First-principles study of the magnetic structures of ordered and disordered Mn-Ir alloys

Electronic and magnetic structures of gamma-phase disordered Mn100-xIrx alloys including L1(2)-type ordered Mn3Ir alloy have been investigated by the first-principles approach using the tight-binding (TB) linear muffin tin orbital (LMTO) method. For the L1(2)-type ordered Mn3Ir alloy, a triangular (T1) magnetic structure is considered to be stable, reflecting in a dip structure around the Fermi level in the density of states. For the gamma-phase disordered Mn3Ir alloy, on the other hand, the most stable structure is suggested to be the 3Q structure among the multiple-Q spin density wave (MSDW) structures. The Neel temperature is estimated to be about 735 K from the effective exchange constant J(0), in good agreement with the experimental value of about 730 K. With decreasing Ir concentration in the gamma-phase disordered Mn100-xIrx alloys, the transition from the 3Q to the 2Q structure takes place in the vicinity of x=13 under the constant lattice parameters with the axial ratio of c/a=1. This critical concentration x is close to the observed concentration at which the axial ratio changes from c/a=1 to c/a>1. However, it should be stressed that these two critical concentrations do not necessarily coincide with each other, that is, the critical concentration of the magnetic structure transition is lower than that of the lattice distortion. This theoretical expectation has been verified by the experimental structural and magnetic data.