huang, O. Tchernyshyov, C. L. Broholm and R. J. Cava
cond-mat.mtrl-sci