  Magnetism-induced negative thermal expansion (NTE) observed in inverse
perovskite antiferromagnets Mn3AN (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 non-coplanar 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.
