Theory of magnetism-driven negative thermal expansion in inverse perovskite antiferromagnets

Magnetism-induced negative thermal expansion (NTE) observed in inverse perovskite antiferromagnets Mn ₃ AN(A=Zn,Ga, etc.) is theoretically studied by a classical spin model with competing bond-length-dependent exchange interactions. We numerically reproduce the crystal-volume expansion upon cooling triggered by a noncoplanar antiferromagnetic order and show that the expansion occurs so as to maximize an energy gain of the nearest-neighbor antiferromagnetic interactions. This mechanism is not specific to inverse perovskite magnets and might also be expected in magnets with other crystal structures. We propose other candidate crystal structures that might exhibit NTE through this mechanism.