Temperature dependence of the soft magnetic character of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 amorphous and nanocrystalline all
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Results on microstructure and coercivity of current-annealed Fe73.5Cu1Nb3Si13.5B9 amorphous alloy treated at different current densities (12–56 A/mm2) and duration (0.5–720 min) are presented. Saturation magnetization and coercivity dependencies with the current density of the nanocrystalline samples is explained by considering the presence of two phases: nanocrystals of Fe(Si) body-centered cubic (bcc) grains and the residual amorphous matrix. An increase in the magnetic hardness observed when the sample was heated by current densities, giving rise to an increase in the sample temperature above the Curie point of the residual amorphous matrix, could be ascribed to exchange and dipolar decoupling of the Fe(Si)-bcc grains.
Fe-based alloys with very fine microstructure, first investigated by Yoshizawa et al.,1 combine high-saturation magnetization with very small coercive field and are, therefore, especially interesting from the standpoint of their use as soft magnetic materials. These soft magnetic nanocrystalline alloys are characterized by the presence of two-coupled phases, a nanocrystalline one with typical grain size on the order of 10 nm, distributed in a soft magnetic amorphous phase. Similar to the amorphous materials, nanocrystalline alloys combine a large exchange correlation length with a reduced magnetic anisotropy. The relevant aspect of magnetic nanocrystals is related to the coincidence of the grain size scale (nanometers) with the typical or critical magnetic length, such as exchange length or exchange correlation length. Such scale coincidence gives rise to critical macroscopic properties. Softness Fe-rich nanocrystalline alloys is also due to a second complementary reason: the opposite magnetostriction of the two constituting phases. The low magnetic anisotropy compared with the exchange interactions has been successfully explained in the framework of the random anisotropy model,2,3 where the effect of the random distribution of the easy axes orientation gives rise to a remarkable decrease of the average structural anisotropy.4 Herzer explained the outstanding magnetic softness of these materials considering the ratio of the exchange correlation length (or domainwall thickness) L to the orientation fluctuation length of randomly distributed local easy axes, which in this case is roughly the average crystallite size D. For L Ⰷ D, the macroscopic structural anisotropy averages out, and the domain wall can move without becoming pinning.
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e-mail: [email protected] J. Mater. Res., Vol. 18, No. 5, May 2003
http://journals.cambridge.org
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The coercivity dependence on the grain size for Febased nanocrystals, obtained by partial devitrification of melt-spun amorphous alloys, has been found to behave as predicted by the approximation developed by Alben and co-workers,3 considering low values for the effective magnetic anisotropy. According to this approximation, there exists long-range ferromagnetic order with low anisotropy as concerns the micromagnetic scale, and therefore, th
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