Magnetic structures in ordered and disordered $\gamma$-FeMn alloys: Ordering due to disorder

We present calculations of the magnetic properties of ordered and disordered $\gamma$-FeMn using a self-consistent tight-binding linearized muffin-tin orbital approach allowing for noncollinear spin structures. In an ordered compound ${\mathrm{Fe}}_{0.5}{\mathrm{Mn}}_{0.5}$ we find a ground state with parallel Fe moments and canted Mn moments forming an angle of $128°.$ If substitutional disorder is present in the system, a uniform collinear antiferromagnetic order builds up. A possible explanation of this result is proposed.