[(Fe 0.5 Co 0.5 ) 0.75 B 0.20 Si 0.05 ] 96 Nb 4 Metallic Glasses with Small Cu Additions
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FE-BASED bulk metallic glasses (BMGs) have a high application potential because of their unique soft magnetic properties, mechanical behaviour, and high corrosion resistance.[1–4] Also, they can be obtained directly in the final shape suitable for use as magnetic sensors, magnetic valves, magnetic clutches, etc. in different devices. Fe-based alloys able to form magnetic BMGs are of the type transition metal–metalloid and often contain five or more elements.[5] Usually, the metalloid content is around 20 at. pct. Multicomponent glassy alloys decrease the critical cooling rate of glass formation and successfully promote the BMG formation. Despite the fact that in amorphous alloys the magnetic crystalline anisotropy is not present, some of the properties such as coercivity (Hc), initial permeability (li), or saturation magnetization (Ms) may be affected by the shape or stress anisotropies induced upon casting, i.e., by the very rapid cooling of the molten alloy, which is one of the requirements for glass formation.[5] The unwanted induced anisotropies can be reduced by thermal annealing at elevated temperatures,[6] but below crystallization temperature Tx, or M. STOICA, Scientific Researcher, Institute for Complex Materials, and S. ROTH, Senior Researcher, Institute for Metallic Materials, are with IFW Dresden, D-01171 Dresden, Germany. Contact e-mail: [email protected] R. LI, formerly Humboldt Fellow, Institute for Complex Materials, IFW Dresden, is with the Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, 100191 Beijing, P.R. China. J. ECKERT, Professor and Director, Institute for Complex Materials, IFW Dresden, is also with the Institute of Materials Science, TU Dresden, D-10162 Dresden, Germany. G. VAUGHAN, Senior Researcher, is with the European Synchrotron Radiation Facility (ESRF), Genoble F-38042, France. A.R. YAVARI, Professor and Senior Researcher, is with SIMaPCNRS, INP Grenoble, St-Martin-d’He´res F38402, France. Manuscript submitted April 29, 2010. Article published online October 8, 2010 1476—VOLUME 42A, JUNE 2011
combination between compositional fine tuning and annealing.[6] The values of Hc depend mostly on surface and volume pinning of magnetic domain walls. It has been reported[7] that due to surface irregularities, Hc is proportional to the ratio of the surface roughness amplitude to the specimen thickness. This contribution to Hc is high in the case of amorphous ribbons and rather low for bulk samples because (1) the surface upon casting is very smooth and without scratches and (2) the thickness is significantly larger than that of rapidly quenched ribbons. The contribution to Hc due to volume pinning results from the presence of internal stress. The stress sources are supposed to have their origin in the partial instability of the free volume below the melting point.[8] This contribution is proportional to the product of saturation magnetostriction and the amplitude of stress fluctuations.[9] This Hc contribution should be low in the ca
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