Magnetic properties and magnetocaloric effect of GdGa compound

Journal of Alloys and Compounds 469 (2009) 15–19

Magnetic properties and magnetocaloric effect of GdGa compound J.Y. Zhang a , J. Luo a , J.B. Li a , J.K. Liang a,b,∗ , Y.C. Wang a , L.N. Ji a , Y.H. Liu a , G.H. Rao a a

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China b International Center for Materials Physics, Academia Sinica, Shenyang 110016, China Received 21 December 2007; received in revised form 23 January 2008; accepted 24 January 2008 Available online 10 March 2008

Abstract Polycrystalline GdGa compound was prepared by arc-melting method. X-ray powder diffraction reveals that GdGa crystallizes in the CrB-type structure. The Curie temperature of the compound is 183 K. A distinct peak associated with spin reorientation transition was shown around 100 K on the magnetization versus temperature curve measured in a field of 0.05 T. Isothermal magnetic entropy change (|SM |) was estimated based on the magnetization isotherms. The maximum value of |SM | is 4.81 J kg−1 K−1 under the applied field changing from 0 T to 5.0 T. The relative cooling power (RCP(S)) of GdGa compound is about 576 J kg−1 under an applied field change of 5 T. The specific heat of the compound has been estimated within the framework of Debye and mean field approximations to evaluate the adiabatic temperature change (Tad ). © 2008 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 75.20.En; 75.30.Sg; 75.60.Ej Keywords: CrB-type structure; Heat capacity; Magnetocaloric effect

1. Introduction Magnetic refrigeration based on magnetocaloric effect (MCE) of materials is an alternative technology for cooling and gas liquefaction, which has great advantages over conventional gas refrigeration, such as superior efficiency and environmental friendliness. Magnetic refrigerators using paramagnetic salts have been employed for a long time to achieve ultralow temperature (<1 K) [1]. However, thermodynamic cycles (simple Brayton, Ericsson, and Carnot) adopted in these refrigerators provide only small temperature spans. For magnetic refrigerators working around room temperature, the active magnetic regenerative (AMR) cycle has been proposed to achieve a large temperature span. In AMR cycle, the magnetic material is used both as the working material and as the thermal regenerator [2]. A typical active magnetic regenerator is composed of layers of porous magnetocaloric materials with their Curie temperatures ranging from ∼20 K to ∼300 K. Richard et al. [3] reported that ∗ Corresponding author at: Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China. E-mail address: [email protected] (J.K. Liang).

0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.01.115

the multilayer AMR regenerators produced a larger temperature span and cooling power, in comparison to the single material regenerators. The rare earth metal Gd has been considered as the best candidate for the room-temperature AMR material [4], except for its high cost, until several new systems were found. In 1997, Gd5 Si2 Ge2 was discovered to exhibit the MCE about twice as large as that of Gd [5]. In the subsequent years, large MCE has been reported in the systems of Ni–Mn–Ga alloys [6,7], La(Fe13−x Six )Hy alloys[8–10], and perovskite manganese oxides [11–13], etc. Among them, perovskite manganites, which exhibit large MCE in response to low applied field, are believed to be good candidates for AMR material in the temperature range of 100–375 K [13]. Binary equiatomic intermetallic compounds of rare earth elements and gallium are all ferromagnetic and have the orthorhombic CrB type structure with the space group of Cmcm [14–16]. Among these compounds GdGa has the highest Curie temperature (near 200 K [17,18]). In addition, gadolinium compounds and alloys were proved to exhibit good performance in AMR due to their larger magnetic entropy density [2,13]. Despite that the structure and magnetic properties of GdGa compound have been studied for decades, few attentions has

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J.Y. Zhang et al. / Journal of Alloys and Compounds 469 (2009) 15–19

been focused on its MCE. In this paper, we report the MCE and specific heat of GdGa compound. 2. Experimental Polycrystalline sample of GdGa was prepared by arc melting the raw materials in a water-cooled copper crucible under high-purity argon atmosphere. The purity of the starting elements was 99.9% for Gd and 99.99% for Ga. To compensate the weight loss during melting and annealing, 3% excess of Gd was added before melting. The alloy was turned over and re-melted at least four times to ensure homogeneity. Titanium was used as an oxygen getter during the melting process. The ingot was wrapped by Ta foil and annealed at 1073 K for 2 weeks in an evacuated quartz tube, and then quenched into water. Crystal structure and phase identification of the sample was examined by powder X-ray diffraction (XRD). The XRD data were collected on a Rigaku D/max-2500 diffractometer with Cu K␣ radiation (45 kV × 250 mA) and a graphite monochromator. A step scan mode was employed with a step width of 2θ = 0.02◦ and a sampling time of 1 s. The temperature dependence of the magnetization (M) of the sample was measured on a superconducting quantum interference device (SQUID) magnetometer at a low field (0.05 T) in the temperature range from 5 K to 300 K. The Curie temperature of the compound was identified as the temperature at which the minimum of dM/dT occurs. In order to determine the magnetocaloric effect of the sample, the magnetization isotherms were measured by the SQUID magnetometer with magnetic fields up to 5 T at different temperatures. The specific heat of the sample was measured using a Modulated Differential Scanning Calorimetry (MDSC) performed on DSC Q200 (TA Instruments) in the temperature range of 273–473 K. Compared to the traditional DSC measurement, this method can obtain heat flow and heat capacity in a single experiment.

Fig. 1. XRD pattern of GdGa. The vertical bars indicate the Bragg positions. The curve at the bottom shows the difference between the observed and the calculated intensities.

free energy (F) of a magnetic system is expressed by the Landau expansion in powers of magnetization M: F=

3. Results and discussion The powder XRD data of the sample at room temperature were analyzed by the Rietveld refinement program FULLPROF [19]. GdGa compound has an orthorhombic CrB-type structure with space group Cmcm. The number of chemical formula unit per unit cell is 4. Gd atoms and Ga atoms occupy two different 4c (0, y, 1/4) equivalent sites [14]. The refinement result shows that yGd = 0.1417(1), yGa = 0.4283(1), lattice parameters ˚ b = 11.0104(3) A, ˚ and c = 4.1058(2) A, ˚ respeca = 4.3388(2) A, tively, with residual factors Rp = 12.4%, Rwp = 14.7%, and Rexp = 5.41%. Fig. 1 shows the Rietveld fitted XRD plots. As can be inferred from the difference plot between the observed and calculated patterns, the sample is a single phase and free from any impurities. Fig. 2 shows the temperature dependence of the magnetization of the sample measured in an applied field of 0.05 T. The Curie temperature was determined as 183 K, which agrees with the previous reports [15,16]. A distinct peak was observed on the M–T curve at about 100 K, which did not appear in the thermomagnetic measurements of previous works [15,18]. To our knowledge, this peak should correspond to a spin reorientation with increasing temperature. The magnetic spin reorientation transition in GdGa compound at 100 K has been reported by Delyagin et al. based on M¨ossbauer spectroscopy measurements [20]. The typical magnetization isotherms of the sample measured from 100 K to 265 K at intervals of 5 K are shown in Fig. 3a. The Arrott plots are depicted in Fig. 3b. The Arrott plots were used to determine the type of phase transition of the sample near TC according to the Inoue-Shimizu model [21]. In this model, the

C1 (T ) 2 C3 (T ) 4 C5 (T ) 6 M + M + M − MH 2 4 6

(1)

It has been pointed out that the type of transition is related to the sign of the Landau coefficient C3 (T) at the Curie temperature (C3 (TC )) [22]. If C3 (TC ) is negative, a first order transition is expected, otherwise the transition will be second order. The sign of C3 (TC ) can be obtained by the Arrott plots [23]. In the case that there are S-shape curves near TC in the Arrott plots, C3 (TC ) is negative, otherwise, it is positive. As seen in Fig. 3b, no S-shaped curve is exhibited near the Curie temperature. Therefore, the magnetic transition in GdGa compound is of second-order.

Fig. 2. Temperature dependence of the magnetization of GdGa measured in an applied field of 0.05 T.

J.Y. Zhang et al. / Journal of Alloys and Compounds 469 (2009) 15–19

17

Fig. 4. Temperature dependence of the isothermal magnetic entropy change |SM | of GdGa under different applied magnetic field.

Fig. 3. Magnetization isotherms of GdGa measured at different temperatures near TC : (a) M vs. H plots and (b) Arrott plots.

Based on the Maxwell relation, the isothermal magnetic entropy change SM (T, H) is given by   H  ∂M dH (2) SM (T, H) = ∂T H 0 where H is the applied field change. For magnetization (M) measured at discrete field and temperature intervals, Eq. (2) can be approximated by the following expression [24]:  1 SM = (Mi+1 − Mi )H Hi (3) Ti+1 − Ti i

where Mi and Mi+1 are the magnetization values measured in a field H at temperatures Ti and Ti+1 , respectively. The temperature dependence of the isothermal magnetic entropy changes of GdGa, calculated from the isothermal magnetization curves by Eq. (3), is presented in Fig. 4 for different magnetic field changes of H = 1 T, 2 T, 3 T and 5 T. The peak values are 4.81 J kg−1 K−1 and 2.33 J kg−1 K−1 for H = 5 T and 2 T, respectively. As shown in Fig. 4, the |SM | decreases gradually below the transition temperature, indicating a spreading distribution of |SM |. Therefore, despite the smaller magnitude of |SM | of the GdGa compound, its |SM | versus T curves

are significantly broader, compared to those of Gd and some first-order transition materials, which is important for AMR [2]. The magnetic cooling efficiency of GdGa compound can be evaluated by the so-called relative cooling power (RCP(S)) defined as RCP(S) = −SM (T,H) × ␦TFWHM , where ␦TFWHM is the full-width at half-maximum of the |SM | versus T curve [13]. The relevant parameters for GdGa and some important AMR materials are listed in Table 1. The RCP(S) of GdGa compound is appreciable and even larger than those of Gd and some first-order transition materials such as Gd5 Si2 Ge2 and La(Fe0.88 Si0.12 )13 . The relatively high RCP(S) arises from the strongly asymmetric distribution of |SM | and large ␦TFWHM . As shown in Table 1, the ␦TFWHM of GdGa is much larger than those of Gd and other compounds. According to the theoretical calculations of Refs. [12] and [25], the asymmetry of the |SM | versus T curve should be observed for a conventional ferromagnet within the mean field theory approximation. The spin fluctuation when approaching the spin reorientation (TSR ≈ 100 K) can enhance the asymmetry below TC and accordingly result in the larger ␦TFWHM . In addition, the composition inhomogeneity and/or non-stoichiometry of the phase may also contribute to the large ␦TFWHM . Nevertheless, the sharp magnetic transition shown in Fig. 2 and the satisfied Rietveld refinement result indicate that such contributions should not be significant. We also estimate the adiabatic temperature change Tad of the sample from the measured SM using the following expression Tad = −SM

T CP,H

(4)

In our calculation, the lattice and magnetic contributions were included in the specific heat CP,H (CP,H = CD + CH ). The Debye specific heat CD was calculated from  CD = 9NkB

T θD

3  0

θD /T

x 4 ex dx (ex − 1)2

(5)

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J.Y. Zhang et al. / Journal of Alloys and Compounds 469 (2009) 15–19

Table 1 The Curie temperature and magnetocaloric parameters of the GdGa compound in comparison with some important magnetic refrigerant candidate materials Composition GdGa

TC (K) 183

H (T) 5 2

−SM (J/kg K) 4.81 2.33

RCP(S) (J/kg)

␦TFWHM (K)

576a

119.7a

221

94.7

Tad (K)

Reference

4.43 2.06

b b

Gd5 Si2 Ge2

276

5 2

18.4 14.1

443 109

24.0 7.7

15.1 7.4

[5] [5]

Gd

294

5 2

10.2 5.0

410 196

40.2 39.1

11.5 5.6

[5,13] [5]

11

DyCo2

140

5

331

–

6.3

[2,29]

TbCo2

227

5

6.5

247

–

3.6

[2,29]

La0.67 Ba0.33 MnO3

292

5

1.48

161

–

–

[13]

La(Fe0.88 Si0.12 )13

193

5

499

21.7

8.6

[9]

23

In the temperature range where the MCE was measured, the |SM | of the GdGa compound under a field change of 5 T has not yet dropped to its half maximum value at 100 K. In estimating RCP(S), the temperature of 100 K was taken as the left endpoint of the full-width at half-maximum of the |SM | vs. T curve. Therefore, both the ␦TFWHM and the RCP(S) values were underestimated. b Present work. a

where N is the number of atoms per unit mass, kB is the Boltzmann constant, and θ D is the Debye temperature. The magnetic contribution to the specific heat CH is expressed as [26] ∂M ∂M 2 1 − Nint (6) ∂T 2 ∂T In Eq. (6), Hext is the external magnetic field, Nint = 3kB TC /Ns g2 μ2B J(J + 1) is the molecular field constant, Ns is the number of spins per unit mass, g is the Land´e factor and J is the total angular momentum. The calculated specific heat is shown in Fig. 5. The measured values in temperature range of 273–473 K are also plotted in the same figure. In the temperature range far above the Curie temperature (273–473 K), the magnetic contribution to the specific heat can be neglected. Therefore, the Debye temperature can be estimated from fitting the experimental specific heat data in the temperature range far above TC . The Debye temperature of GdGa compound used in the specific heat calculation was estimated as 310 K. The estimated adiabatic temperature changes for a magnetic field variation of 1 T, 2 T, 3 T and 5 T are shown in Fig. 6. The CH = −Hext

Fig. 6. The estimated adiabatic temperature change for different field changes.

maximum temperature change is about half of that for Gd, but close to the values of some manganese oxides due to its smaller heat capacity [27]. 4. Conclusion

Fig. 5. The calculated specific heat of GdGa along with the measured values between 273 K and 473 K.

Polycrystalline sample of GdGa has been prepared by arcmelting method. The structural, magnetic and magnetocaloric properties were studied. The sample crystallizes in a single phase with the CrB structure. The crystal structure was refined by Rieveld profile fitting method. A spin reorientation transition at 100 K was observed on the M–T curve. The magnetic entropy changes of the sample were determined by the magnetization measurements. Adiabatic temperature changes were estimated based on the calculated specific heat. It is worth noting that Eq. (4) for an estimation of the adiabatic temperature change is essentially a rough approximation. First, the specific heat is estimated using a combined experimental and theoretical approach which has some limitations. In addition, Eq. (4) itself is simply a very rough approximation especially near TC as pointed

J.Y. Zhang et al. / Journal of Alloys and Compounds 469 (2009) 15–19

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