Strong spin-orbit effects in double perovskites
One of the interests of our group is in systems where spin-orbit coupling plays a dominant role. This is not usual in d electron materials, but can happen when there are some heavy elements are the electronic kinetic energy or exchange is relatively weak. One class of materials where this seems fairly common is ordered double perovskites. In the double perovskites we consider (called "rock salt ordered double perovskites"), half the sites of a cubic lattice are occupied by magnetic ions, forming an fcc lattice. Because they are spread fairly far apart, these ions have relatively weak exchange and hence spin-orbit can dominate. Here the magnetic (call B') ion is often a heavy 4d or 5d atom, which also helps.
A number of these materials have been studied experimentally, with mysterious unidentified transitions seen, unconventional ferromagnetism detected, and in some cases spin liquid ground states postulated. But there has been very little theory.
^ TOPOur work:
In two papers, we developed basic microscopic spin Hamiltonians for these materials, for the cases where the B' ion has the d1 or d2 configuration. In these cases, orbital and spin degeneracy combine to generate effective total angular momentum spins, j=3/2 and j=2, respectively. The interactions amongst these spins, however, are quite unusual, and contain large multipolar couplings. As a result, the phases predicted by this model are radically different from weak spin-orbit systems.
Read our first paper and our second paper.
