Flattening of isothermal magnetic entropy change versus temperature curve in amorphous Fe–Mn–Mo–B ribbons

Abstract

Microstructure, Curie temperature and isothermal magnetic entropy change of the amorphous Fe70Mn10Mo5B15 alloy in the as-quenched state and after annealing within the amorphous state at Ta1=723K and after the accumulative annealing at Ta1=723K and then at Ta2=753K for 0.5 h have been studied. The transmission Mössbauer spectra recorded at 300K are typical of a paramagnetic amorphous alloy and do not reveal distinct changes on annealing. The spectra in all cases are decomposed into a set of symmetric quadrupole doublets ascribed to the amorphous phase and a single line stemming from medium range ordered (MRO) regions which are paramagnetic at room temperature. The volume fraction of such MRO regions is small and increases only slightly on annealing. The Curie temperature of the amorphous phase decreases from 298 K in the as-quenched state to 268 K after the annealing at Ta1 and then increases to 282 K after the accumulative heat treatment at Ta1 and Ta2 due to invar effect. The isothermal magnetic entropy change of the alloy exhibits the “caret like” behavior with the peak values near the Curie points of the amorphous phase. The composites consisted of two adherent ribbons with different mass fraction (α) subjected to the annealing and made of two ribbons in the as-quenched state and after the heat treatment at Ta1 show the flat maxima of the magnetic entropy change in the temperature spans 14K and 34K for α=0.7 and α=0.5 in the former and latter composite, respectively. Parameter α denotes the mass fraction of the ribbon annealed at Ta1 in the composites. The flat maximum of the magnetic entropy change is obtained for the composite made of two ribbons with the same chemical composition but different thermal history. The refrigerant capacity (RC) of the amorphous Fe70Mn10Mo5B15 alloy is about 115 J/kg in the as-quenched state and decreases slightly on annealing. RC is enhanced for the composites in comparison with their components.