ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 304 (2006) e871–e873 www.elsevier.com/locate/jmmm A systematic study of giant magnetoimpedance of Cr-substituted Fe73.5ÀxCrxSi13.5B9Nb3Au1 (x ¼ 1, 2, 3, 4, 5) alloys N.D Thoa, N Chaua, S.C Yub,Ã, H.B Leec, N.D Thea, N.Q Hoaa 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 27 March 2006 Abstract The magnetoimpedance of Fe76.5ÀxCrxSi13.5B9Nb3Au1 (x ¼ 1, 2, 3, 4, 5) alloys has been measured to investigate the influence of crystallization process on the soft magnetic properties and magnetic anisotropy after thermal treatment Annealing performed at 540 and 550 1C for 30 in a vacuum indicated that ultrasoft magnetic properties of nanocomposite materials are obtained The magnetoimpedance ratio (MIR) and incremental permeability ratio (PR) measurements show that the maximum values of MIR and PR increase drastically in the heat-treated samples It means that the samples are softened by nanocrystallization The PR curves become narrower and sharper due to the decrease of anisotropy The MIR behavior related with the softness of magnetic properties of heattreated samples and the incremental PR strongly changes with external magnetic field r 2006 Elsevier B.V All rights reserved PACS: 75.50.Tt Keywords: Amorphous; Nanocrystalline; Magnetoimpedance; Permeability The magnetoimpedance (MI) effect is the change of the impedance experienced by an AC current flowing through a magnetic material when an external DC magnetic field is applied In recent years, many works on MI effect has stimulated considerable attention because of its technological applications in magnetic sensors and devices [1,2] The influence of various substituting alloying elements for Fe in FeCuNbSiB composition has been also widely investigated Recently, it has been reported that the partial substitution of Fe by Cr enhances the thermal stability of the amorphous alloy against nanocrystallization and lowers the Curie temperature of both, as-cast amorphous alloys and the residual amorphous phase in the nanocomposite [3] Other authors [4] have found that partially substituting Fe by Co in Fe73.5ÀxCoxSi13.5B9Nb3Cu1 alloys leads to the increasing of magnetic moment and the Curie point of precipitated crystalline phase More recently, it was reported that the Cr substitution in Fe-based 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.2006.03.021 amorphous alloys can also improve the mechanical or electrical properties without deteriorating the magnetic properties [5] In this study, we systematically investigate the giant magnetoimpedance (GMI) effect and the effect of Cr substitution for Fe on the soft magnetic properties of Fe73.5ÀxCrxSi13.5B9Nb3Au1 (x ¼ 1, 2, 3, 4, 5) alloys with different annealing temperatures Amorphous ribbons (7 mm wide, 16.8 mm thick) with nominal compositions Fe73.5ÀxCrxSi13.5B9Nb3Au1 (x ¼ 1, 2, 3, 4, 5) were 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 structures of both asquenched and annealed samples were examined by X-ray diffractometer (D5005, Bruker) For the MI measurement the external field applied by a solenoid can be swept through the entire cycle equally divided by 800 intervals from À300 to 300 Oe The frequency of MI measurement was ranging from to 10 MHz, and the AC current was fixed at 10 mA for all measurements ARTICLE IN PRESS e872 N.D Tho et al / Journal of Magnetism and Magnetic Materials 304 (2006) e871–e873 The X-ray diffraction analysis shows that the asquenched samples are amorphous The DSC measurements on the as-quenched ribbons were performed at a heating rate of 20 1C/min The results indicate that the devitrification of the investigated alloys takes place in two main stages The first one corresponds to the crystallization of the a-Fe(Si) soft magnetic phase, while the second is related with the appearance of boride-type phase (Fe3B or Fe2B) It was found that the addition of Cr produces a slight stabilization of the amorphous alloy against the crystallization, which can be detected as a linear increase of the peak temperature of the first exothermal The MIR as a function of the applied field, H, is defined with respect to the maximum applied field (H max ¼ 300 Oe in the present work) as DZ/Z(%) ¼ DZ/Z(Hmax) ¼ 1À|Z(H)/Z(Hmax)| The PR can be defined as PR(H) ¼ Dm/m (Hmax) ¼ 1À|m (H)/m (Hmax)| In amorphous ribbon materials, the MI effect is favored by the decrease of resistivity resulting from the crystallization process This strong reduction in the resistivity of the nanocrystalline ribbons reduces the skin depth and Fig The MIR versus the external field H measured at various frequencies in Fe72.5Cr4Si13.5B9Nb3Au1 alloy annealed at (a) 540 1C and (b) 550 1C increases the MIR of the samples [6] From the nearly zero MIR values in the as-quenched samples it is evident that the samples not show soft magnetic properties However, the maximum MIR value increases drastically in the samples annealed at 540 and 550 1C, indicating that the studied samples are ultra-softened by the crystallization, see example in Fig for sample with x ¼ From Fig 1, the frequency dependence of MIR in the samples shows a typical behavior where the shapes of the MIR curves are generally getting broader with the increment of the measuring frequency throughout all samples The rather high values and sharpness of the MIR curves in the annealed samples compared to the as-quenched ones indicate that structure changes such as the nanocrystallization have occurred in the annealed samples Among the investigated samples, the largest GMI effect was observed in the nanocrystalline sample containing 5% of Cr (x ¼ 5) and reached the value of 190% at frequency of MHz This is ideal for developing high-performance magnetic sensor The MI effect at high frequency can be explained in terms of an external field dependence of impedance as a result of the circumferential magnetization with respect to Fig The PR curves of as-quenched samples and the annealed samples Fe72.5ÀxCrxSi13.5B9Nb3Au1 (x ¼ 1, 2, 3, 4, 5) measured at MHz ARTICLE IN PRESS N.D Tho et al / Journal of Magnetism and Magnetic Materials 304 (2006) e871–e873 the current direction running through the sample and the skin effect of AC current At the frequency below MHz, the maximum value of GMI was relatively low (not shown here) and we suppose that it is due to the contribution of the induced magnetoinductive voltage to MI [6] When the frequency is in the range MHzpfp5 MHz, skin effect is dominant and larger GMI values were found The highest MI ratio is observed for nearly zero saturation magnetostriction after annealing at about 550 1C At this annealing temperature the effective magnetostriction is fully compensated so that the effective magnetoelastic anisotropy is minimum Moreover, transverse magnetic anisotropy is additionally induced on the ribbon’s surface during the annealing process This typical anisotropy controls the magnetization vector in the transverse domain and thus improves the magnetic response of the soft magnetic nanocrystalline material Therefore, the GMI effect is enhanced as a direct consequence of the higher mobility of domain walls as correlated with transverse permeability [6] The PR curves measured at MHz in the as-quenched samples and the samples annealed at 550 1C are plotted in Fig The large changes of the magnitude and field shape of the PR in the nanocrystalline alloys compared with as-quenched samples indicate that structural change such as crystallization has occurred To determine the structural changes of the samples, X-ray diffraction measurements have been performed The X-ray patterns e873 show the appearance of a-Fe(Si) peaks in the annealed samples The sharpness of PR curves after annealing implies the decrease of local anisotropy distribution by the nanocrystallization and indicates that the magnetization can be saturated under very low external field In general, the changes of the MI are closely related to the changes of the longitudinal incremental permeability Therefore, magnetic softness can be estimated from the MIR or the PR 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] R.S Beach, A.E Berkowitz, Appl Phys Lett 76 (1994) 6209 [2] R.L Sommer, C.L Chien, Appl Phys Lett 67 (1995) 3652 [3] J Fuzer, P Matta, P Kollar, P Sovak, M Konc, J Mag Mag Mater 158 (1996) 205 [4] N Chau, N.X Chien, N.Q Hoa, P.Q Niem, N.H Luong, N.D Tho, V.V Hiep, J Mag Mag Mater 282 (2004) 174 [5] N Bayri, H.I Adiguzel, S Atalay, P Sovak, Phys Status Solidi A 189 (2002) 805 [6] M Knobel, M.L Sanchez, C Gomez-Polo, P Marin, M Vazquez, J Appl Phys 79 (1996) 1646  ... nanocrystallization and indicates that the magnetization can be saturated under very low external field In general, the changes of the MI are closely related to the changes of the longitudinal... 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... DSC measurements on the as-quenched ribbons were performed at a heating rate of 20 1C/min The results indicate that the devitrification of the investigated alloys takes place in two main stages