ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 304 (2006) e195–e197 www.elsevier.com/locate/jmmm Annealing effect on soft magnetic properties and magnetoimpedance of Finemet Fe73.5Si13.5B9Nb3Au1 alloy N.D Thoa, N Chaua, S.C Yub,Ã, H.B Leec, N.D Thea, L.A Tuanc a Center for Materials Science, Vietnam National University, 334 NguyenTrai, Hanoi, Vietnam b Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea c Departement of Physics, Kongju National University, Kongju 314-701, South Korea Available online March 2006 Abstract Effect of annealing on the soft magnetic properties of Fe73.5Si13.5B9Nb3Au1 amorphous ribbon has been investigated by means of structure examination, magnetoimpedance ratio (MIR) and incremental permeability ratio (PR) spectra measured in the frequency range of 1–10 MHz at a fixed current of 10 mA X-ray diffraction analysis showed that the as-cast sample was amorphous and it became nanocrystalline under a proper heat treatment When annealing amorphous alloy at 530 1C for 30, 60, 90 min, soft magnetic properties have been improved drastically Among the samples investigated, the sample annealed at 530 1C for 90 showed the softest magnetic behavior The MIR and PR curves revealed the desirable changes in anisotropy field depending upon annealing r 2006 Elsevier B.V All rights reserved PACS: 75.50.Tt; 73.63.Bd Keywords: Amorphous magnetic meterials; Nanocrystalline materials; Magnetoimpedance; Permeability The investigation of ultra soft magnetic materials has been extensively carried out in the recent years Their extremely soft magnetic behaviors achieved upon suitable partial nanocrystallization by heat treatment Magnetic interaction among nanocrystalline grains via the intervening amorphous grain boundary phase results in improving soft magnetic properties Among these materials, the nanocrystalline Finemet alloy with composition of Fe73.5Si13.5B9Nb3Cu1 has found wide applications in generators and magnetic sensors [1–3] It was shown that the role of Cu and Nb played to maximize the density of crystal nuclei and to retard grain growth, respectively, leading to an ultrafine grain structure Consequently, ultrasoft magnetic properties of Finemet alloy are obtained [1,2] The influence of partial substitution of Fe with various alloying elements in Finemet alloy has been widely investigated It was found [4] that the partial substitution ÃCorresponding author Tel.: +82 43 2612269; fax: +82 43 2756415 E-mail address: scyu@chungbuk.ac.kr (S.C Yu) 0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2006.01.159 of Fe by Co leads to the increasing of magnetic moment and Curie point of the amorphous phase In the previous report, we have studied the crystallization in Finemet with Ag substituted for Cu [5] and showed that the crystallization of a-Fe(Si) phase is more stronger than that in pure Finemet In this paper, we present our study on the influence of annealing temperature and annealing time on the soft magnetic properties and magnetoimpedance effect of Finemet Fe73.5Si13.5B9Nb3Au1 alloy Amorphous ribbon (7 mm wide, 16.8 mm thick) with nominal composition Fe73.5Si13.5B9Nb3Au1 was obtained by rapid quenching from the melt spinning technique The crystallization behaviors of the samples were investigated by DSC (SDT-2960 TA Instruments) measurements The phase structure of both as-quenched and annealed samples was examined by X-ray diffractometer (D5005, Bruker) For MI measurement the external field applied by a solenoid can be swept through the entire cycle equally devided by 800 intervals from À300 to 300 Oe The frequency of MI measurement was ranging from to ARTICLE IN PRESS e196 N.D Tho et al / Journal of Magnetism and Magnetic Materials 304 (2006) e195–e197 10 MHz, and the AC current was fixed at 10 mA for all measurements The XRD pattern for as-quenched ribbon (Fig 1) exhibited only one broad peak around 2y ¼ 451, which is often known as diffuse halo, indicating that the ribbon is amorphous To find out a proper annealing regime for amorphous ribbon, we carried out DSC measurements (not shown here) The obtained results show that the substitution of Cu by Au does not change desirably the shape and peak position of DSC curves in comparison with those of Finemet Namely, there are two exothermal peaks, the first peak corresponds to the nanocrystallization of the a-Fe(Si) soft magnetic phase and the second one relates to the appearance of boride-type phases (Fe3B or Fe2B) and recrystallization phenomena It is well known in Finemet alloy, the roles of Cu to maximize the density of crystal nuclei of a-Fe(Si) phase and Cu-enriched regions are observed at the grain boundaries [6] We suppose that the role of Au in our studied sample is similar to that of Cu in Finemet The crystallization kinetics of ribbon can be observed by measurement of thermomagnetic curve (not shown here) It was found that there is single phase structure in the M(T) curve measured along cooling cycle whereas in case of Ag substituted for Cu in Finemet, the multiphase structure is observed [5] Based on the DSC measurements, the ribbon has been annealed in vacuum at temperature range T a ¼ 5202550 1C for different keeping time t ¼ 30, 60 and 90 to achieve the nanocrystalline material with a-Fe(Si) phase The structure of annealed samples has been determined by XRD, an example for sample optimum annealed at 530 1C in 90 is shown also in Fig The XRD results indicated that a-Fe(Si) phase is detected in all samples annealed at different regimes It is evident that, upon a proper heat treatment, the as-quenched amorphous state transformed into bcc nanograins with excellent soft magnetic properties (mmax has reached 99,000) Using Scherrer expression, the grain size of crystallites is determined and showed to be 10.8 nm GMI profiles were measured as a function of frequency and annealing conditions Fig presents the MIR of samples annealed at 530 1C for different keeping time 30, 60 and 90 min, respectively It was found that the GMI profiles show a single-peak behavior at low frequency (f p1 MHz) At frequency below MHz, the maximum value of GMI was relatively low due to the contribution of induced magneto-inductive voltage to MI When frequency in range MHzpf p5 MHz, the skin effect is dominant, a higher maximum of GMI value was found Among the annealed samples, the highest MIR is observed for sample annealed at 530 1C in 90 This behavior can be understood that the crystallization volume fraction of aFe(Si) phase increases with increasing of annealing keeping time Moreover, the optimal annealing leading to the lowest value of net magnetostriction of nanocomposite material, therefore soft magnetic properties are improved and MIR value is increased The PR curves measured in the frequency range for asquenched sample and sample annealed at 530 1C in 90 are plotted in Fig The large changes of magnitude and field shape of PR in the nanocrystalline alloy compared with those of as-quenched one indicate that the sample is ultra softened by crystallization and the structure change by such annealing has been occurred The sharpness of PR curves after annealing implies the decrease of local anisotropy distribution due to nanocrystallization and also indicates that the magnetization can be saturated under very low external field This behavior is helpful to examine the soft magnetic properties in nanocrystalline alloy In Fig X-ray diffraction pattern of as-quenched and annealed ribbon: T a ¼ 530 1C in 90 Fig The MIR versus the external field H measured in sample annealed at 530 1C and keeping time 30, 60, 90 min, respectively ARTICLE IN PRESS N.D Tho et al / Journal of Magnetism and Magnetic Materials 304 (2006) e195–e197 e197 general, the changes of MI are closely related to the change of longitudinal incremental permeability Therefore, the magnetic softness of material can be estimated from the MIR or PR profiles Research at Chungbuk National University was supported by the Korea Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam University Research at Center for Materials Science was supported by Vietnam National Fundamental Research Program, Grant no 811204 References [1] [2] [3] [4] Y Yoshizawa, S Oguma, K Yamauchi, J Appl Phys 64 (1988) 6044 P Marin, A Hernando, J Magn Magn Mater 215 (1995) 729 D.C Jiles, Acta Mater 51 (2003) 5907 N Chau, N.X Chien, N.Q Hoa, P.Q Niem, N.H Luong, N.D Tho, V.V Hiep, J Magn Magn Mater 282 (2004) 174 [5] N Chau, N.Q Hoa, N.H Luong, J Magn Magn Mater 290–191 (2005) 1547 [6] K Hono, D.H Ping, S Hirosawa, MRS Symp Proc 577 (1999) 507 Fig The PR curves of as-quenched sample (a) and annealed sample (b) versus frequency  ... peak corresponds to the nanocrystallization of the a-Fe(Si) soft magnetic phase and the second one relates to the appearance of boride-type phases (Fe3B or Fe2B) and recrystallization phenomena... large changes of magnitude and field shape of PR in the nanocrystalline alloy compared with those of as-quenched one indicate that the sample is ultra softened by crystallization and the structure... magnetization can be saturated under very low external field This behavior is helpful to examine the soft magnetic properties in nanocrystalline alloy In Fig X-ray diffraction pattern of as-quenched and