Structural-magnetic phase transition in the theta-NiMn subjected to irradiation with fast neutrons

The neutron diffraction studies of the structural and magnetic phase transi tions in theta-NiMn were performed after irradiation with fast neutrons to a fluence of 2 x 10(24) m(-2) at 350 K. The structural phase transition fro m the ordered tetragonal phase of the CuAu I type to an fee disordered phas e with elements of short-range and long-range order of the initial phase is shown to occur already at a fluence of 1 x 10(24) m(-2). The doubled neutr on fluence did not lead to a further substantial atomic redistribution in t he alloy. The magnetic structure of the theta-NiMn was established to chang e under irradiation from a collinear to a noncollinear structure with a reo rientation of manganese magnetic moments from the (110) direction in the or dered tetragonal phase to the (1 1 I) direction in the disordered cubic pha se. The local magnetic moment of manganese atoms varies from 3.8 to 1.8 mu( B). The influence of the neutron irradiation on the ordered alloys is discu ssed in the framework of the statistical theory, from which it follows that , on irradiation with fast neutrons, the free energy of the alloy always in creases owing to a decrease in the number of pairs of neighboring unlike at oms, which is equivalent to atomic disordering up to a complete separation of the alloy into components.