ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 310 (2007) 2483–2485 www.elsevier.com/locate/jmmm The crystallization, magnetic and magnetocaloric properties in Fe76.5ÀxNbxSi15.5B7Au1 ribbons N.Q Hoaa,b, D.T.H Gama, N Chaua, N.D Thea,b, S.-C Yub,Ã a Center for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Road, Hanoi, Vietnam b Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea Available online 27 November 2006 Abstract Fe76.5ÀxNbxSi15.5B7Au1 ribbons (x ¼ 0.0, 1.5, 3.0, 4.5) have been fabricated by rapid quenching technique The DSC measurements indicated that both first exothermal peak Tp1 (of a-Fe(Si) phase) and second peak Tp2 (of boride phase) as well as crystallization activation energy increase with increasing Nb content substituted, whereas saturation magnetization of samples decreases with x, due to ferromagnetic dilution Besides, Curie temperature of amorphous phase decreases with x, i.e Nb stabilizes amorphous structure of ribbons The investigation of magnetic entropy change of studied samples showed that it may lead to magnetocaloric effect around respective Curie temperature of amorphous phase r 2006 Elsevier B.V All rights reserved PACS: 75.50.Tt; 75.30.Sg; 75.50.Kj; 75.50.Bb Keywords: Soft magnetic amorphous system; Melt-spun; Magnetocaloric effect; Nanocrystalline matirials It was shown that the crystallization of Fe–Si–B amorphous alloys containing Nb and Cu causes the formation of nanoscale BCC structure [1] Cu and Nb atoms play a very important role in producing the nanocrystalline structure A small amount of Cu (with melting temperature Tm ¼ 1083 1C) to form a-Fe(Si) phase as crystallization nucleation, but Nb with high melting temperature (Tm ¼ 2468 1C) ascribed to hinder the grain growth, which are decisive factors to achieve the ultra-soft magnetic properties In the previous papers, we have studied the influence of Ag, Zn and Au in Finemet [2–4] on the properties of ribbons This article presents study on influence of Nb content substituted for Fe in Fe76.5Àx NbxSi15.5B7Au1 alloys on their structure, crystallization and properties Fe76.5ÀxNbxSi15.5B7Au1 ribbons (x ¼ 0.0, 1.5, 3.0, 4.5) have been fabricated by rapid quenching with 25 mm thickness and mm wide Structure of as-cast samples was checked by X-ray diffractometer D5005 and the results 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.2006.11.088 (Fig 1) show that beside sample x ¼ which is partly crystallized, the containing Nb alloys are fully amorphous The DSC measurements indicated that first exothermal peak corresponding to crystallization of a-Fe(Si) phase increased from 502 to 555 and 576 1C for ribbons x ¼ 1.5, 3.0, 4.5, respectively A similar tendency is found for second crystallization peak of boride phase (676, 716 and 722 1C, respectively) The DSC curves were measured in different heating rates from 10 to 50 1C/min and from the Kissinger plots, the crystallization activation energy for a-Fe(Si) phase, Ea1, as well as for boride phase, Ea2, has been determined and shown to be increased, both, with increasing Nb content existed in the samples The magnetic measurements showed that saturation magnetization of as-cast ribbons consequently reduced with higher value of x, namely, Ms ¼ 147 (x ¼ 1.5), 134 (x ¼ 3.0) and 112 emu/g (x ¼ 4.5) due to ferromagnetic dilution The M(T) curves of samples measured in low field of mT with heating and cooling cycle are presented in Fig We define DTcr ¼ TonsetÀTC as the stabilizing temperature ARTICLE IN PRESS 2484 N.Q Hoa et al / Journal of Magnetism and Magnetic Materials 310 (2007) 2483–2485 Fig X-ray diffraction patterns of as-cast ribbons Fe76.5ÀxNbxSi15.5 B7Au1 Fig Thermomagnetic curves of studied samples Fe76.5ÀxNbxSi15.5 B7Au1 in field of mT, (1) heating cycle, and (2) cooling cycle Table Several characteristics of Fe76.5ÀxNbxSi15.5B7Au1 ribbons Sample x ¼ 1.5 x ¼ 3.0 x ¼ 4.5 Ea1 (eV) Ea2 (eV) Ms (as-cast) (emu/g) Ms (annealed) (emu/g) TC (1C) DTcr (1C) D (nm) a (nm) jDSmjmax (J/kg K) 1.92 3.40 147 160 385 80 21.5 0.2840 2.4 2.50 4.62 134 142 345 220 10.8 0.2842 2.3 2.62 4.84 112 139 307 320 8.6 0.2845 2.0 region of amorphous phase, in which no phase transition is detected Table collects parameters of alloys including DTcr and obviously DTcr increases with increasing Nb content, namely DTcr ¼ 80 (x ¼ 1.5), 220 (x ¼ 3.0) and 320 1C (x ¼ 4.5), i.e Nb plays the role of stabilizing amorphous structure Fig Hysteresis loops of samples Fe76.5ÀxNbxSi15.5B7Au1 (x ¼ 1.5) The samples are annealed at appropriate temperatures and keeping time with crystallization volume fraction of around 82% After annealing, materials become nanocomposites with nanocrystallites a-Fe(Si) embedded in remaining amorphous matrix Once again from Table 1, we can see that Nb plays the role of limitation of grain growth, namely, particle size D ¼ 21.5 (x ¼ 1.5), 10.8 (x ¼ 3.0) and 8.6 nm (x ¼ 4.5) From Table 1, we also recognize that lattice parameter of annealed samples increases with increasing Nb content Yavari et al pointed out that possibility of some atomic percentages of niobium, which are solved in the nanocrystallites, might be taken into account [5] During the crystallization process, it is assumed that niobium is rejected into the amorphous boundary phase causing an inhomogeneous distribution, consequently, an accumulation of Nb at the boundary of the nanocrystalline grains occurred Hence, it seems that the higher value of the lattice constant was originated from the diffusion of Nb into the Fe–Si crystallites (see Table 1) Because of nanosize and appropriate volume fraction of a-Fe(Si) phase, the soft magnetic properties of annealed sample have been remarkably improved comparing with those of as-cast sample (Fig 3) Magnetic entropy change, |DSm|, as a function of temperature was evaluated by measuring a series of isothermal magnetization curves around Curie temperature of amorphous phase The results indicate that |DSm| established as high as around respective Curie temperature of amorphous phase (see Table 1) In conclusion, the structure, crystallization and magnetic properties of Fe76.5ÀxNbxSi15.5B7Au1 alloys have been examined The crystallization temperature, crystallization activation energy, stabilizing temperature region of amorphous phase as well as lattice parameter increase with increasing Nb content in the samples, whereas saturation magnetization, Curie temperature of amorphous phase as well as particle size decrease The role of Nb is discussed in detail ARTICLE IN PRESS N.Q Hoa et al / Journal of Magnetism and Magnetic Materials 310 (2007) 2483–2485 We acknowledge Korean Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University and Vietnam National Fundamental Research Program for Natural Sciences for financial support of this work References [1] Y Yoshizawa, S Oguma, K Yamauchi, J Appl Phys 64 (1988) 6044 [2] N Chau, N.Q Hoa, N.H Luong, J Magn Magn Mater 290 (2005) 1547 2485 [3] N Chau, N.Q Hoa, N.D The, L.V Vu, J Magn Magn Mater 303 (2006) e415 [4] N Chau, N.Q Hoa, N.D The, P.Q Niem, J Magn Magn Mater 304 (2006) e179 [5] A Reza Yavari, G Fish, S.K Das, L.A Davis, Mater Sci Eng A 181/A182 (1994) 1415 ... accumulation of Nb at the boundary of the nanocrystalline grains occurred Hence, it seems that the higher value of the lattice constant was originated from the diffusion of Nb into the Fe–Si crystallites... crystallization activation energy, stabilizing temperature region of amorphous phase as well as lattice parameter increase with increasing Nb content in the samples, whereas saturation magnetization,... Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University and Vietnam National Fundamental Research Program for Natural Sciences