Magnetic anisotropy of the noncollinear antiferromagnet IrMn3

The magnetic anisotropy of antiferromagnets plays a crucial role in stabilizing the magnetization of many spintronic devices. In noncollinear antiferromagnets such as IrMn, the symmetry and temperature dependence of the effective anisotropy are poorly understood. Theoretical calculations and experimental measurements of the effective anisotropy constant for IrMn differ by two orders of magnitude, while the symmetry has been inferred as uniaxial in contradiction to the assumed relationship between crystallographic symmetry and temperature dependence of the anisotropy from the Callen-Callen law. In this Rapid Communication, we determine the effective anisotropy energy surface of L12-IrMn3 using an atomistic spin model and constrained Monte Carlo simulations. We find a unique cubiclike symmetry of the anisotropy not seen in ferromagnets and that metastable spin structures lower the overall energy barrier to a tenth of that estimated from simple geometrical considerations, removing the discrepancy between experiment and theory. The temperature scaling of the anisotropy energy barrier shows an exponent of 3.92, close to a uniaxial exponent of 3. Our results demonstrate the importance of noncollinear spin states on the thermal stability of antiferromagnets with consequences for the practical application of antiferromagnets in devices operating at elevated temperatures.