Journal of Magnetism and Magnetic Materials 307 (2006) 178–185 Soft magnetic properties and giant magneto-impedance effect of Fe 73.5x Cr x Si 13.5 B 9 Nb 3 Au 1 (x ¼ 1–5) alloys Anh-Tuan Le a,b , Chong-Oh Kim a , Nguyen Chau c , Nguyen Duy Cuong b , Nguyen Duc Tho c , Nguyen Quang Hoa c , Heebok Lee d,Ã a Research Center for Advanced Magnetic Materials (ReCAMM), Chungnam National University, Taejon 305-764, Korea b Department of Materials Engineering, Chungnam National University, Taejon 305-764, Korea c Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam d Department of Physics Education, Kongju National University, Kongju 314-701, Korea Received 20 December 2005; received in revised form 24 March 2006 Available online 2 May 2006 Abstract In this paper, the effect of microstructural and surface morphological developments on the soft magnetic properties and giant magneto-impedance (GMI) effect of Fe 73.5x Cr x Si 13.5 B 9 Nb 3 Au 1 (x ¼ 1, 2, 3, 4, 5) alloys was investigated. It was found that the Cr addition causes slight decrease in the mean grain size of a-Fe(Si) grains. AFM results indicated a large variation of surface morphology of density and size of protrusions along the ribbon plane due to structural changes caused by thermal treatments with increasing Cr content. Ultrasoft magnetic properties such as the increase of magnetic permeability and the decrease of coercivity were observed in the samples annealed at 540 1C for 30 min. Accordingly, the GMI effect was also observed in the annealed samples. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.50.Tt; 75.75.+a Keywords: AFM; Surface topography; Microstructure; Magnetic properties; Nanocrystalline alloys 1. Introduction Recently, the nanocrystalline soft magnetic alloys have attracted much research interest due to their fundamental scientific interest and their potential applications [1]. A special attention is paid on the nanocrystalline Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 alloy, named as FINEMET. The discovery of this alloy established a new approach to develop soft magnetic materials with high magnetic flux density (B s ) and high effective permeability (m e ) in which the magneto- crystalline anisotropy can be reduced by refining the grain size in less than a few tens of nanometers [2]. The optimum nanocrystallized state is obtained by isothermal annealing of the as-quenched amorphous ribbon above its dynamic crystallization temperature, typically in the range from 773 to 818 K for 1 h [1–3]. After such a heat treatment the material shows a uniform structure of ultrafine crystallites (BCC FeSi) with average diameter of 10–20 nm embedded in the residual amorphous matrix. Structural changes, induced by annealing, modify the macroscopic magnetic behavior of the constituent material. Accordingly, the microstructure dependence of the magnetic properties in nanocrystalline magnetic materials was explained by the random anisotropy model, which is proposed for amorphous ferromagnets by Alben et al. [4] and developed by Herzer [5]. According to this model, when the grain size is less than the ferromagnetic exchange length (L ex ), the exchange interaction dominates the anisotropy energy and forces the magnetization vectors to be parallel to each other over several grains. Under this condition, the effective anisotropy is averaged out and thus it leads to ultrasoft magnetic properties, i.e., low coercivity and very high permeability. The ultrasoft magnetic properties observed at room temperature are due to nanostructure effects which result in magnetic coupling between the grains through the ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.03.066 Ã Corresponding author. Tel: +82 41 850 8276; fax: +82 41 850 8271. E-mail address: tuanitims@yahoo.com.au (H. Lee).