Abstract
Nanocrystalline BaCo0.90Mn0.10O3-δ (BCMO) was synthesised under normal atmospheric condition using sol-gel method. The X-ray diffraction patterns show the crystal structure changes from 2H- to 12H-type hexagonal phase depending on annealing temperature. The 2H-type hexagonal phase was observed for 900 °C annealed BCMO and 12H-type phase was formed for 1100 °C annealed BCMO sample. Temperature dependence of the zero-field-cooled and field cooled magnetization data shows the ferromagnetic to paramagnetic (FM-PM) ordering at ∼49 K in a 12H-type phase, whereas weak ferromagnetic ordering is observed in 2H-type phase. The saturation magnetization increases as phase changes from 2H to 12H type-hexagonal phase. The magnetocaloric effect (MCE) was studied near the magnetic ordering temperature for both type+s of phases (2H and 12H). The maximum magnetic entropy changes (|ΔSMmax|) were found to be 0.28 Jkg−1K−1 and 2.50 Jkg−1K−1 for a magnetic field change of 5 T for 2H- and 12H-type phases sample respectively. The critical exponent and theoretical analysis of the MCE using Landau theory of phase transition confirm that the contributions to the free energy in the presence of ferromagnetic clusters are strongly influencing the MCE by coupling with the order parameter around the ordering temperature for 12H-type hexagonal phase sample.
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