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

Electronic and magnetic structures of γ-phase disordered (formula presented) alloys including (formula presented)-type ordered (formula presented) alloy have been investigated by the first-principles approach using the tight-binding (TB) linear muffin tin orbital (LMTO) method. For the (formula presented)-type ordered (formula presented) alloy, a triangular (formula presented) magnetic structure is considered to be stable, reflecting in a dip structure around the Fermi level in the density of states. For the γ-phase disordered (formula presented) alloy, on the other hand, the most stable structure is suggested to be the (formula presented) structure among the multiple-Q spin density wave (MSDW) structures. The Néel temperature is estimated to be about 735 K from the effective exchange constant (formula presented) in good agreement with the experimental value of about 730 K. With decreasing Ir concentration in the γ-phase disordered (formula presented) alloys, the transition from the (formula presented) to the (formula presented) structure takes place in the vicinity of (formula presented) under the constant lattice parameters with the axial ratio of (formula presented) This critical concentration x is close to the observed concentration at which the axial ratio changes from (formula presented) to (formula presented) 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.