Ferromagnetism in CdCr2Se4 and CdCr2S4

Abstract

A study has been made of the magnetic properties of the spinels ACr2X4, where A is diamagnetic zinc or cadmium, and X is oxygen, sulfur, or selenium. CdCr2Se4 and CdCr2S4 are apparently ferromagnetic with moments somewhat below the predicted value of 6.0 μB/molecule. By contrast, ZnCr2Se4 and ZnCr2S4 are antiferromagnetic at low temperature despite having positive asymptotic Curie points (TA). We found very little thermal variation of cell size in these materials, which further corroborates Lotgering's conclusion that the antiferromagnetism is due to next-nearest-neighbor interactions. Disagreement between experimental values of TA/TC and the theoretical value calculated assuming nearest-neighbor interactions indicate that next-nearest-neighbor interactions are playing a role in the ferromagnetic materials as well. Comparisons of TA and TN in these materials indicate that the magnetic interactions are strongly dependent upon the diamagnetic A-site ion. However, the data cannot uniquely attribute this effect to an exchange coupling directly involving the A-site cation.