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

We present calculations of the magnetic properties of ordered and disordere d gamma-FeMn using a self-consistent tight-binding linearized muffin-tin or bital approach allowing for noncollinear spin structures. In an ordered com pound Fe0.5Mn0.5 we find a ground state with parallel Fe moments and canted Mn moments forming an angle of 128 degrees. If substitutional disorder is present in the system, a uniform collinear antiferromagnetic order builds u p. A possible explanation of this result is proposed.