The magnetocaloric effect and critical behavior in amorphous Gd 60Co 40-xMn x alloys.

The magnetocaloric effect and critical behavior in amorphous Gd60Co40−xMnx alloys Z. G. Zheng, X. C. Zhong, H. Y. Yu, V. Franco, Z. W. Liu, and D. C. Zeng Citation: Journal of Applied Physics 111, 07A922 (2012); doi: 10.1063/1.3673860 View online: http://dx.doi.org/10.1063/1.3673860 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/111/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magneto-caloric effect of a Gd50Co50 amorphous alloy near the freezing point of water AIP Advances 5, 097122 (2015); 10.1063/1.4930832 Critical behavior and magnetocaloric effect of Gd65Mn35− x Ge x (x = 0, 5, and 10) melt-spun ribbons J. Appl. Phys. 112, 033903 (2012); 10.1063/1.4740062 Tunable magnetocaloric effect in Gd-based glassy ribbons J. Appl. Phys. 110, 053920 (2011); 10.1063/1.3632983 Magnetocaloric effect in Fe–Zr–B–M ( M = Mn , Cr, and Co) amorphous systems J. Appl. 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Box 1065, 41080 Sevilla, Spain (Presented 31 October 2011; received 23 September 2011; accepted 1 November 2011; published online 29 February 2012) The amorphous alloys Gd 60 Co 40x Mn x (x ¼0, 5, 10, 15) were prepared by melt spinning. The Curie temperature, T c , increases monotonously with Mn addition, ranging from 198 K for x ¼0to 205 K for x ¼15, while the maximum values of DS M under the applied field change from 0 to 5 T are 7.7, 7.1, 6.2 and 5.4 Jkg 1 K 1 for x ¼0, 5, 10, and 15, respectively. All samples undergo a second order ferri-paramagnetic phase transition. The critical behavior around the transition temperature is investigated in detail, using both the standard Kouvel-Fisher procedure as well as the study of the field dependence of the magnetocaloric effect. Results indicate that the obtained critical exponents are reliable, and that the present alloys exhibit local magnetic interaction. V C2012 American Institute of Physics. [doi:10.1063/1.3673860] The magnetic materials with large magnetocaloric effect (MCE) have attracted considerable attention for magnetic refrigeration applications. In recent years, MCE has been found in various intermetallic alloys, such as Gd 5 Si 2 Ge 2 , 1 La(Fe,Si) 13 , 2,3 MnAs 1x Sb x , 4 MnFeP 1x Si x , 5 Ni-Mn-In, 6 and amorphous alloys Fe 882x Co x Ni x Zr 7 B 4 Cu 1 . 7 However, a material with a large DS m does not necessarily have a large refrigeration efficiency. The refrigerant capacity (RC) can provide an estimate of the performance of a magnetocaloric material. 8 Recently, Gd based soft magnetic amorphous alloys, a type of second order phase transition (SOT) materials, have been proposed as the promising candidates for magnetic refrigerant. Owing to the amorphous nature, they exhibit some special advantages such as the negligible magnetic hysteresis loss, higher electrical resistivity, tunable Curie temperature T c ,high corrosion resistance, and large refrigerant capacity. Zhong et al. 9 found that amorphous Gd 68x Ni 32þx have large refrigerant capacity (RC) and the maximum RC value is 724 Jkg 1 with DH¼5T.ForGd 71 Co 29 amorphous alloys, 10 the magnetic entropy changes DS M under a magnetic field of 10 kOe is 3.1 Jkg 1 K 1 .Fuet al. 11 investigated the magnetic bulk metallic glass Gd 52.5 Co 16.5 Al 31 ,whichhasT c of 95 K and DS max of 9.8 Jkg 1 K 1 under a field change of 5 T. In this work, the effects of Mn on magnetic and magnetocaloric properties of Gd 60 Co 40x Mn x alloys are investigated. To investigate the nature of the ferrimagnetic (FM) to paramagnetic (PM) phase transition, we also performed a critical exponent study using the isothermal magnetization data around T c . The Gd 60 Co 40x Mn x (x ¼0, 5, 10, 15) alloy buttons were prepared by arc-melting a mixture of pure Gd (99.95 wt%), Co (99.9 wt%) and Mn (99.5 wt%) in an argon atmosphere. To ensure composition homogeneity, the ingots were repeatedly melted at least four times. The crashed pieces of ingot were then melt-spun into ribbons (with a thickness of about 20–30 lm and a width of 3 mm) on a copper wheel with a speed of 50 m/s using a single-roller meltspinner in an argon atmosphere. The microstructure of the alloys was characterized by a Philips X’pert Pro MPD X diffractometer, using a monochromatized X-ray beam with nickel filtered Cu Karadiation (1.54056 A ˚). The temperature and magnetic field dependences of magnetization were measured by a Physical Properties Measurement System (PPMS-9, Quantum Design Co.). For the X-ray diffraction patterns of all ribbon samples (not shown here), only one broad peak appears between 2h of 30 and 40 degrees in all alloys, and no obvious diffraction peaks corresponding to crystalline phase are observed, indicating an amorphous structure in the Gd 60 Co 40x Mn x ternary alloys. For x ¼0, there is a small peak near 2h¼28, indicating that it may be caused by some nanocrystal Gd. Figure 1presents the magnetization as a function of temperature for Gd 60 Co 40x Mn x amorphous alloys from 5 to 300 K under an applied field of 500 Oe. With the increase of temperature, the magnetization decreases, resulting from the magnetic transition from FM ordered state to PM ordered state. The Curie temperatures, T c , determined as the temperature at the maximum of jdM/dTjversus Tplot shown in the insert, are 198 K, 202 K, 204 K and 205 K for x ¼0, x ¼5, x¼10 and x ¼15, respectively. The magnetocaloric effects of the series of alloys was calculated in terms of isothermal magnetic entropy change using Maxwell’s equation. Figure 2shows the temperature dependency of DS M under the applied magnetic field changes of 2 T and 5 T. The peak values of DS M are found to be 7.7, 7.1, 6.2 and 5.4 Jkg 1 K 1 for x ¼0, 5, 10, and 15 respectively. These values are comparable to or even higher than those of Gd 65 Fe 20 Al 8 B 7 amorphous alloy (5.21 Jkg 1 K 1 at 5 T) 12 and Gd 60 Al 10 Mn 30 nanocomposite (3.3 Jkg 1 K 1 at 5 T). 13 The Arrott plots of the Gd 60 Co 40x Mn x (x ¼5) amorphous ribbons are displayed in Fig. 3. The isothermal magnetization M-H curves are presented in the inset. A typical ferrimagnetic a) Authors to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected]. 0021-8979/2012/111(7)/07A922/3/$30.00 V C2012 American Institute of Physics111, 07A922-1 JOURNAL OF APPLIED PHYSICS 111, 07A922 (2012) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.116 On: Wed, 27 Jan 2016 15:10:13 transition is evident in the vicinity of T c .FromFig.3,noinflection or negative slope is observed as an indication that FM – PM transition is of second-order. 12,14 Similar behavior has also been found for the whole series of alloys studied in this work. The second-order transition from FM to PM near the Curie point is characterized by a set of critical exponents: b (associated with the spontaneous magnetization), c(relevant to the initial susceptibility), and d(associated with the critical magnetization isotherm). They are defined as: 15 Msð0;TÞ¼m0t jj b;t0 (1) v1 0ð0;TÞ¼h0 m0t jj c;t0 (2) H¼DMd;t¼0 (3) where tis the reduced temperature (t¼T/T c 1), and m 0 ,h 0 and Dare the critical amplitudes. M s (0, T) is the spontaneous magnetization at different temperatures; v1 0is the inverse of the susceptibility. Although the different critical exponents can be determined independently from experimental measurements, they are related to each other, which is taken in most cases to be of the form bþc¼bd. In order to properly determine the T c , as well as the critical exponents b,c, and d, several methods can be used, including modified Arrott plots technique (MAPs), Kouvel–Fisher (KF) method and Widom scaling relation. 16 More recently, it has been shown that the field dependence of the magnetocaloric effect can be used to determine the critical exponents of materials. 17,18 For the Kouvel–Fisher method, two additional magnitudes are defined as: YðTÞ¼Ms@Ms @T1¼TTc b(4) XðTÞ¼v1@v1 @T1¼TTc c(5) By plotting Y(T) and X(T) as a function of temperature, one should get straight lines around T c with the slopes of 1/band 1/c, respectively, and the intercepts on the temperature axes are equal to T c . The exponents obtained from KF method are as follows: b¼0.311, c¼1.306 with T c ¼203.9 K. All these critical exponents derived from various methods, including the KF method and scaling of the magnetocaloric effect, are given in Table I along with the theoretically predicted values for different models, such as Mean-field theory, three-dimensional (3D) Heisenberg theory and three-dimensional (3D) Ising theory. It should be noted that the 3D Heisenberg model or 3D Ising model is based on the short-range magnetic coupling, and mean-field theory is based on the long-range magnetic coupling. The magnetic interactions in the present alloys exhibit local magnetic interactions. It is clear from Table Ithat, with increasing Mn contents, the values of critical exponent band cincrease continuously. Physically, bdescribes how the ordered moment grows below T c while cdescribes the divergence of the magnetic susceptibility at T c . The smaller the value of b, the faster the growth of the ordered moment. The bvalue increases with increasing Mn contents, reflecting a slower growth of the ordered moment with decreasing temperature. This tendency is in agreement with magnetization intensity below T c ,whichdecreaseswith increasing Mn contents, as shown in the Fig. 1. Also, the FIG. 1. (Color online) Temperature dependences of magnetization for Gd 60 Co 40x Mn x amorphous alloys under an applied field of 500 Oe. FIG. 2. (Color online) Magnetic entropy changes in the Gd 60 Co 40x Mn x amorphous ribbons under the magnetic field changes of 2 T and 5 T. FIG. 3. (Color online) The Arrott plots of Gd 60 Co 40x Mn x (x ¼5) amorphous alloy. The insert is isothermal magnetization curves as a function of magnetic field. 07A922-2 Zheng et al. J. Appl. Phys. 111, 07A922 (2012) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.116 On: Wed, 27 Jan 2016 15:10:13 change in the values of critical exponents is due to the formation of finite magnetic clusters in the infinite ferromagnetic network. 19 In terms of renormalized magnetization m¼M t jj band renormalized field h¼H tjj bþd, the scaling equation of state can be written as: m¼f6ðhÞ(6) Equation (6) implies that for the true scaling relations and the right choice of b,c,anddvalues, scaled mplotted as a function of scaled hwill fall on two universal curves: one above T c and another below T c . In order to check if these critical exponents can generate the scaling equation of state Eq. (6) for Gd 60 Co 40x Mn x , the scaled mversus scaled his plotted in Fig. 4taking critical exponents from Table I. It is rather significant that all the data collapse into two separate branches: one below T c and another above T c . This clearly indicates that the obtained critical exponents are reliable, and the interactions get properly renormalized in critical regime following scaling equation of state. Similar behavior has also been found for the whole series of alloys studied in this work. The Curie temperature of amorphous Gd 60 Co 40x Mn x (x ¼0, 5, 10, 15) alloys can be tuned from 198 K to 205 K with increasing Mn content. The maximum values of DS M are 7.7, 7.1, 6.2 and 5.4 Jkg 1 K 1 for Gd 60 Co 40x Mn x with x¼0, 5, 10, and 15, respectively, under the applied field change from 0 to 5 T. The Gd 60 Co 40x Mn x amorphous alloys exhibit second order magnetic phase transition from FM to PM state. The values of critical exponents for describing the FM to PM second order transition are extracted using KF method and field dependence of MCE. The critical exponent values obtained for all samples are comparable to the values predicted by the 3D Ising model, and have also been verified by the scaling equation of state. This work is supported by Guangdong Provincial Science and Technology Program (Grant No. 2010B050300008), the Fundamental Research Funds for the Central Universities, SCUT (Grant Nos. 2012ZZ0013 and 2011ZM0014), China Postdoctoral Science Foundation (20110490875). V. F. acknowledges the financial support of the Spanish Ministry of Science and Innovation and EU FEDER (Project MAT 2010- 20537), the PAI of the Regional Government of Andalucı´a (Project FQM-6462), and the United States Office of Naval Research (Project N00014-11-1-0311). 1 H. Fu, X. T. Zu, and T. D. Shen, Thermochim. Acta 445, 53 (2006). 2 Z. Lin, S. Li, S. Tsai, J. Duh, M. Liu, and F. Xu, J. Magn. Magn. Mater. 323, 1741 (2011). 3 M. Zou, J. A. Sampaio, V. K. Pecharsky, and K. A. Gschneidner, Phys. Rev. B 80, 172403 (2009). 4 L. 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Leslie-Pelecky, Phys. Rev. B 77, 184415 (2008). TABLE I. The values of the exponents b,c, and das determined from KouvelFisher method for Gd 60 Co 40x Mn x . The theoretically predicted values of exponents for various universal classes are also given for the sake of comparison. Composition bc dReference x¼0 0.309[0.308] 1.252[1.417] 5.051 a [5.601] This work x¼5 0.311[0.315] 1.306[1.733] 5.120 a [6.502] This work x¼10 0.322[0.328] 1.362[1.872] 5.229 a [6.707] This work x¼15 0.326[0.329] 1.580[1.954] 5.846 a [6.940] This work Mean-field theory 0.5 1.0 3.0 20–22 3D Heisenberg theory 0.365 1.336 4.8 20–22 3D Ising theory 0.325 1.24 4.82 20–22 a The values calculated using the relation bþc¼bd. Values without brackets are calculated using the Kouvel-Fisher method; square brackets indicate that the calculation is done using the scaling of the magnetocaloric effect. FIG. 4. (Color online) The scaled mare plotted as a function of scaled hfor Gd 60 Co 40x Mn x (x ¼10) alloy. The plot shows all the data collapse into two distinct branches: one below T c and another above T c . Inset shows the same plot on loglog scale. 07A922-3 Zheng et al. J. Appl. Phys. 111, 07A922 (2012) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.116 On: Wed, 27 Jan 2016 15:10:13