Magnetic exchange interactions in SrMnO3

We calculate Heisenberg-type magnetic exchange interactions for SrMnO3 under isotropic volume expansion by 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 e(g) 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 e(g) and t(2g) contributions to the exchange interactions decay exponentially for large interatomic distances, the e(g) contribution remains surprisingly strong over relatively large distances along the crystal axes.