Superparamagnetic behavior and magnetic entropy change in partially crystallized Fe–Mo–Cu–B alloy



The microstructure and some thermomagnetic properties of the amorphous Fe76Mo10Cu1B13 alloy in the as-quenched state and after annealing at 713 K for 0.5 h are studied. High-resolution electron microscopy reveals the existence of medium-range ordered (MRO) regions 1–2 nm in size within the amorphous phase that become the nuclei of the grains during primary crystallization at 713 K for 0.5 h. The α-Fe grains with average diameter of 6 nm and mean distance between them of 26 nm behave like an assembly of noninteracting, single-domain superparamagnetic particles. This is confirmed by the Mössbauer spectroscopy, magnetization versus temperature and magnetizing field measurements above the Curie point of the amorphous remainder. The partial crystallization of the Fe76Mo10Cu1B13 alloy is accompanied by giant magnetic hardening. The maximum of the magnetic entropy change (math formula) in the as-quenched state occurs at temperatures around the Curie point of the amorphous phase and equals to 0.88 J kg−1 K−1. After the annealing a threefold decrease of math formula is observed with the flat maximum in the temperature range of 280–310 K and this may be attributed to the magnetic hardening. The magnetic entropy change depends on both the temperature and the maximum of magnetizing field induction according to math formula. For the annealed samples the exponent n is close to 1 below the Curie point of the amorphous phase and does not reach 2 above these temperatures. Such behavior is ascribed to the α-Fe granular phase embedded in the amorphous remainder after annealing at 713 K for 0.5 h. In this case, the material can be treated at least as biphasic one.