Huge magnetic hardening ascribed to metastable crystallites during first stages of devitrification of amorphous FeSiBNbS
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M. Va´zquez, A. Asenjo, and J.M. Garcia Instituto de Magnetismo Aplicado and Instituto de Ciencia de Materiales, P.O. Box 155, 28230 Las Rozas (Madrid), Spain
G. Abrosimova and A. Aronin Institute of Solid State Physics, RAS 142432, Chernogolovka, Russia (Received 3 August 1999; accepted 19 June 2000)
A huge magnetic hardening (i.e., increase of coercivity from 5 A/m up to 4.4 kA/m) was reported for Fe75Si11B10 Nb3Sn alloy ribbons during their very first stage of crystallization from an initial amorphous state. In contrast, Fe78Si11B10Sn1 showed no such hardening, while a moderate hardening was observed for Fe76Si11B10 Nb3. This outstanding change was ascribed to the generation of metastable nanocrystallites that disappear upon heating at higher temperatures. A noticeable softening was then recovered (with a decrease of coercivity down to 0.8 kA/m) once the optimum homogeneous conventional nanocrystalline phase was achieved. Both structure and magnetic evolutions are followed by different techniques. I. INTRODUCTION
It is well known that devitrification of typically very soft ferromagnetic metallic glasses leads to their final magnetic hardening. This is a consequence of the overall structure transformation from a quasi-ideal disorder in the as-cast amorphous state into a heterogeneous structure consisting of several crystalline phases with each one exhibiting a peculiar magnetic hardness. Crystalline grains additionally exhibit an extended range in the size distribution. Both kinds of heterogeneities introduce magnetic defects which hinder the magnetization process; therefore, acting as pinning centers for domain wall displacements and inducing magnetic anisotropies (crystalline, elastic, or local stray fields) results in an overall magnetic hardening of the macroscopic magnetic material.1,2 However, as shown in systematic studies for some particular Fe base alloys with given additives (i.e., containing elements as Nb, Cu, Zr, etc.), it is possible to achieve a stable and homogeneously distributed nanocrystalline microstructure embedded in a residual amorphous phase upon thermal treatments at adequate conditions.3,4 Such stable and homogeneous nanostructure is outstandingly soft magnetically owing to the drastic reduction of internal stresses and, consequently, of magnetoelastic anisotropy. It is also due to the averaging out of the magnetocrystalline anisotropy that arises from the nanocrystallites as a consequence of the smallness of both the size and the distance between grains in comparison to the magnetic exchange correlation length.5,6 1936
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J. Mater. Res., Vol. 15, No. 9, Sep 2000 Downloaded: 18 Mar 2015
Nevertheless, at the very early stages of the devitrification process, a relatively modest magnetic hardening has been observed in some cases.7,8 The latter has been ascribed to the appearance of pinning centers created by the very first growing of fine grains together with the lack of magnetic coupling among neighboring grains now far away distances greater than the excha
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