Magnetic Properties of Annealed Amorphous Alloys (Fe-rich) Obtained by Gas Atomization Technique

Abstract

Soft magnetic Fe-based Fe-Si-B amorphous alloys have been widely used as magnetic components in high frequency transformers, inductors, and sensors due to their magnetic behavior [1], [2]. These materials are typically produced by the “melt-spinning” technique, involving the rapid solidification process. It is important to remark that the amorphous ribbons obtained by the melt-spinning technique were widely introduced as soft magnetic materials in the 70s. One of the ways to advance in the research field of magnetic materials involves the exploration of new routes to fabricate them. Thus, a novel technique of rapid solidification that we have successfully used to produce soft magnetic amorphous alloys is gas atomization [3], which produces the material in powder form. In this technology, it is possible to reach average cooling rates of up to $10 ^{6}\mathrm {K} /\mathrm {s}$, depending on processing conditions and the atomizing gas. The soft magnetic character of gas atomized powders with composition Fe 70 Si 18 B 12 was reported by the authors in [3]. It was showed that particles $\lt 10 \mu \mathrm {m}$ were amorphous and exhibited a low coercive field of around 7 Oe. Recently, the authors have produced a gas atomized powder of composition Fe 72.5 Si 12.5 B 15 that is fully amorphous for the whole particle size distribution, whose 90th percentile is $48.7 \mu \mathrm {m}$. Particles with a diameter $\lt 20 \mu \mathrm {m}$ exhibit a coercivity of 3.26 Oe. It is well known that a thermal treatment below the crystallization temperature leads to structural relaxation with a significant improvement of the soft magnetic character [4]. In this work, we report the effect of the thermal treatment (at 250, 350, and $450 ^{o}\mathrm {C}$ for 0 and 1 h) on the magnetic behavior of amorphous powder Fe 72.5 Si 12.5 B 15 with a particle size $\lt 20 \mu \mathrm {m}$. The annealing time of 0 h means that the sample was heated up to the annealing temperature and immediately cooled down without any holding. After such thermal treatments, the amorphous character of the annealed alloys was checked by X-ray diffraction technique. Fig. 1 shows the hysteresis loops of the annealed samples measured at room temperature, denoting the soft magnetic character associated with a very low value of coercive field. In fact, coercive field significantly decreases (see Fig. 2) from 3.26 Oe (as-atomized) to 0.44 Oe (annealed at $450 ^{o}\mathrm {C})$. There is an influence of the annealing time in this drop, with lower values of coercivity in the samples treated for 1 hour, except for the samples treated at $450 ^{o}\mathrm {C}$, whose coercivity is practically the same. The above mentioned behavior of the coercivity should be ascribed to the structural relaxation associated with thermal annealing without crystallization, decreasing the internal stresses and leading to a significant reduction of the magnetoelastic anisotropy.