Anomalous Temperature Dependence of Coercivity at Low Temperature in $L1_{0}$ FePt Thin Films

L1₀ FePt thin films of varying composition were epitaxially grown at different substrate temperatures (400-600° C) on MgO (100) single crystal substrate by magnetron co-sputtering technique. Processing conditions were optimized to produce films with either (002) or a mixture of (002) and (200) crystallites. Effects of two different crystallographic orientations, i.e., (200) and (002) on the magnetic properties of L1₀ FePt were studied. Anomalous temperature variation of coercivity (H_c) (i.e., a decrease in H_c with decreasing temperature) was observed at low temperatures for the films having comparable intensities of (200) and (002) diffraction peaks in the X-ray diffraction patterns. We have identified the cause for this anomalous behavior in H_c. Strong inter-particle exchange interaction between (200) and (002) crystallites is shown to be responsible for the anomalous temperature variation in H_c.