Paramagnetic-to-antiferromagnetic phase transformation in sputter-deposited Ni-Mn thin films

Sputter-deposited, equiatomic Ni-Mn thin films were observed to possess a metastable, nanocrystalline, chemically disordered, fcc (Al) structure. Grain growth and a phase change to a chemically ordered, antiferromagnetic Ll0 structure were identified by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) experiments revealed exothermic signals that correspond to the grain growth and phase transformation reactions. The enthalpy of transformation for the Al to L10 phase change was calculated as -3.5 kJ/mol, which agrees with thermodynamic modeling. An activation energy of 139 kJ/mol was calculated for the phase transformation by the Kissinger method.