Sensitivity of the magnetic properties of the ZnCr₂O₄ and MgCr₂O₄ spinels to nonstoichiometry

We report that small amounts of metal atom nonstoichiometry are possible in the ZnCr₂O₄ and MgCr₂O₄ spinels. The nonstoichiometry, though less than 2%, significantly impacts T_N and the nature of the magnetic correlations above T_N. The Zn _1+xCr_2-xO₄ spinel is particularly sensitive. While stoichiometric ZnCr₂O₄ displays antiferromagnetic short-range correlations in the susceptibility above T_N, ferromagnetic correlations are observed in nonstoichiometric, hole-doped Zn _1+xCr_2-xO₄. The Mg_1+xCr _2-xO₄ spinels are less profoundly affected by non-stoichiometry, though significant changes are also observed. We contrast the magnetic properties of Zn_1+xCr_2-xO₄ and Mg_1+xCr_2-xO₄ (x = 0, 0.02, 0.04) with those of materials with the equivalent amounts of isovalent nonmagnetic Ga3⁺ substituted on the Cr3⁺ site to separate the effects of static site disorder and hole doping.