ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 304 (2006) 36–40 www.elsevier.com/locate/jmmm The existence of giant magnetocaloric effect and laminar structure in Fe73.5ÀxCrxSi13.5B9Nb3Cu1 N Chaua,Ã, P.Q Thanhb, N.Q Hoaa, N.D Thea a Center for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam b Department of Physics, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam Available online March 2006 Abstract Amorphous soft magnetic ribbons Fe73.5ÀxCrxSi13.5B9Nb3Cu1 (x ¼ 1–5) have been fabricated by rapid quenching on a single copper wheel The differential scanning calorimetry (DSC) patterns showed that the crystallization temperature of a-Fe(Si) phase is ranging from 542 to 569 1C, a little higher than that of pure Finemet (x ¼ 0) With the same annealing regime, the crystallization volume fraction as well as the particle size of a-Fe(Si) crystallites decreased with increasing Cr amount substituted for Fe in studied samples Especially, the interesting fact is that the laminar structure of heat-treated ribbons on the surface contacted to copper wheel in the fabricating process has been firstly discovered and explained to be related to the existence of Cr in studied samples The hysteresis loop measurement indicated that there is the pinning of displacement of domain walls The giant magnetocaloric effect (GMCE) has been found in amorphous state of the samples After annealing, the soft magnetic properties of investigated nanocomposite materials are desirably improved r 2006 Elsevier B.V All rights reserved PACS: 75.50.Tt; 75.30.Sg; 71.55.Jv; 73.63.Bd Keywords: Soft magnetic amorphous system; Nanocrystalline materials; Magnetocaloric effect; Magnetic properties of nanostructures Introduction A great interest has been paid to nanocrystalline Finemet Fe73.5Si13.5B9Nb3Cu1 [1] With two existing ordered magnetic phases, the material almost vanishes effective magnetostriction and magnetocrystalline anisotropy The local anisotropy is averaged out over several grains, which reduces effective anisotropy significantly owing to the grain size smaller than the ferromagnetic exchange interaction lengths [2], Finemet has excellent soft magnetic properties In the previous papers, we have examined the influence of P substituted for B [3], Ag for Cu [4] and Co for Fe [5] in Finemet on the crystallization and properties of materials The partial substitution of Fe by Al has been reported [6–8] The effect of inclusion of Cr in Finemet has been investigated [9] and the authors showed that there is the enrichment in Cr of residual amorphous matrix in the ÃCorresponding author Tel.: +84 5582216; fax: +84 8589496 E-mail address: chau@cms.edu.vn (N Chau) 0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2006.02.006 FeCr samples Gomez-Polo et al studied Fe73.5ÀxCrx Si13.5B9Nb3Cu1 [10] and reported that the inclusion of Cr atoms promotes a magnetic hardening at room temperature for the highest Cr concentration x ¼ 10 associated to a drastic decrease of the Curie point of residual amorphous phase in alloy Recently, magnetic properties of Mn-doped Finemet nanocrystalline alloy have been examined [11] In this paper, we present our study of influence of partial substitution of Fe by Cr on the structure and properties of Finemet-type alloys and for the first time we also show that there is giant magnetocaloric effect (GMCE) as well as laminar structure occurring in these alloys Experiment Alloy ribbons (6–8 mm wide, 20 mm thick) with nominal compositions Fe73.5ÀxCrxSi13.5B9Nb3Cu1 (x ¼ 1–5) were prepared by rapid quenching on a single copper wheel Structure of the ribbons was studied using a X-ray diffractometer Bruker D5005 with Cu–Ka radiation The ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 304 (2006) 36–40 evolution of crystallization process was monitored by differential scanning calorimetry (DSC) SDT 2960 TA Instruments Microstructure of annealed samples was studied in a 5410 LV Jeol scanning electron microscope (SEM) The magnetic properties of ribbons were measured using a vibrating sample magnetometer (VSM) DMS 880 and Permagraph AMH 401 A Walker 37 587°C 750°C 50°C/min Heat Flow (a.u.) 579°C 723°C 40°C/min 576°C 739°C 562°C 30°C/min 732°C 557°C Results and discussion 721°C 10°C/min 400 200 800 600 1000 T (°C) (a) -9.4 -9.6 -9.8 -10.0 ln (/βTp2) The XRD patterns of as-cast samples exhibit only one broad peak around 2y ¼ 451, showing that the as-cast ribbons are amorphous DSC measurements on amorphous ribbons were performed with heating rate of 20 1C/min in Ar atmosphere and Fig presents the results There are clearly two separated exothermal peaks Tp1 and Tp2 on these curves, ascribed to the precipitation of bcc-Fe(Si) and boride phase, respectively Tp1 is ranging from 542 to 569 1C (depending on Cr content), a little higher than that of pure Finemet (x ¼ 0) [5] We suppose that it is related to the higher melting temperature of Cr (1857 1C) compared with that of Fe (1536 1C) The DSC measurements for all compositions were also examined with heating rate from 10 to 50 1C/min and Fig shows an example for ribbon x ¼ As heating rate increased, the primary crystallization peak shifts to higher temperatures (Fig 2a) due to thermal activation necessary for crystallization Kissinger technique has been used to find activation energy and the plot in Fig 2(b) shows this value to be Ea1 ¼ 3.06 eV The crystallization activation energy for bcc-Fe(Si) phase (Ea1) and boride phase (Ea2) of the rest samples are also determined and shown in Table 20°C/min -10.2 -10.4 -10.6 -10.8 -11.0 -11.2 1.162 1.169 1.176 1.183 1.190 1.197 1.204 1/Tp (*10-3 K-1) (b) Fig DSC curves with different heating rates (a) and Kissinger plot (b) of sample x ¼ 569°C Table Crystallization activation energies of first peak, Ea1, and of second peak, Ea2, on DSC curves 562°C 732°C 709°C Heat Flow (a.u.) 561°C 732°C x=5 Ea1 (eV) Ea1 (eV) 2.73 2.95 2.91 4.08 3.06 4.36 3.15 4.57 3.30 4.70 560°C 705°C x=4 542°C x=3 690°C x=2 x=1 200 x 400 600 800 1000 T (°C) Fig DSC curves of as-cast ribbons Fe73.5ÀxCrxSi13.5B9Nb3Cu1 (x ¼ 1–5) with heating rate of 20 1C/min The formation of ferromagnetic phase during primary crystallization is critical for obtained excellent soft magnetic properties since other phases might act as pinning sites for domain walls Another important feature forming the desired properties is proper nanocrystallization, namely the particle size as well as crystallization volume fraction DSC apparatus has been used to estimate the crystallization volume fraction [12]: wf ¼ DH a À DH t DH a (1) ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 304 (2006) 36–40 where DH a and DH t are crystallization enthalpy of as-cast and annealed alloy for time t, respectively Fig shows the DSC curves for ribbon x ¼ (as-cast and annealed at 540 1C for 15 min) Using expression (1) we derived the crystallization volume fraction of a-Fe(Si) phase at the peak Tp1 to be wf ¼ 84% for sample x ¼ This value decreased consequently to 60, 58 and 32% for samples x ¼ 2, 3, and 4, respectively At this annealing condition, in the sample x ¼ the crystallization only partly occurred Increasing keeping time to ta ¼ 30 min, wf showed to be 88%, 86%, 83%, 77% and 69% for samples x ¼ 1–5, respectively We note that the higher Cr content substituted for Fe in the Finemet, the lower crystallization ability at the same annealing condition occurred due to the same reason as the increasing Tp1 with x (see Fig 1) Fig shows the X-ray diffraction patterns of annealed ribbons and insert part in this figure is the average value of particle size determined according to Scherrer expression Nanocrystalline alloys, because of their small grain size, exhibit significant broadening in the Bragg diffraction peaks The calculations show that the particle size decreases with increasing Cr content in studied samples, perhaps originating from high melting temperature as well as high hardness of Cr Both the surfaces of annealed ribbons (wheel-contacted and air-contacted surfaces in fabricating process) are observed by SEM One very interesting feature is that for the first time the clear laminar (or cellular) structure of grains with thickness of less that 25 nm and average length of around 300 nm has been observed Fig presents as an example for sample x ¼ It is also surprising that plans of laminas are oriented almost perpendicularly to the surface of ribbons The air-contacted surface (with lower cooling x =1, dg = nm x =2, dg = 8.6 nm x =3, dg = nm x =4, dg = nm x =5, dg = 6.5 nm Intensity (a.u.) 38 x=5 x=4 x=3 x=2 x=1 20 30 40 50 Theta (deg.) 60 70 Fig X-ray diffraction patterns of annealed ribbons Fe73.5ÀxCrxSi13.5B9Nb3Cu1 (Ta ¼ 540 1C–30 min) 542°C Heat Flow (a.u.) as-cast ribbon ∆Ha Fig SEM micrograph of annealed sample x ¼ at the wheel-contacted surface (Ta ¼ 540 1C–30 min) annealed ribbon 598°C ∆Ht 200 400 600 T (°C) 800 1000 Fig DSC curves of as-cast and annealed sample (x ¼ 1) for estimating crystallization volume fraction rate) of this ribbon exhibits only granular structure with average grain size of 25 nm We suppose that a small amount of Cr, which is forming FeCr nanograins is the reason for laminar structure on the annealed ribbon surface which has higher cooling rate in rapid quenching technology The investigation of a FeCrCuNbSiB nanocomposite alloy showed an atomic segregation during crystallization and the absence of chromium in crystalline phase [13] As in Ref [9], we suppose that beside residual amorphous phase, there is coexistence of bcc-Fe(Si) nanograins and small amount of FeCr nanograins ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 304 (2006) 36–40 12 x=1 (annealed) x=5 (annealed) x=5 (as-cast) B (kG) The thermomagnetic curves of studied samples are measured in applied field of 200 Oe and Fig shows the M(T) curves of as-cast ribbons x ¼ and One can see that magnetization is fastly reduced at TC of amorphous phase, after that materials are in superparamagnetic state, then magnetization starts to develop corresponding to the crystallization of a-Fe(Si) phase The M(T) measurement along cooling cycle exhibits clearly a two-step curve relating to multiphase structure of the ribbon It is suggested that in this cycle, beside a-Fe(Si) phase and remaining amorphous phase, a little amount of boride phase also co-exists Experiments showed that the Curie temperature of amorphous state significantly decreased with increasing Cr content (see Fig and Table 3), in good agreement with results obtained in Ref [9] The hysteresis loops of as-cast and annealed ribbons have been measured and Fig presents the results for compositions x ¼ and As seen from this figure, different from pure Finemet, here hysteresis loops of ascast ribbons with Cr doping have quite high rectangular coefficient of more than 90% showing the pinning of displacement of domain walls It could be related to the existence of high gradient of mechanical strain by Cr doping after rapid quenching After annealing, the soft magnetic properties of the studied nanocomposite samples are desirably improved (see Fig and Table 2) From a series of isotherms M(H) measured at different temperatures around TC of respective amorphous phase, the magnetic entropy change, |DSm|, of studied samples has been determined Fig and Table show the results and clearly that we can control the maximum value and the peak of |DSm| by Cr doping content These |DSm|max indicate that the studied samples have GMCE and according to our knowledge, this effect was firstly discovered by us for Finemet [14] 39 x=1 (as-cast) -4 -8 -12 -1.0 -0.5 0.0 Fig Hysteresis loops of studied samples x ¼ and (as-cast and annealed at 540 1C–30 min) Table Magnetic characteristics of Fe73.5ÀxCrxSi13.5B9Nb3Cu1 annealed ribbons (measured in maximal magnetic field of Oe) Sample mmax Hc (Oe) Bmax (kG) x¼1 x¼2 x¼3 x¼4 x¼5 50,000 53,900 42,700 37,400 38,800 0.069 0.031 0.080 0.054 0.057 10.8 9.3 6.8 6.9 6.6 12 160 x= x=1 x=5 120 x=2 |∆Sm| (J/kgK) M (emu/g) 1.0 0.5 H (Oe) 80 40 TC,a TC,a x=3 TC,c 200 x=4 x=5 TC,c 400 600 800 T (°C) Fig Thermomagnetic curves of as-cast samples with x ¼ and in applied magnetic field of 200 Oe 100 150 200 250 300 350 T (°C) Fig Magnetic entropy change, |DSm|, as a function of temperature of amorphous samples Fe73.5ÀxCrxSi13.5B9Nb3Cu1 ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 304 (2006) 36–40 40 Table Curie temperature, TC, and maximum magnetic entropy change, |DSm|max, of Fe73.5ÀxCrxSi13.5B9Nb3Cu1 samples x o TC ( C) |DSm|max (J/kg K) 327 9.8 310 7.7 265 5.9 232 5.6 170 5.1 Conclusions The substitution of Cr for Fe in Finemet leads to several interesting behaviors such as the increase of crystallization temperature and decrease of crystallization volume fraction and particle size of a-Fe(Si) phase, significantly reducing Curie temperature of amorphous state, the existence of pinning centers, and furthermore the laminar (or cellular) structure and GMCE are discovered These materials have been combined almost the advantages of well-known MCE materials because of high magnetization (high Fe content), sharp FM-PM transition (high atomic homogeneity), high working temperature as well as low heat capacity (metallic nature) The studied samples could be considered as promising candidates for magnetic refrigerant materials working at high temperature Acknowledgement The authors are grateful to the Vietnam National Fundamental Research Program for financial support of the Project 811204 References [1] Y Yoshizawa, S Oguma, K Yamauchi, J Appl Phys 64 (1988) 6044 [2] G Herzer, IEEE Trans Magn 26 (1990) 397 [3] N Chau, N.H Hoang, N.X Chien, P.Q Thanh, L.V Vu, Physica 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.X Chien, N.Q Hoa, P.Q Niem, N.H Luong, N.D Tho, V.V Hiep, J Magn Magn Mater 282 (2004) 174 [6] B.T Tate, B.S Parmar, I Todd, H.A Davies, M.R.J Gibbs, R.V Major, J Appl Phys 83 (1998) 6335 [7] S.H Lim, W.K Pi, T.H Noh, H.J Kim, I.K Kang, J Appl Phys 73 (1993) 6591 [8] I Todd, H.A Davies, M.R.J Gibbs, D Kendal, R.V Major, MRS Symp Proc 577 (1999) 493 [9] P Marin, M Lopez, A Hernando, Y Iqbal, H.A Davies, M.R.J Gibbs, J Appl Phys 39 (2002) 374 [10] C Gomez-Polo, J.I Perez-Landazabal, V Recarte, IEEE Trans Magn 39 (2003) 3019 [11] C Gomez-Polo, J.I Perez-Landazabal, V Recarte, P Mendoze Zelis, Y.F Li, M Vazquez, J Magn Magn Mater 290–291 (2005) 1517 [12] M.S Leu, T.S Chin, MRS Symp Proc 577 (1999) 557 [13] A Slawska-Waniewska, M Gutowski, M Kuzminski, E Dynowska, H.K Lachowicz, in: G.C Hadjipanayis, R.W Siegel (Eds.), Nanophase Materials, Kulver Academic, The Netherlands, 1994, p 721 [14] Nguyen Chau, Ngo Duc The, Cao Xuan Huu, Proceedings of the Second International Workshop on Nanophysics and Nanotechnology (IWONN’04), Hanoi, Vietnam, October 22–23, 2004, p 51 ... size of a-Fe(Si) phase, significantly reducing Curie temperature of amorphous state, the existence of pinning centers, and furthermore the laminar (or cellular) structure and GMCE are discovered These... surfaces in fabricating process) are observed by SEM One very interesting feature is that for the first time the clear laminar (or cellular) structure of grains with thickness of less that 25 nm and. .. amount of Cr, which is forming FeCr nanograins is the reason for laminar structure on the annealed ribbon surface which has higher cooling rate in rapid quenching technology The investigation of