The magnetic and structural properties of highly ordered (S ~ 0.82) epitaxial Fe50-xMnxPt50 thin films were investigated. We report the change in the magnetic properties of Mn doped FePt epitaxial thin films. This study differs from the earlier experimental studies on Mn doped FePt based alloys. Ordered L10 Fe50-xMnxPt50 (x=0, 6, 9, 12 and 15) thin films with a constant thickness of 45 nm were prepared by co-sputtering Fe50Pt50 and Mn50Pt50 on to MgO (100) single crystal substrate. We find a significant increase in the coercivity for Fe-Mn-Pt thin films. We have shown that this increase in magnetic properties coincide with the tetragonal distortion, while the recent first principles study of Mn doped FePt showed the sub lattice ordering of ferromagnetically aligned Mn atoms would lead to increase in magnetic properties in the FeMnPt ternary alloy system with fixed Pt concentration. At x=12 the coercivity has increased by 46.4 % when compared to Fe50Pt50. The increase in magnetic properties in Fe50-xMnxPt50 is due to the tetragonal distortion as experimental c/a ratio is larger than the expected c/a ratio for ferromagnetically ordered Mn atoms in the sublattice at the concentration x=12. Thus we show that high temperature deposition and high temperature annealing is one of the methods to achieve large coercivity in Mn doped FePt as it leads to tetragonal distortion.
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