Influence of ERTA on magnetocaloric properties of Sr doped BaFe12O19 thin films

The effect of electron beam rapid thermal annealing (ERTA) on the crystallographic, structural, magnetic and magnetocaloric properties of Sr doped M-type barium hexaferrite thin films (Ba1-xSrxFe12O19) deposited with set optimized parameters on silicon (001) substrates were investigated. Amorphous to crystalline phase transitions occurred when the as-deposited thin films were annealed, however crystallinity reduced as a function of Sr concentration. The as-deposited thin films showed uniform grain size and distribution with reduction in grain size as a function of Sr concentration, whereas annealed thin films showed increase in agglomerated structures as a function of Sr concentration. As-deposited thin films with x = 0, 0.05 and 0.1 showed weak ferromagnetic response. Magnetic hysteresis loops and magnetic force microscopy images indicate improved c-axis in-plane magnetization with reduced coercivity for Sr doped thin films and saturation magnetization for thin films subjected to ERTA. The Arrott plots in positive quadrant suggested a typical M-type ferrite behaviour with second-order ferromagnetic phase transitions around the Curie temperature. The entropy changes associated with magnetic phase transitions were determined using magnetization measurements performed in the range of 50-105 K for as-deposited and 65-125 K for ERTA samples under different magnetic fields. The calculated maximum relative cooling power (RCP) of the as-deposited and ERTA thin films are 1941 mJ/cc and 3065 mJ/cc for 50 kOe magnetic fields. This suggests that Ba1-xSrxFe12O19 with x = 1 is a promising candidate for micro and nano magnetic refrigeration systems.