Orbital coupling of noncollinear antiferromagnets to magnetic fields

The same symmetry considerations that imply a non-zero anomalous Hall effect in certain noncollinear antiferromagnets also imply non-zero total spin magnetization due to canting and finite orbital magnetization. It has been understood recently that the orbital magnetization of periodic crystals has a so-called "itinerant" contribution related to the Berry curvature of Bloch bands, which is not necessarily small, in contrast to the canting-induced total spin magnetization. We have explicitly calculated the orbital magnetization of several noncollinear antiferromagnets. In all cases we find that they are orders of magnitude larger than the total spin magnetization. Coupling between orbital magnetization and external magnetic fields is thus expected to be dominant in switching the direction of the magnetic order, and hence the anomalous Hall effect. Our calculation points to the important role of the transverse spin-orbital susceptibility in noncollinear antiferromagnets, compared to spin-spin and orbital-orbital susceptibilities. We use simple models as well as first-principles calculations to demonstrate a number of unique behaviors associated with magnetic field-induced order parameter switching in noncollinear antiferromagnets, and discuss their experimental implications.