Giant magnetic anisotropy of the bulk antiferromagnets IrMn and ${\text{IrMn}}_3$ from first principles

We perform an ab initio study of the ordered phases of IrMn and ${\text{IrMn}}_3$, the most widely used industrial antiferromagnets. Calculation of 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 Néel temperature. Our most challenging result is the extremely strong second-order anisotropy for ${\text{IrMn}}_3$ appearing in its frustrated triangular magnetic ground state, which is surprising since the ordered $L{1}_2$ 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 sublattices.

Figure 1

(Color online) Sketch of the ${\text{IrMn}}_3$ unit cell. Dark spheres represent three Mn atoms corresponding to the antiferromagnetic sublattices. The solid arrows indicate the local easy axes and the dotted arrows indicate the spin direction in the $T1$ ground state.

Figure 2

(Color online) Calculated change in energy of the $L{1}_2$ ${\text{IrMn}}_3$ system when rotating the triangular $T1$ spin structure around the (111) axis (circles) and the (110) axis (squares). The solid lines display appropriate fits to $K_{\text{eff}}{\text{sin}}^2\left(\phi \right)$ and the function in Eq. (4).

Figure 3

(Color online) Isotropic exchange interactions $J_{ij}$ between the Mn atoms in IrMn alloys calculated from the corresponding ground-state magnetic configurations by using the torque method (Ref. 17).

Figure 4

(Color online) Staggered magnetizations $M_s$ as a function of temperature obtained using Langevin dynamics over 20 ps with system sizes of $24000$ sites $\left(L{1}_2\right)$ and $70000$ sites $\left(L{1}_0\right)$ and using periodic boundary conditions.