  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.
