Magnetic exchange interactions in SrMnO$_3$

Abstract

We calculate Heisenberg-type magnetic exchange interactions for SrMnO3 under isotropic volume expansion using an approach that is based on total energy variations due to infinitesimal spin rotations around a given reference state. Our total energy calculations using density functional theory (DFT) indicate a transition from antiferromagnetic to ferromagnetic coupling for increasing interatomic distances, corresponding to a sign change of the nearest neighbor exchange interaction. This sign change cannot easily be understood from a standard superexchange mechanism. Furthermore, the exchange interaction strongly depends on the corresponding reference state. This "non-Heisenberg" behavior increases with increasing volume and is also confirmed through noncollinear DFT calculations. An orbital-and energy-resolved decomposition of the exchange coupling suggests that an increased partial occupancy of eg orbitals near the Fermi level is crucial both for the sign change and the non-Heisenberg behavior of the nearest neighbor interaction. Furthermore, even though both eg and t2g contributions to the exchange interactions decay exponentially for large inter-atomic distances, the eg contribution remains surprisingly strong over relatively large distances along the crystal axes.