Magnetic phase transitions and magnetocaloric

JOURNAL OF APPLIED PHYSICS 109, 07A919 (2011)

Magnetic phase transitions and magnetocaloric properties of (Gd12-xTbx)Co7 alloys Z. G. Zheng, X. C. Zhong, H. Y. Yu, Z. W. Liu,a) and D. C. Zengb) School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China

(Presented 15 November 2010; received 24 September 2010; accepted 10 November 2010; published online 29 March 2011) The structure and magnetocaloric properties of polycrystalline (Gd12-xTbx)Co7 (x ¼ 0, 4, and 8) alloys with Ho12Co7-type monoclinic structure have been investigated. A spin reorientation transition occurs at about 123 K for the alloy with x ¼ 0. The peak values of magnetic entropy change DSM under a magnetic field change DH of 5 T is 8.8 Jkg1K1 at 160.8 K (Tc), 8.2 Jkg1K1 at 140.8 K (Tc), and 7.1 Jkg1K1 at 118.9 K (Tc) for the alloys with x ¼ 0, 4 and 8, respectively. The refrigerant capacities reach 478, 327, and 160 Jkg1 for x ¼ 0, 4, and 8, respectively. The relatively large magnetic entropy change and reversible magnetization behavior make these alloys a good choice for magnetic refrigeration applications in the temperature range of C 2011 American Institute of Physics. [doi:10.1063/1.3551736] 118–160 K. V

I. INTRODUCTION

In recent years, the magnetic materials with high magnetocaloric effect (MCE) have attracted considerable attention as refrigerants for magnetic refrigeration. A large isothermal magnetic entropy change (DSM) has been obtained in the materials such as Gd5(Ge2Si2),1 La(Fe,Si)13,2 MnFe(P,As),3 etc. Due to their large magnetic moments, heavy rare-earth transition metal compounds are considered as one of the best materials for achieving large MCE. In particular, the Gd based compounds, such as Gd6Co2Si3,4 GdFeAl,5 GdTX (T ¼ Mn, Fe, Ni, Pd, X ¼ Al, In)6 etc. have been investigated widely. For Gb-Co based alloys, Chen et al.7 reported that Tc of Gd12Co7 is 163 K and the maximum entropy change is about 4.6 Jkg1K1 for a field change of 2 T. Zhou et al.8 investigated the (GdxTb1-x)Co2 alloys and found that they have large MCE near room temperature. As well known, the magnetic properties of rare earth intermetallics are primarily governed by the RKKY type exchange interaction and magnetocrystalline anisotropy.9 Hence it is possible to modify the magnetic phase transition and magnetic entropy change in rare earth intermetallics by using two different rare earth elements mixed with transition metal. In this work, the influence of Tb on the magnetic properties and magnetocaloric effect of (Gd12-xTbx)Co7 (x ¼ 0, 4, and 8) alloys were investigated. II. EXPERIMENTAL

The polycrystalline (Gd12-xTbx)Co7 (x ¼ 0, 4, and 8) alloys were prepared by arc-melting a mixture of pure Gd (99.95 wt. %), Tb (99.95 wt. %), and Co (99.9 wt. %) in argon atmosphere. To ensure compositional homogeneity, the ingots were repeatedly melted at least four times. Before characteri-

zation, the ingots were wrapped in Ta foil and annealed at 500 C for 5 days. The structure of the samples was identified by Philips X’pert Pro MPD X diffractometer, using a monochromatized x-ray beam with nickel filtered Cu Ka1 radiation ˚ ) generated at 40 kV and 30 mA. The temperature (1.54056 A and magnetic field dependences of magnetization were measured by a physical properties measurement system (PPMS-9, Quantum Design Co.). III. RESULTS AND DISCUSSION

XRD patterns for all experimental alloys were subjected to Rietveld refinement. Figure 1 shows the Rietveld refinement result for (Gd8Tb4)Co7 alloy at room temperature. The sample was formed in a single phase with monoclinic Ho12Co7-type crystal structure (space group P121=c1).10 Similar results were obtained for other (Gd12-xTbx)Co7 (x ¼ 0 and 8) alloys. The lattice constants a, b, c, and the unit-cell volume V derived from the XRD patterns are listed in Table I. Lattice parameters and unit cell volume are in good agreement with the reported data.7 The results indicate that the structure of alloys does not change with Tb addition. With increasing Tb content, the lattice constant a decreases but b, c, and V increase. The temperature (T) dependences of magnetization (M) of (Gd12-xTbx)Co7 (x ¼ 0, 4, and 8) alloys under a 500 Oe applied field in “zero-field cooling” (ZFC) and “field cooling” (FC) modes are shown in Fig. 2. The Curie temperature TC, which is defined as the temperature at the maximum of jdM=dTj vs T plot, for Gd12Co7, Gd8Tb4Co7, and Gd4Tb8Co7

a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. b) Electronic mail: [email protected]. 0021-8979/2011/109(7)/07A919/3/$30.00

FIG. 1. (Color online) Experimental XRD data (dots) and Rietveld refinement results for (Gd8Tb4)Co7 alloy.

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Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

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TABLE I. The room temperature crystallographic data of (Gd12-xTbx)Co7 (x¼0, 4, 8) compounds obtained by the Rietveld refinements. Here a–c are the lattice parameters; V is the lattice volume and RWP, RP, and REXP are the reliability factors of the fittings. x

˚) A (A

˚) B (A

˚) C (A

˚ 3) V (A

RWP

RP

REXP

0 4 8

8.5546(8) 8.2596(8) 8.2511(5)

11.3126(4) 11.3396(1) 11.4055(3)

13.8134(9) 14.0009(4) 14.0096(7)

859.6809(5) 873.1828(3) 881.5531(3)

1.96 3.88 4.32

1.21 2.64 3.26

1.75 2.57 3.35

are 160.8, 140.8, and 118.9 K, respectively. All alloys undergo a ferromagnetic (FM) to paramagnetic (PM) order transition. The magnetization changes rapidly with the change of temperature in the vicinity of TC, indicating that a large entropy change may be observed near Curie temperature. Furthermore, a spin reorientation behavior with TSR ¼ 123 K is observed for the (Gd12-xTbx)Co7 alloy with x ¼ 0, which should result from the competition between magnetic exchange and the crystalline electric field.11 The FC and ZFC data branch out from each other in the vicinity of Tc, indicating thermomagnetic irreversibility for this alloy. To study the magnetic phase transition of Gd8Tb4Co7 alloy, the Arrott plot (H=M versus M2) is depicted in Fig. 3. It is deduced from the isothermal magnetization curves (Fig. 3, inset). It is widely accepted that the negative slopes or inflection points in the Arrott plots are related to firstorder magnetic transition, while the positive slope and linear behavior near Tc often mean that the phase transition is second-order phase transition.12,13 Neither inflection nor a negative slope in the Arrott plot of Gd8Tb4Co7 is observed near the TC in Fig. 3, which indicates that the alloy undergos a second-order magnetic transition. The magnetocaloric effects of (Gd12-xTbx)Co7 (x ¼ 0, 4, and 8) alloys were calculated in terms of isothermal magnetic entropy change by using Maxwell’s equation.14 Figure 4 shows the temperature dependencies of DSM under the applied magnetic field change from 0 to 5 T. The peak values of DSM are found to be 8.8, 8.2, and 7.1 Jkg1K1 for the alloys with x ¼ 0, 4, and 8, respectively. These values are relatively larger than the DSM reported for Gd65Fe20Al12B3,15 Pr0.7Sr0.3MnO3,16 etc.

FIG. 2. (Color online) Temperature dependencies of magnetization for (Gd12-xTbx)Co7 (x¼0, 4, and 8) alloys measured in a 500 Oe applied magnetic field.

FIG. 3. (Color online) The Arrott plots of Gd8Tb4Co7 alloy around Tc of 137.8 K. The temperature step is 2 K in the vicinity of TC and 5 K in other ranges.

Refrigerant capacity (RC) is another important parameter to evaluate the MCE of the magnetocaloric materials. RC can be measured in different ways. In this work, the RC values are calculated by numerically integrating the area under the (DSM)T curves using the full width at half maximum of theÐ peaks as the integration limit, that is, T RC ¼ T12 DSM ðT ÞdT. The RC values of alloys are 478.4, 327.2, and 160.2 J=kg for (Gd12-xTbx)Co7 alloys with x ¼ 0, 4, and 8, respectively. IV. CONCLUSION

The polycrystalline compounds (Gd12-xTbx)Co7 (x ¼ 0, 4, 8) with Ho12Co7-type monoclinic structure are prepared by arc-melting. The magnetic and magnetocaloric properties of alloys have been investigated. The Curie temperature TC can be tuned from 160.8 to 118.9 K with increasing Tb content. A second-order phase transition from the FM to PM state occurs at the vicinity of Tc. A maximum value of DSM is 8.8, 8.2, and 7.1 Jkg1K1 for (Gd12-xTbx)Co7 with x ¼ 0, 4, and 8, respectively, in applied field change from 0 to 5 T. For the same field change, the refrigerant capacities of these alloys reach 478, 327, and 160 Jkg1, respectively. The relatively large value and reversible

FIG. 4. (Color online) Temperature dependences of isothermal magnetic entropy changes (DSM) for (Gd12-xTbx)Co7 (x¼0, 4, and 8) alloys under magnetic field change of 0-5 T


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behavior for the field increasing=decreasing cycles make these alloys good candidate materials for magnetic refrigeration in the temperature range of 118–160 K. ACKNOWLEDGMENTS

This work is supported by Guangdong Provincial Science and Technology Program (Grant Nos. 2010B050300008 and 2007B010600043) and the Fundamental Research Funds for the Central Universities, SCUT (Grant No. 2009ZM0291). 1

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