Magnetic entropy changes at early stages of nanocrystallization in amorphous Fe 90Zr 7B 3 ribbons

Abstract

Microstructure, revealed by transmission electron microscopy and conventional Mössbauer spectroscopy, magnetization versus magnetizing field induction and temperature and isothermal magnetic entropy changes in the as-quenched and subjected to annealing at T a1 = 723 K for 2 or 3 h and at T a2 = 743 K for 2.5 h of Fe 90Zr 7B 3 amorphous alloy are studied. In the as-quenched state the medium range ordered regions are observed. The annealing at T a1 leads to early stages of crystallization and nanograins with different diameter embedded in amorphous matrix are formed. At the Curie point of the amorphous phase they are magnetically decoupled and behave like superparamagnetic particles. The Curie point of the residual amorphous phase shifts towards higher temperature as compared to the as-quenched state due to the Invar like effect. The peak of the isothermal magnetic entropy changes appears at the Curie temperature of the main amorphous phase. Their values at the maximum applied field of 0.75 T equals to 0.32 J/kg K −1 in the as-quenched alloy and remain almost unchanged after early stages of nanocrystallization. After the annealing at T a2 the peak of the entropy changes distinctly decreases. Such behavior is ascribed to the biphasic character of the sample. The main amorphous phase and ordered one, which in some circumstances can be treated as an assembly of superparamagnetic particles, contribute to the total magnetic entropy changes.