Materials Science and Engineering A 449–451 (2007) 360–363 The discovery of the colossal magnetocaloric effect in a series of amorphous ribbons based on Finemet N Chau a , N.D The a,b , N.Q Hoa a,c , C.X Huu a , N.D Tho a , S.-C Yu c,∗ a Center for Materials Science, College of Science, Vietnam National University, 334 Nguyen Trai, Hanoi, Vietnam b Department of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom c Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea Received 23 August 2005; received in revised form 10 February 2006; accepted 24 February 2006 Abstract A large number of amorphous ribbons based on Finemet have been prepared by rapid quenching on a single copper wheel with linear speed of v = 25–30 m/s The ribbons are 20–25 ␮m thick and 6–8 mm wide All as-cast samples are amorphous Two criteria producing the colossal magnetocaloric effect (CMCE) in magnetic materials working as magnetic refrigerants are high saturation magnetization and sharp ferromagnetic–paramagnetic phase transition The Fe-based amorphous ribbons fit these cretia Thermomagnetic curves as well as isothermal magnetization curves around the Curie temperature of all the studied samples have been determined The results show that the magnetic entropy change, | Sm |, belongs to a class of materials with CMCE and the | Sm |max values have been obtained at a moderately low magnetic field change of 1.35 T, moreover | Sm |max occurred at quite high temperature © 2006 Elsevier B.V All rights reserved Keywords: Amorphous magnetic materials; Magnetocaloric effect; Nanocrystalline materials Introduction When an external magnetic field is applied to a material, magnetic moments in material attempt to align with the magnetic field, thereby reducing the magnetic entropy of the spin system If this process is performed adiabatically, the reduction in spin entropy is offset by an increase in lattice entropy, and temperature of the sample will rise When an applied magnetic field is removed, the temperature of specimen will drop This magnetocaloric effect (MCE), or adiabatic temperature change, which is detected as the heating or cooling of magnetic materials, is due to the varying magnetic field Magnetic refrigeration provides an alternative method for cooling Recently, there has been interest in extending the magnetic refrigeration technique to near and higher than room temperature region because of the desire to eliminate chlorofluoro-carbons present in high-temperature gas-cycle systems and to save energy [1] As we well known, the adiabatic magnetic entropy change, Sm , and temperature change, Tad , are correlated with magnetization, magnetic field change, heat capacity and absolute ∗ 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.2006.02.354 temperature by Maxwell’s fundamental relations [2]: Hmax Sm (T, H) = Hmax Tad (T, H) = − ∂M(T, H) ∂T T C(T, H)H dH (1) H ∂M(T, H) ∂T dH (2) H where Hmax is the final applied magnetic field From Eq (1), we see that two criteria forming large MCE in magnetic materials are a high saturation magnetization and a sharp change in magnetization at the ferromagnetic–paramagnetic (FM–PM) phase transition The prototype material for room temperature range is lanthanide metal Gd which orders ferromagnetically at 294 K [3] A series of Gd5 (Ge1−x Six )4 alloys was reported [4,5] to display a Sm at least two times larger than that of Gd near room-temperature The compound La(Fe,Co)11.83 Al1.17 has also showed a considerable MCE near room temperature [6] Recently, a new class of magnetic refrigerant materials MnFe(P, As) and related compounds for room-temperature applications have been discovered [7,8], also, interstitial modifications of compounds La(Fe, Si)13 with hydrogen, carbon and nitrogen [9,10] have attracted much attention These new N Chau et al / Materials Science and Engineering A 449–451 (2007) 360–363 361 Fig X-ray diffraction patterns of as-cast samples Fe73.5−x Crx Si13.5 B9 Nb3 Au1 materials have important advantages over existing magnetic coolants: they exhibit a large MCE and the operating temperature can be ranged from below 200 to about 400 K by adjusting the chemical composition or the content of interstitial atoms Another class of materials also displaying a large MCE is based on perovskite [11,12], where we examined the positive entropy change in manganite with charge-ordering [13] In this report we present our discovery of colossal magnetocaloric effect (CMCE) in a series of amorphous ribbons based on Finemet Fig Magnetization as a function of applied field of the sample Fe73.5 Si13.5 B9 Nb3 Au1 at different temperatures The structure of the ribbons was examined by X-ray diffrac´˚ tometery (D5005 Bruker) with Cu K␣ radiation (λ = 1.54056 A) Isothermal magnetization curves and thermomagnetic curves were measured by vibrating sample magnetometer (VSMDMS 880, Digital Measurement Systems) with maximal Experiments Amorphous ribbons with nominal compositions (number indicate at.%) No 1: Fe73.5 Si13.5 B9 Nb3 Au1 ; No 2: Fe73.5−x Crx Si13.5 B9 Nb3 Cu1 (x = 1–9); No 3: Fe73.5−x Crx Si13.5 B9 Nb3 Au1 (x = 1–5); No 4: Fe73.5−x Mnx Si13.5 B9 Nb3 Cu1 (x = 1, and 5) have been prepared by rapid quenching melting alloys (using elements of 99.9% purity) on a copper wheel with wheel speeds v = 25–30 m/s The ribbons are 20–25 ␮m thick and 6–8 mm wide Fig Thermomagnetic curves of as-cast ribbon Fe73.5 Si13.5 B9 Nb3 Au1 : (1) heating cycle and (2) cooling cycle Fig Thermomagnetic curves of as-cast ribbons Fe73.5−x Crx Si13.5 B9 Nb3 Cu1 (a) and magnetic entropy change, | Sm |, vs temperature (b) 362 N Chau et al / Materials Science and Engineering A 449–451 (2007) 360–363 applied 13.5 kOe The heat capacity measurements studied samples were performed by DSC TA 2960 ments and the results showed that these values order of that of pure Fe (the maximum values of ity reached at respective Curie temperatures, C were 400 J/kg K) of the Instruare in capacaround magnetic field up to 13.5 kOe Fig shows the results for sample No When magnetization is measured at a small discrete field and temperature interval, Sm could be determined from Eq (1) by formula: Sm = Results and discussion Fig shows the XRD patterns of as-cast ribbons No These patterns exhibit only one broad peak around 2θ = 45◦ , showing that the samples are amorphous The same behavior is observed for all ribbons studied The thermomagnetic curves of all samples have been measured at a low applied magnetic field of 50 Oe Fig presents the M(T) curves for as-cast ribbon No as an example It is clear that at the Curie temperature, TC , of amorphous state, magnetization suddenly decreases, after that the sample is in a (super)paramagnetic state to above 550 ◦ C, then starts to increase due to crystallization To study the magnetocaloric effect of the samples, a series of isothermal magnetization curves around their respective TC have been measured in a Mi − Mi+1 Hi Ti − Ti+1 (3) where Mi and Mi+1 are the experimental values of magnetization at Ti and Ti+1 , respectively, under an applied magnetic field of Hi The magnetic entropy change, | Sm |, of sample No has been calculated and has a maximum value of 7.8 J/kg K This value indicated that the mentioned sample has colossal magnetocaloric effect (CMCE) We note that this CMCE was reached at a quite low magnetic field variation of 13.5 kOe Figs 4–5 display the thermomagnetic curves of as-cast ribbons as well as | Sm | versus temperature of the other samples These figures show that the doping of Cr and Mn in Finemettype alloys significantly decreased the Curie temperature of amorphous state of respective compositions In the systems with Cr doping this could be explained by ferromagnetic dilution as well as by the existence of FeCr at the grain boundary [14] In the case of Mn substituted for Fe in Finemet, the authors of [15] reported that the migration of Mn atoms to the grain boundary region would promote a reduction of the magnetic coupling in the system We also see that all studied samples exhibit a large MCE and the temperature at which | Sm | reached a maximum (close to the respective TC of amorphous phase) could be controlled by adjusting the doping content According to our knowledge, CMCE was first discovered by us for amorphous phase of Finemet compound with very high | Sm |max = 13.9 J/kg K [16] The studied samples in the present work belong to ultrasoft nanocomposite materials after appropriate annealing similar to that obtained in the ribbons Fe73.5−x Cox Si13.5 B9 Nb3 Cu1 [17] and in Finemet with Cu substituted by Ag [18] Conclusions The amorphous magnetic alloys based on Finemet have essential advantages: high saturation magnetization Ms , sharp change of Ms at FM–PM phase transition of the amorphous state, high working temperature (∼TC ) and low heat capacity (400 J/kg K) Therefore they are very well adapted for magnetocaloric materials and: Fig Temperature dependence of magnetic entropy change, | Sm |, of ribbons Fe73.5−x Crx Si13.5 B9 Nb3 Au1 : (a) and Fe73.5−x Mnx Si13.5 B9 Nb3 Cu1 (b) (i) colossal magnetic entropy change, | Sm |, is discovered in a large number of amorphous ribbons; (ii) | Sm |max occurred at quite high temperature, which could be controlled by substitution effect; (iii) the | Sm |max value has been obtained at a moderately low magnetic field change of 1.35 T and as a consequence, the studied samples could be considered as promising magnetic refrigerant materials working in the high temperature region N Chau et al / Materials Science and Engineering A 449–451 (2007) 360–363 Acknowledgements The authors express sincere thanks to Vietnam National Fundamental Research Program for financial support and this work was supported by Korea Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University References [1] V.K Pecharsky, K.A Gschneidner Jr., J Magn Magn Mater 167 (1997) 2179 [2] A.H Morrish, The Physical Principles of Magnetics, Willey, New York, 1963 (Chapter 3) [3] S.Yu Dankov, A.M Tishin, V.K Pecharsky, K.A Gschneidner Jr., Phys Rev B 57 (1998) 3478 [4] V.K Pecharsky, K.A Gschneidner Jr., Phys Rev Lett 78 (1997) 4494 [5] L Morellou, J Blasco, P.A Algarabel, M.R Ibarra, Phys Rev B 62 (2000) 1022 [6] F.X Hu, B.G Shen, J.R Sun, Z.H Cheng, Phys Rev B 64 (2001) 012409 [7] O Tegus, E Bruck, K.H.J Buschow, F.R de Boer, Nature 415 (2002) 150 363 [8] E Bruck, M Ilyn, A.M Tishin, O Tegus, J Magn Magn Mater 290–291 (2005) [9] Y.F Chen, F Wang, B.G Shen, J.R Sun, G.J Wang, F.X Hu, Z.H Cheng, T Zhu, J Appl Phys 93 (2003) 6981 [10] S Fujieda, A Fujita, K Fukamichi, Appl Phys Lett 81 (2002) 1276 [11] L.E Hueso, P Sande, D.R Miguens, J Rivas, F Rivaldulla, M.A LopezQuintela, J Appl Phys 91 (2002) 9943 [12] Md.A Choudhury, J.A Akhter, D.L Minh, N.D Tho, N Chau, J Magn Magn Mater 272–276 (2004) 1295 [13] N Chau, D.H Cuong, N.D Tho, H.N Nhat, N.H Luong, B.T Cong, J Magn Magn Mater 272–276 (2004) 1292 [14] P Marin, M Lopez, A Hernando, Y Iqbal, H.A Davies, M.R.J Gibbs, J Appl Phys 39 (2002) 374 [15] C Gomez-Polo, J.I Perez-Landajabal, V Recarte, P Mendoza Zelis, Y.F Li, M Vazquez, J Magn Magn Mater 290–291 (2005) 1517 [16] Nguyen Van Hieu, Phan Hong Khoi, Nguyen Xuan Phuc, et al., Vietnam Academy of Science and Technology Press, Hanoi, Vietnam, October 2004 [17] 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 [18] N Chau, N.Q Hoa, N.H Luong, J Magn Magn Mater 290–294 (2005) 1547  ... substituted by Ag [18] Conclusions The amorphous magnetic alloys based on Finemet have essential advantages: high saturation magnetization Ms , sharp change of Ms at FM–PM phase transition of the amorphous. .. our discovery of colossal magnetocaloric effect (CMCE) in a series of amorphous ribbons based on Finemet Fig Magnetization as a function of applied field of the sample Fe73.5 Si13.5 B9 Nb3 Au1 at. .. temperature, TC , of amorphous state, magnetization suddenly decreases, after that the sample is in a (super)paramagnetic state to above 550 ◦ C, then starts to increase due to crystallization To