Theory of successive transitions in vanadium spinels and order of orbitals and spins

TL;DR

This paper provides a theoretical analysis of the successive structural and magnetic transitions in vanadium spinel oxides, highlighting the interplay of spin, orbital, and lattice degrees of freedom on a frustrated pyrochlore lattice.

Contribution

It introduces a model combining Jahn-Teller coupling and spin-orbital superexchange to explain the thermodynamics of vanadium spinels, considering quantum and relativistic effects.

Findings

Competition between Jahn-Teller and superexchange interactions explains phase transitions.

Quantum fluctuations influence the stability of orbital and spin orders.

Relativistic spin-orbit coupling affects magnetic properties.

Abstract

We have theoretically studied successive transitions in vanadium spinel oxides with (t_2g)^2 electron configuration. These compounds show a structural transition at ~ 50K and an antiferromagnetic transition at ~ 40K. Since threefold t_2g orbitals of vanadium cations are occupied partially and vanadiums constitute a geometrically-frustrated pyrochlore lattice, the system provides a particular example to investigate the interplay among spin, orbital and lattice degrees of freedom on frustrated lattice. We examine the models with the Jahn-Teller coupling and/or the spin-orbital superexchange interaction, and conclude that keen competition between these two contributions explains the thermodynamics of vanadium spinels. Effects of quantum fluctuations as well as relativistic spin-orbit coupling are also discussed.