Disentangling orbital and spin exchange interactions for Co$^{2+}$ on a rocksalt lattice

TL;DR

This study uses neutron spectroscopy to analyze and disentangle the complex spin and orbital exchange interactions in Co$^{2+}$ ions within a MgO lattice, revealing a hierarchy of magnetic interactions influenced by orbital degeneracy.

Contribution

It provides the first detailed extraction of multiple exchange constants for Co$^{2+}$ in a rocksalt lattice, highlighting the role of orbital degeneracy and GKA rules in magnetic interactions.

Findings

Dominant antiferromagnetic next nearest neighbor exchange

Presence of dual ferromagnetic and antiferromagnetic interactions

Hierarchy of exchange interactions influenced by orbital degeneracy

Abstract

Neutron spectroscopy was applied to study the magnetic interactions of orbitally degenerate Co$^{2+}$ on a host MgO rocksalt lattice where no long range spin or orbital order exists. The paramagnetic nature of the substituted monoxide Co$_{0.03}$Mg$_{0.97}$O allows for the disentanglement of spin-exchange and spin-orbit interactions. By considering the prevalent excitations from Co$^{2+}$ spin pairs, we extract 7 exchange constants out to the fourth coordination shell. An antiferromagnetic next nearest neighbor 180$^{\circ}$ exchange interaction is dominant, however dual ferromagnetic and antiferromagnetic interactions are observed for pairings with other pathways. These interactions can be understood in terms of a combination of orbital degeneracy in the $t_{2g}$ channel and the Goodenough-Kanamori-Anderson (GKA) rules. Our work suggests that such a hierarchy of exchange interactions exists in transition metal-based oxides with a $t_{2g}$ orbital degeneracy.