Spin-lattice coupling in frustrated antiferromagnets

TL;DR

This paper reviews how spin-lattice coupling relieves frustration in pyrochlore antiferromagnets, predicts lattice distortions leading to chiral structures, and explains the emergence of spiral magnetic states in spinel oxides.

Contribution

It introduces a general theory of the collective spin-Jahn-Teller effect in pyrochlore lattices, including novel predictions of chiral lattice distortions and spiral magnetic states.

Findings

Chiral pyrochlore lattice distortions break inversion symmetry.

Long-period spiral states are induced by spin-orbit coupling.

Models using local phonon modes explain observed phenomena.

Abstract

We review the mechanism of spin-lattice coupling in relieving the geometrical frustration of pyrochlore antiferromagnets, in particular spinel oxides. The tetrahedral unit, which is the building block of the pyrochlore lattice, undergoes a spin-driven Jahn-Teller instability when lattice degrees of freedom are coupled to the antiferromagnetism. By restricting our considerations to distortions which preserve the translational symmetries of the lattice, we present a general theory of the collective spin-Jahn-Teller effect in the pyrochlore lattice. One of the predicted lattice distortions breaks the inversion symmetry and gives rise to a chiral pyrochlore lattice, in which frustrated bonds form helices with a definite handedness. The chirality is transferred to the spin system through spin-orbit coupling, resulting in a long-period spiral state, as observed in spinel CdCr2O4. We discuss explicit models of spin-lattice coupling using local phonon modes, and their applications in other frustrated magnets.