Disentangling Spin-Orbital Interactions - Extraction of far reaching exchange interactions in Cobalt Oxide*

Cobalt oxide (CoO) is a classical example where the magnetic superexchange is sufficiently strong such that a significant molecular field induced mixing between single-ion spin-orbit manifolds occurs. This makes the extraction of all multi-neighbor exchange interactions and comparison with first principles theory nearly intractable. We disentangle these interactions via neutron spectroscopic measurements on the diluted structural analog Co_0.03Mg_0.97O, thereby switching off the magnetic-order-induced molecular field that is the origin of this strong admixture. By considering the prevalent `dimer' response, we extract 7 exchange constants out to the fourth coordination shell with a dominant antiferromagnetic next nearest neighbor 180^o superexchange interaction, in agreement with Kanamori's original predictions. Other interactions appear in ferromagnetic/antiferromagnetic pairs, consistent with the Goodenough-Kanamori-Anderson rules and account for the Neel and Curie-Weiss temperatures obtained from thermodynamic measurements.