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.