Giant magnetic anisotropy of the bulk antiferromagnets IrMn and IrMn3

TL;DR

This study uses ab-initio calculations to reveal unexpectedly strong magnetic anisotropy in bulk antiferromagnets IrMn and IrMn3, with implications for their stability in sensor devices.

Contribution

It provides the first detailed theoretical analysis of magnetic anisotropy in IrMn and IrMn3, linking local symmetry breaking to large anisotropy effects.

Findings

IrMn3 exhibits extremely strong second order anisotropy.

The large anisotropy arises from local symmetry breaking despite cubic overall symmetry.

The effective spin model aligns well with experimental data.

Abstract

Theoretical predictions of the magnetic anisotropy of antiferromagnetic materials are demanding due to a lack of experimental techniques which are capable of a direct measurement of this quantity. At the same time it is highly significant due to the use of antiferromagnetic components in magneto-resistive sensor devices where the stability of the antiferromagnet is of upmost relevance. We perform an ab-initio study of the ordered phases of IrMn and IrMn3, the most widely used industrial antiferromagnets. Calculating the form and the strength of the magnetic anisotropy allows the construction of an effective spin model, which is tested against experimental measurements regarding the magnetic ground state and the Neel temperature. Our most important result is the extremely strong second order anisotropy for IrMn3 appearing in its frustrated triangular magnetic ground state, a surprising fact since the ordered L12 phase has a cubic symmetry. We explain this large anisotropy by the fact that cubic symmetry is locally broken for each of the three Mn sub-lattices.