Influence of ERTA on magnetocaloric properties of Sr doped BaFe_(12)O_(19) thin films

摘要

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.