Materials Science and Engineering A 449–451 (2007) 364–367 The crystallization and properties of alloys with Fe partly substituted by Cr and Cu fully substituted by Au in Finemet N.Q Hoa a , N Chau a , S.-C Yu b,∗ , T.M Thang a , N.D The a , N.D Tho a a Center for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam b Department of Physics, Chungbuk National University, Cheongju 361-736, Republic of Korea Received 23 August 2005; received in revised form March 2006; accepted 20 March 2006 Abstract The structure, crystallization and magnetic properties of ribbons obtained by first making amorphous ribbons and then objecting them to a crystallization annealing have been published elsewhere by us previously In the present work the soft magnetic ribbons Fe73.5−x Crx Si13.5 B9 Nb3 Au1 (numbers indicate at.%, x = 1–5) are prepared by fast quenching on a single copper wheel X-ray diffraction patterns show that the as-cast samples are amorphous Differential scanning calorimetry analysis indicates that the crystallization temperature of the ␣-Fe(Si) phase is a little higher than that of pure Finemet With the same annealing conditions, the crystallization volume fraction decreases with increasing Cr content substituted for Fe Hysteresis loops of as-cast samples measured by Permagraph show that domain walls are pinned After appropriate annealing, the ultrasoft magnetic properties of nanocomposite materials are established The magnetic entropy change, | Sm |, of studied samples has been determined, and a giant magnetocaloric effect is found Our materials could be considered as promising magnetic refrigerants working at high temperatures (several hundreds ◦ C) © 2006 Elsevier B.V All rights reserved Keywords: Nanocrystalline alloy; Soft magnetic amorphous system; Magnetocaloric effect Introduction The excellent soft magnetic properties of Finemet Fe73.5 Si13.5 B9 Nb3 Cu1 (numbers indicate at.%) are directly correlated to its ultrafine structure composed of bcc-Fe rich crystals of nanometer size surrounded by a residual amorphous matrix [1,2] This magnetic softening is mainly ascribed to the averaging out of magnetocrystalline anisotropy via ferromagnetic interaction between two constituent magnetic phases, and is reinforced by the negligible magneto elastic contribution due to desirable reduction of both internal quenched stresses as well as effective magnetostriction Recent studies have been devoted to investigate the substitution effect of P for B [3], Ag for Cu [4], Au for Cu [5] in Finemet, Co for Fe [6,7], Cr for Fe [8,9] as well as of Mn for Fe [10] on the structural, crystallization and magnetic properties of these alloys When Cu substituted by Ag, there is very sharp exothermal peak exhibited in differential scanning calorimetry ∗ Corresponding author Tel.: +82 43 2612269; fax: +82 43 2756415 E-mail address: scyu@chungbuk.ac.kr (S.-C Yu) 0921-5093/$ – see front matter © 2006 Elsevier B.V All rights reserved doi:10.1016/j.msea.2006.03.139 (DSC) [4], also the crystallization temperature of ␣-Fe(Si) phase as well as crystallization activation energy showed to be higher than those of pure Finemet If Au is substituted for Cu, the crystallization temperature of the bcc-Fe(Si) phase is a little higher but crystallization activation energy is found to be less than that of pure Finemet If Co is substituted for Fe, saturation magnetization, Ms , Curie temperature of amorphous state, TC , showed to be higher than those of pure Finemet [6,7] but if Cr is substituted for Fe, TC is drastically decreased [8,9] The aim of this work is to present our study of the inclusion effect of Cr and Au on crystallization and properties of Finemettype alloys Experiment Amorphous ribbons with nominal compositions Fe73.5−x Crx Si13.5 B9 Nb3 Au1 (x = 1–5) have been prepared by rapid quenching on a copper wheel The ribbons are 14.3–17 m thick and mm wide The structure of samples was examined by Xray diffraction (XRD; D 5005, Bruker) with Cu K␣ radiation The evolution of crystallization process was studied on differential scanning calorimetry (SDT 2900, TA Instruments) The N.Q Hoa et al / Materials Science and Engineering A 449–451 (2007) 364–367 365 ribbons were annealed in vacuum Thermomagnetic curves were measured by a vibrating sample magnetometer (VSM-DMS 880, Digital Measurement Systems) Hysteresis loop parameters were carried out using automatic magnetic hysteresis graph (AMH-401A, Walker) Results and discussion The XRD patterns of as-cast samples showed one broad peak centered at approximately 2θ = 45◦ which appears to be typical of an amorphous phase DSC measurement on amorphous ribbons was performed in Ar atmosphere and Fig presents these results There are clearly two separated exothermal peaks Tp1 and Tp2 on DSC curves, ascribed to the precipitation of bcc-Fe(Si) and boride phase, respectively The first peak Tp1 is ranging from 564 to 579 ◦ C depending on the Cr content substituted for Fe, it is a little higher than that of pure Finemet [7] and of Finemet with Cu substituted by Au [5] From Fig we can see also that the second peak Tp2 exhibited with high sharpness relating to strong crystallization of boride phase These results are fully agree with those reported in [9] Fig presents the linear dependence of crystallization Fig DSC curves of as-cast ribbons Fe73.5−x Crx Si13.5 B9 Nb3 Au1 (x = 1–5) with heating rate of 20 K/min Fig Dependence of crystallization activation energies at the first peak Ea1 and the second peak Ea2 on Cr concentration Fig Thermomagnetic curves of the samples x = and x = measured in magnetic field of 50 Oe activation energy of ␣-Fe(Si) phase (Ea1 ) and of boride phase (Ea2 ) on Cr content Crystallization kinetics of samples could be studied by measurements of thermomagnetic curves at low field Fig shows the results for samples x = and x = We can see from this figure that at Curie temperature, TC , of amorphous phase, magnetization suddenly decreases then material is in superparamagnetic state up to temperature region starting to crystallize Magnetization curves measured along cooling cycle showing that the samples are of single phase in contrary with that of Ag substituted for Cu [4] Insert of this figure is the magnetization curves of studied samples The drastic decrease of TC and Ms could be explained by ferromagnetic dilution due to Cr substitution for Fe as also observed in [8,9] To promote the characteristic nanocrystalline structure, the samples were submitted to isothermal annealing in vacuum for 45 The DSC apparatus has been used to estimate the crystallized volume fraction (χf ) of ␣-Fe(Si) phase [8] and the results showed that at the same annealing conditions χf decreases with increasing Cr content substituted for Fe which confirming that Cr atoms enhances crystallization temperature as observed from Fig and it could be associated with atomic rearrangements within the amorphous and FeCr phase which give rise to the formation of bcc-Fe(Si) nanograins, which coexist with the FeCr nanograins as assumed in [11] Fig shows the XRD patterns of studied samples annealed at 550 ◦ C for ta = 45 The mean crystalline size (dg ) of ␣-Fe(Si) phase determined by XRD peak broadening analysis, using the Scherrer expression, is also indicated in Fig dg decreased from 11.7 nm for sample x = to 8.2 nm for sample x = showing one again that Cr hinders crystallization Fig shows hysteresis loops of as-cast and annealed ribbon x = (Ta = 550 ◦ C, ta = 45 min) measured at low magnetic field Similar to that of Fe73.5 Si13.5 B9 Nb3 Ag1 alloy [4] (but different for Finemet) the as-cast sample exhibits high rectangular coefficient of hysteresis loop We suppose as in [8] that the FeCr nanograins, present in sample from as-cast state could act as wall pinning centers during magnetization process As we see from this figure, after annealing, the soft magnetic properties 366 N.Q Hoa et al / Materials Science and Engineering A 449–451 (2007) 364–367 Table The magnetic characteristics (μi , μmax = initial and final permeability, Hc = coercive field) of as-cast and Fe73.5−x Crx Si13.5 B9 Nb3 Cu1 annealed samples (Ta = 550 ◦ C, ta = 45 min) Fig X-ray diffraction patterns of studied samples annealed at 45 550 ◦ C for of sample significantly improved The magnetic parameters of as-cast and annealed samples are collected in Table We can see from Table that the ultrasoft magnetic properties are established in the whole series of samples after the same annealing condition The calculations of magnetic entropy change, Sm , associated with the second-order phase transition were performed using isothermal magnetization curves measured around the respective Curie temperatures of amorphous-state samples and Fig shows the | Sm |(T) curves of studied samples It seems from Fig that quite high values of | Sm |max have been achieved in all samples and the studied ribbons could be considered as materials with giant magnetocaloric effect (GMCE) By doping of Cr, we can shift the peak of | Sm | to lower temperature similar to that of TC depending on x In conclusion, the Fe73.5−x Crx Si13.5 B9 Nb3 Au1 samples are prepared with amorphous structure While crystallization temperature of bcc-Fe(Si) phase increases with increasing of Cr content, the crystallization volume fraction, particle size of crystallites, Ms and TC decrease with increasing of Cr content substituted for Fe The doping of Cr leading to pinning of displacement of domains walls in as-cast samples The samples Sample μI μmax Hc (Oe) x=1 As-cast Annealed 832 11,800 16,200 28,000 0.188 0.035 x=2 As-cast Annealed 850 11,000 15,000 40,100 0.250 0.033 x=3 As-cast Annealed 950 23,000 11,900 50,500 0.200 0.028 x=4 As-cast Annealed 980 13,000 12,000 22,100 0.230 0.029 x=5 As-cast Annealed 856 12,200 16,000 19,000 0.200 0.031 Fig The | Sm |(T) curves of studied samples measured in 13.5 kOe magnetic field change exhibited a GMCE and they could be considered as promising magnetic refrigerants working at high temperatures Ultrasoft magnetic properties of studied nanocomposite materials are established Acknowledgements The authors express their sincere thanks to the National Research Program in Natural Science for financial support of the Project 811204 References Fig Hysteresis loops of as-cast and annealed ribbons of the sample x = [1] Y Yoshizawa, S Oguma, K Yamauchi, J Appl Phys 64 (1988) 6044 [2] G Herzer, Mater Sci Eng A133 (1999) [3] N Chau, N.H Luong, N.X Chien, P.Q Thanh, L Van Vu, Phys B 327 (2003) 241 [4] N Chau, N.Q Hoa, N.H Luong, J Magn Magn Mater 290–294 (2005) 1547 [5] N Chau, N.Q Hoa, N.D The, P.Q Niem, in press N.Q Hoa et al / Materials Science and Engineering A 449–451 (2007) 364–367 [6] J.S Blazquez, J.M Borrego, C.F Conde, A Conde, J.M Grenche, J Phys.: Condens Matter 15 (23) (2003) 3957 [7] 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 [8] P Marin, M Lopez, A Hernando, Y Iqbal, H.A Davies, M.R.J Gibbs, J Appl Phys 39 (2002) 374 367 [9] C Gomez-Polo, J.I Perez-Landazabal, V Recarte, IEEE Trans Magn 39 (2003) 3019 [10] C Gomez-Polo, J.I Perez-Landazabal, V Recarte, P.M Zelis, Y.F Li, M Vazquez, J Magn Magn Mater 290–291 (2005) 1517 [11] H.K Lachowicz, A Slawska-Winiewska, J Magn Magn Mater 133 (1994) 238 ... phase increases with increasing of Cr content, the crystallization volume fraction, particle size of crystallites, Ms and TC decrease with increasing of Cr content substituted for Fe The doping of. .. than that of pure Finemet [7] and of Finemet with Cu substituted by Au [5] From Fig we can see also that the second peak Tp2 exhibited with high sharpness relating to strong crystallization of boride... heating rate of 20 K/min Fig Dependence of crystallization activation energies at the first peak Ea1 and the second peak Ea2 on Cr concentration Fig Thermomagnetic curves of the samples x = and