Antiferromagnetism in γ-Phase Mn–Ir Alloys

Abstract

Crystallographic and magnetic properties were studied in the disordered γ-phase Mn_1−xIr_x (0.05<x<0.35) alloys. In Mn rich alloys, tetragonal distortion with axial ratio c⁄a>1 occurs at a temperature below their Néel temperature, T_N. The tetragonal distortion temperature, T_d in the antiferromagnetic ordered state decreases rapidly with increasing Ir and the alloys with x>0.15 remain cubic down to low temperatures. On the other hand, T_N increases with alloying Ir at a rate of about 9 K/at%Ir up to 730 K at x=0.25 and it is extrapolated to 500±20 K for fcc Mn. It was concluded that the anomalous rise of T_N is not mainly attributed to atomic order nor to the enhancement of exchange interaction due to lattice expansion, but that the perturbation of band structure due to alloying leads to stronger magnetic interaction. A Cu₃Au type atomic ordering in the vicinity of x=0.25 was found to raise the T_N by about 200 K. The origin of tetragonal distortion is discussed.