ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 290–291 (2005) 1547–1550 www.elsevier.com/locate/jmmm The crystallization in Finemet with Cu substituted by Ag N ChauÃ, N.Q Hoa, N.H Luong Center for Materials Science, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam Available online December 2004 Abstract The amorphous ribbon Fe73.5Si13.5B9Nb3Ag1 has been prepared by rapid cooling on a single copper wheel The DSC curves show the very sharp peak at 593–617 1C depending on the heating rate from 10 1C/min to 50 1C/min, which is higher than that for pure Finemet (540–572 1C, respectively) The crystallization activation energy of a-Fe(Si) phase has been determined and shown to be E ¼ 4:09 eV; higher than that of pure Finemet E ẳ 3:25 eVị: After annealing at 550 1C for 10 min, the crystallization volume fraction of a-Fe(Si) phase reaches the value of 78.4% with average grain size of 32 nm The M(T) measurement along cooling cycle exhibits clearly two-step curve corresponding to the multiphase structure of the ribbon The annealing of ribbon leads to the occurrence of nanocomposite state and improvement of soft magnetic properties of the material r 2004 Elsevier B.V All rights reserved PACS: 75.50.Tt Keywords: Crystallization kinetics; Grain size; Nanocrystalline materials Recently, a great effort has been devoted to the development of a new type of high-functional highstrength materials by utilizing the formation of crystallization-induced nanostructure It has been found that the crystallization of Fe–Si–B amorphous alloys containing Nb and Cu causes the formation of nanoscale BCC structure and the BCC alloys exhibit good soft magnetic properties of 1.2X1.4 T for saturation magnetic fluxdensity (Bs) and high effective permeability (me) [1] The excellent soft magnetic properties of these alloys have been explained by Herzer [2] on the randomanisotropy model (RAM) first proposed for amorphous, soft magnetic alloys by Alben et al [3] This model relates the exchange correlation length (or domain wall thickness), Lex, to the fluctuation length of the local easy ÃCorresponding author Tel.: +84 5582216; +84 8589496 E-mail address: chau@cms.edu.vn (N Chau) fax: axis orientation which, in the case of the nanocomposite alloys, approximates to the average grain diameter, dg When Lexbdg the macroscopic anisotropy average out to give an effective anisotropy /KS which is approximately three orders of magnitude smaller In addition, it is considered that the negative saturation magnetostriction, ls ; of the high Si nanocrystalline phase is closely counterbalanced in volume terms by larger positive ls for the glassy matrix so that the net ls is very small These two factors contribute to the low coercivity ($0.01 Oe) and high permeability ($105 at 50 Hz) for Finemet It was shown that Cu and Nb play a very important role in producing the nanocrystalline structure A small amount of Cu facilitates to form a-Fe(Si) phase as crystallization nucleation but Nb with high melting temperature is ascribed to hinder the grain growth, which are decisive factors to achieve the excellent soft magnetic properties Many attempts have been made to improve properties of Finemet 0304-8853/$ - see front matter r 2004 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2004.11.245 ARTICLE IN PRESS 1548 N Chau et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1547–1550 Fe73.5Si13.5B9Nb3Cu1 by investigating the effect of partial substitution or addition of alloying elements for Fe (e.g Co [4]), for B (e.g P [5]) In this work, we present our study on the influence of Ag substituted for Cu in Finemet on the crystallization and properties of alloy Fe73.5Si13.5B9Nb3Ag1 The amorphous ribbon Fe73.5Si13.5B9Nb3Ag1 has been prepared by rapid cooling on a single copper wheel The ribbon had a width of mm and thickness of approximately 25 mm The structure of the as-cast and annealed ribbons was studied using X-ray diffractometer Bruker D5005 with Cu-Ka radiation The crystallization phase transitions were examined in Differential Scanning Calorimetry (DSC) SDT 2960—TA Instruments The microstructure of annealed ribbons was studied in a 5410 LV Jeol Scanning Electron Microscope (SEM) The magnetic properties of ribbon were measured using a vibrating sample magnetometer DMS—880 Digital Measurement Systems and permagraph AMH—410A Walker The X-ray diffraction analysis shows that the as-cast sample is amorphous The DSC measurements on as-cast ribbons were performed with a heating rate from 10 to 50 1C/min (Fig 1) It is clear that a very sharp exothermal peak of a-Fe(Si) phase occurs in the range of T p1 ¼ 593617 C; around 50 1C higher than that of pure Finemet [4] Based on the DSC curves in Fig we can estimate the crystallization activation energy according to the Kissinger relation [6]: ! b E Ln ẳ ỵ const; (1) kB T p T 2p where b is the heating rate, T p is temperature at exothermal peak, kB is the Boltzmann constant and E is the crystallization activation energy Fig presents the linear dependence of Ln(b/T2p) on 1/Tp From this figure, the value of E has been determined and shown to be 4.09 eV, higher than that of pure Finemet (3.25 eV) [4] Usually in the DSC curves for Finemet there are the second exothermal peaks corresponding to the crystallization of boride phase In the ribbon with Ag substituted for Cu, we only see a very fuzzy maximum after T p1 (Fig 1) From Figs and we can conclude that in the composition Fe73.5Si13.5B9Nb3Ag1, the crystallization of a-Fe(Si) phase occurs more strongly than that in pure Finemet and with higher priority among the another phases It is not easy to find out the optimum annealing regime in order to obtain high-performance soft magnetic properties of studied nanocomposite The DSC apparatus has been used for estimating the crystallization volume fraction and Fig shows the DSC scan of as-cast as well as annealed ribbon with Ta ¼ 585 1C, 30 Using Leu and Chin expression [7], we derived a crystallization volume fraction of a-Fe(Si) phase to be 78.4% for ribbon annealed at Ta ¼ 550 1C for 10 min, and 83.6% for ribbon annealed at Ta ¼ 585 1C for 30 (Fig 3) It is well known that the appropriate crystallization volume fraction leads to the compensation of magnetostriction of a-Fe(Si) phase and of the remaining amorphous one with different signs of magnetostriction Particle size of the grains formed in annealed ribbon (Ta ¼ 550 1C, 10 min) has been determined by SEM picture (Fig 4) to be of 32 nm, less than the ferromagnetic exchange length in Finemet [8] (35 nm) The determination of grain sizes using SEM picture and X-ray analysis based on Scherrer expression [9] gives similar results Experiments showed that the Fig DSC curves of ribbon Fe73.5Si13.5B9Nb3Ag1 with different heating rates Fig Kissinger plot for determination of crystallization activation energy of a-Fe(Si) phase ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1547–1550 1549 Different from pure Finemet, here hysteresis loop of as-cast ribbon has quite a high rectangular coefficient of more than 90% (Fig 6), showing the pinning of displacement of domain walls due to high gradient of domain wall along its displacement direction Perhaps the large and heavy silver atoms located at the grain boundary contribute to high gradient of mechanical strain as well as high gradient of area of domain walls The annealing of ribbon leads to the occurrence of the nanocomposite state and improvement of, but not very, high-performance soft magnetic properties of material (Fig and Table 1) In conclusion, Fe73.5Si13.5B9Nb3Ag1 ribbon has been prepared with an amorphous structure The crystallization of a-Fe(Si) phase has occurred strongly at higher temperature and higher crystallization activation energy than those of original Finemet M(T) curve Fig DSC patterns for as-cast and annealed ribbon Ta ¼ 585 1C, 30 Fig Thermomagnetic curves of the ribbon (1: heating cycle, 2: cooling cycle) Fig SEM micrograph of annealed ribbon: Ta ¼ 550 1C, 10 crystallization volume fraction as well as the nanosize of crystallites are more sensitive on the annealing temperature and keeping time than those of pure Finemet The crystallization kinetics of ribbon can be observed by measurement of thermomagnetic curve The thermomagnetic measurement at low field (50 Oe) shows that the M(T) curve has the dramatical decrease at 310 1C corresponding to the Curie temperature of the amorphous phase and the ribbon is in the paramagnetic state up to 610 1C, then magnetization starts to increase due to the crystallization (Fig 5) The M(T) measurement along cooling cycle exhibits clearly a two-step curve corresponding to multiphase structure of the ribbon It is suggested that in this cycle, beside a-Fe(Si) phase, a large amount of boride phase also coexists Fig Hysteresis loops of the ribbon ARTICLE IN PRESS 1550 N Chau et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1547–1550 References Table The magnetic characteristics of the studied sample Sample mo mmax Hc (Oe) As-cast Annealed Ta ¼ 585 1C, 30 8300 23000 16000 31000 0.20 0.12 measured along the cooling cycle shows that there is the multiphase structure in the sample The substitution of Ag for Cu in Finemet leads to the pinning of displacement of domain wall The authors are grateful to the Vietnam National Fundamental Research Program for financial support of the Project 811204 [1] Y Yoshizawa, S Oguma, K Yamauchi, J Appl Phys 64 (1988) 6044 [2] G Herzer, IEEE Trans Magn MAG26 (1990) 1347 [3] R Alben, J.J Becker, M.C Chi, J.Appl Phys 49 (1978) 1653 [4] 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.H Luong, N.X Chien, P.Q Thanh, L.V Vu, Physica B 327 (2003) 241 [6] H.E Kissinger, Anal Chem 29 (1957) 1702 [7] M.S Leu, T.S Chin, MRS Symp Proc 577 (1999) 557 [8] G Herzer, J Magn Magn Mater 112 (1992) 258 [9] B.D Cullity, Element of X-ray Diffraction, 2nd Ed, Addison-Wesley Publishing Company, Inc., Reading, MA, 1978, p 102 ... pure Finemet (3.25 eV) [4] Usually in the DSC curves for Finemet there are the second exothermal peaks corresponding to the crystallization of boride phase In the ribbon with Ag substituted for Cu, ... that the M(T) curve has the dramatical decrease at 310 1C corresponding to the Curie temperature of the amorphous phase and the ribbon is in the paramagnetic state up to 610 1C, then magnetization... along the cooling cycle shows that there is the multiphase structure in the sample The substitution of Ag for Cu in Finemet leads to the pinning of displacement of domain wall The authors are grateful