Tunable magnetic interactions in mono-transition metal spinels MgCr₂X₄ (X= O,S,Se) and ACr₂S₄ (A=Ca,Sr,Ba)

The electronic and magnetic properties of the spinel materials MgCr₂X₄ (X=O,S,Se) and ACr₂S₄ (A=Ca,Sr,Ba), a special type of mono-transition metal spinel material, are investigated using first-principles density-functional theory calculations. The magnetic properties of these materials are understood by inspecting the competition between the 90-degree ferromagnetic superexchange interactions and the antiferromagnetic direct exchange interactions. We found that the direct exchange interaction is extremely sensitive to the distance between the Cr ions. The overall magnetic interaction can be transformed from antiferromagnetic at short Cr-Cr distances to ferromagnetic at larger Cr-Cr distances. Therefore, we can realize different magnetic states in the same material by controlling the Cr-Cr distance with external strain. Furthermore, the ferromagnetism is strengthened as tensile strain is employed, and a method to improve the Curie temperature, which is essential for spintronics devices, is proposed.