Magnetic entropy change in amorphous and partially crystallized Fe–Mo–Cu–B alloy

Abstract

Microstructure of the amorphous Fe76Mo10Cu1B13 ribbons in the as-quenched state and after the annealing at 723K for 0.5h is studied by transmission electron microscopy and Mössbauer spectroscopy. In the as-cast state α-Fe medium range ordered (MRO) regions are revealed and become the nuclei of crystalline grains. Nanograins 6nm in the average diameter are observed in the sample subjected to the heat treatment. Mössbauer spectrum at 300K of the annealed sample is decomposed into three subspectra ascribed to the amorphous paramagnetic and ferromagnetic phases and interface. At room temperature nanograins do not contribute to the spectra in the form of a single sextet due to magnetic relaxations. At 77K the single sextet can be evidently introduced and its hyperfine parameters indicate the existence of the α-Fe(Mo) crystalline phase. The Curie point (TC) of the amorphous phase in the annealed samples shifts from 277K in the as-cast state to 320K after the annealing. The maximum of the magnetic entropy change (−ΔSM) in the as-quenched state occurs at temperature around TC of the amorphous phase and distinctly decreases after the annealing and shifts towards higher temperature but lower than TC of the amorphous remainder. −ΔSM in the superparamagnetic temperature range, i.e. above the Curie temperature of the amorphous phase obeys the phenomenological relation: −ΔSM=a(T)Bm/T+bB.m2/(T−Θ)2. The excellent |ΔSM|×Bm−2=f(Bm−1) linear dependences for both, as-quenched and annealed samples are observed.