Pressure-enhanced direct exchange coupling observed in chromium spinels

Magnetic properties of chromium spinels CdCr2O4, HgCr2O4, and ZnCr2S4 are examined under various pressures. By applying pressure, Curie-Weiss temperatures are reduced indicating enhancement of antiferromagnetic interactions for all these three. For HgCr2O4, the field at which magnetic transition to a half moment state occurs raises almost proportional to the Curie-Weiss temperature as pressure increases. This implies nearest-neighbor antiferromagnetic interaction governs the field-induced transition. In addition, we found that the nature of magnetic transition of ZnCr2S4 at low temperature is drastically changed from spiral to antiferromagnetic when further nearest-neighbor interactions dominate the nearest neighbor ones, which are suppressed by pressure.