J. Cent. South Univ. Technol. (2008) 15: 429-433

DOI: 10.1007/s11771-008-0080-1

Influence of Tb substitution on low-field magnetocaloric effect in Gd₅Si_1.72Ge_2.28 alloy

DENG Jian-qiu(邓健秋)¹, ZHUANG Ying-hong(庄应烘)², WANG Ri-chu(王日初)¹,

YANG Zhen(阳 震)¹, XU Bin(徐 斌)¹

(1. School of Materials Science and Engineering, Central South University, Changsha 410083, China;

2. Key Laboratory of New Processing Technology for Nonferrous Metal and Materials, Ministry of Education,

Guangxi University, Nanning 530004, China)

Abstract: The lattice parameters, magnetic phase transition, Curie temperature and magnetocaloric properties for (Gd_1-xTb_x)₅Si_1.72- Ge_2.28 alloys with x = 0, 0.15, 0.20 and 0.25 were investigated by X-ray powder diffractometry and magnetization measurements. The results show that suitable partial substitution of Tb in Gd₅Si_1.72Ge_2.28 compound remains the first-order magnetic-crystallographic transition and enhances the magnetic entropy change, although Tb substitution decreases the Curie temperature (T_C) of the compounds. The magnetic entropy change of (Gd_1-xTb_x)₅Si_1.72Ge_2.28 alloys retains a large value in the low magnetic field of 1.0 T. The maximum magnetic entropy change for (Gd_0.80Tb_0.20)₅Si_1.72Ge_2.28 alloy in the magnetic field from 0 to 1.0 T reaches 8.7 J/(kg?K), which is nearly 4 times as large as that of (Gd_0.3Dy_0.7)₅Si₄ compound (|?S_max| = 2.24 J/(kg?K), T_C = 198 K).

Key words: rare earth alloy; magnetic entropy change; magnetic phase transition; magnetocaloric effect (MCE)

1 Introduction

The discovery of the giant magnetocaloric effect (GMCE) in Gd₅Si₂Ge₂ by PECHARSKY and GSCHNEIDNER^[1] in 1997 makes Gd₅Si₂Ge₂ a promising candidate for near room temperature magnetic refrigeration material. In recent years, the GMCE and magnetic/structural phase transition in Gd₅(Si_xGe_1-x)₄ have been investigated intensively^[2-8]. The system Gd₅(Si_xGe_1-x)₄ (0.24≤x≤0.50) undergoes a magnetic/ crystallographic phase transition, i.e. the low-temperature orthogonal ferromagnetic phase transforms to the high-temperature monoclinic paramagnetic phase with change of temperature in the applied magnetic field, and exhibits a GMCE.

Gd₅(Si_xGe_1-x)₄ alloys exhibit GMCE at 0.3＜x＜0.5, but increasing the Curie temperature by increasing the ratio Si to Ge leads to the decrease in GMCE^[6]. At room temperature the GMCE of the monoclinic Gd₅(Si_xGe_1-x)₄ alloys is significantly affected by impurities in the starting materials. For the alloy Gd₅Si₂Ge₂ prepared from commercial Gd (molar fraction of 95%-98%) the GMCE is about 1/3 smaller than that prepared from AMES laboratory Gd (molar fraction about 99.8%) and the T_C is shifted from about 280 K for AMES laboratory Gd to about 300 K for the commercial Gd^[3]. The introduction of C, H or O atoms into Gd₅(Si_xGe_1-x)₄ can also increase its T_C, but at the same time, can significantly decrease or destroy the GMCE due to the loss of the first order nature of the magnetic/structural transition^[9-11]. For the Gd₅(Si_xGe_1-x)₄ alloys, the influence of alloying substitution on the GMCE or Curie temperature have been studied. The substitutions of Fe, Co, Ni, Cu or Al for Si and Ge in Gd₅Si₂Ge₂ compound increase its T_C, but have a deleterious effect on the magnetocaloric properties of Gd₅Si₂Ge₂ due to the loss of the first order nature of the structural/magnetic transition of the compound^[9]. Recently, SHULL et al^[12] have reported that doping the Gd₅Ge₂Si₂ compound with approximately 1%(molar fraction) of Cu, Co, Ga, Mn, or Al results in eliminating the field-induced transformation and the hysteresis. The net refrigeration capacity (NRC) values for the doped compounds are about 1.5 times larger than those of the undoped compounds. In addition, the magnetocaloric effect of (Gd_1-xR_x)₅Si₄ (R = Pr, Dy and Tb) alloys have also been reported in Refs.[13-17]. The (Gd_xTb_5-x)Si₄ alloys exhibit magnetocaloric effect in low magnetic field compared with those of pure Gd metal^[13]. NOBREGA et al^[14] calculated the magnetocaloric effect in compound (Gd_0.6Tb_0.4)₅Si₄ using a HAMILTONIAN model of interacting 4f spin and treated the 4f spin-spin interaction in the MONTE-CARLO simulation. The theoretically calculated results are in good agreement with the available experimental data. For (Gd_1-xDy_x)₅- Si₄ alloys, with an increase of the Dy content, the Curie temperature decreases linearly from 338 to 140 K and the magnetic entropy change (MEC) in low magnetic fields retains a high value^[15]. The similar results were reported by SHIMOTOMAI and IDO^[16]. The magnetocaloric effect of Gd_5-xDy_xSi₄ in the vicinity of T_C is found to be comparable to that of Gd metal. Since the Curie temperature of 335 K for Gd₅Si₄ is decreased by the Dy substitution, it is possible to fabricate composite material for magnetic refrigerant by making use of Gd_5-xDy_xSi₄, which are useful in a wide range of temperature in the vicinity of room temperature. However, Dy substitution in compound Gd₅Si₂Ge₂ leads to the loss of the first order magnetic transition, and hence to an overall reduction in MEC compared with that of the parent compound^[17].

In previous work, the magnetic and magnetocaloric properties of (Gd_1-xTb_x)₅Si_1.72Ge_2.28 alloys with x = 0, 0.02, 0.04, 0.06, 0.08, 0.10 and 0.26 were investigated^[18-19]. Tb substitution evidently increases the MEC in alloy Gd₅Si_1.72Ge_2.28. The maximum MECs in an applied field change of 2.0 T for (Gd_0.94Tb_0.06)₅Si_1.72Ge_2.28 and (Gd_0.74Tb_0.26)₅Si_1.72Ge_2.28 alloys reach 25.13 and 18.89 J/(kg?K), respectively. But the magnetic capacities of the compounds were not reported in previous publications. So it is necessary to further study the influence of Tb substitution on the magnetic refrigeration capacities of Gd₅Si_1.72Ge_2.28. The magnetic refrigeration capacities and magnetocaloric effect in alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 (x = 0.15, 0.20 and 0.25) were reported in this work.

2 Experimental

Four (Gd_1-xTb_x)₅Si_1.72Ge_2.28 polycrystalline alloys with x = 0, 0.15, 0.20 and 0.25 were prepared in an electric arc furnace under an argon atmosphere using a non-consumable tungsten electrode and a water-cooled copper tray. Gd, Tb (purity of 99.9%), Si and Ge (purity of 99.999%) were used as starting materials. Ti was used as an O getter during the melting process. To ensure homogeneity, the alloys were re-melted five times to achieve complete fusion and homogeneous composition. Because the as-cast alloy Gd₅Si_1.72Ge_2.28 exhibits GMCE^[2], no heat treatment was performed for all the alloys studied. The phases, Curie temperature and magnetocaloric properties of the alloys were investigated by X-ray powder diffractometry, metallographic analysis and magnetic measurements.

The X-ray powder diffraction data were collected at room temperature on a Rigaku D/Max 2500 diffractometer with Cu K_α radiation and graphite monochromator operated at 40 kV and 200 mA. 2θ was between 20?and 60? with data collection step of 0.02?. The materials data Inc. software JADE 5.0 and the powder diffraction file were used for the phase analysis. The lattice parameters of alloys with Gd₅Si₂Ge₂-type structure in the P1121/a space group symmetry were refined by using the Rietveld technique. Magnetization measurements were carried out using a vibrating-sample magnetometer (VSM, Lake Shore 7410) in an applied field up to 1.0 T. The Curie temperature was identified as the minimum in the first derivative of the (M—T) curve which was obtained at an applied field of 0.01 T. The magnetocaloric effect was evaluated from the calculated the maximum MEC |?S_max (T, B)|, in the vicinity of the Curie temperature according to the thermodynamic Maxwell relation.

3 Results and discussion

3.1 Structure analysis

The X-ray powder diffraction patterns for the as-cast (Gd_1-xTb_x)₅Si_1.72Ge_2.28 alloys with x = 0, 0.15, 0.20 and 0.25 are shown in Fig.1. According to Fig.1, the alloys consist of the (Gd, Tb)₅Si_1.72Ge_2.28 phase with monoclinic Gd₅Si₂Ge₂-type structure and small but detectable fraction of (Gd, Tb)₅Si₄ phase at room temperature. The optical metallographic micrographs of the alloys show the same results that the alloys contain a small amount of impurity phase. Gd₅Si₃-type phase is not found in Gd₅Si_1.72Ge_2.28 with high purity Gd (purity of 99.94%) and Gd(Si, Ge) phase only presents an antiferro- magnetism to paramagnetism transition at the Neel temperature about 57 K^[20]. However, the small drops in M—T curves are present near 280 K. So the second phase in alloys may be Gd₅Si₄-type structure. The similar results have been reported in as-prepared Gd₅Si₂Ge₂ alloys^[3].

Fig.1 X-ray diffraction patterns for (Gd_1-xTb_x)₅Si_1.72Ge_2.28 alloys with x = 0, 0.15, 0.20 and 0.25

Fig.2 shows the optical metallographic micrograph of alloy (Gd_0.80Tb_0.20)₅Si_1.72Ge_2.28. It can be observed that the microstructure is composed of two parts: bright   (Gd, Tb)₅Si_1.72Ge_2.28 matrix phase and dark impurity

Fig.2 Optical metallographic micrograph of (Gd_0.80Tb_0.20)₅- Si_1.72Ge_2.28 alloy

phase (Gd, Tb)₅Si₄. The refined lattice parameters of phase with Gd₅Si₂Ge₂-type structure in these alloys analyzed by using the Rietveld technique are listed in Table 1. It can be seen that the lattice compresses slightly when partial amount of Tb displaces into the lattice. This behavior is expected from the difference between metallic radii of Gd (0.180 1 nm) and Tb (0.178 3 nm). The results indicate that the Tb atom (atomic radius is 0.251 0 nm) may substitute the larger Gd atom (atomic radius is 0.254 0 nm) in alloy Gd₅Si_1.72Ge_2.28. A similar variation has been reported in alloy (Gd_xTb_1-x)₅Si₄^[13]. 330 K for alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 with x = 0, 0.15, 0.20 and 0.25. The Curie temperature T_C was determined by minimum of the derivative of the M—T curve for each alloy. For  alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 the first large drops of M—T curves at 196-247 K correspond to the first order magnetic/structural transitions which can be induced by the applied magnetic field and temperature. The small drops near 280 K are due to the presence of a small amount of (Gd, Tb)₅Si₄ phase. The phenomenon has also been found in alloy Gd₅Si₂Ge₂^[3]. The XRD patterns for alloy (Gd_0.80Tb_0.20)₅Si_1.72Ge_2.28 taken at 260 and 180 K are shown in Fig.4, which indicates that the structural transition from the monoclinic Gd₅Si₂Ge₂type structure to the orthorhombic Gd₅Si₄-type structure exists in alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 during cooling. Partial Tb substitution in Gd₅Si_1.72Ge_2.28 compound remains the first order magnetic-crystallographic transition.

Table 1 Lattice parameters of the monoclinic Gd₅Si₂Ge₂-type phase cell refinement for (Gd_1-xTb_x)₅Si_1.72Ge_2.28 alloys with x = 0, 0.15, 0.20 and 0.25

3.2 Magnetocaloric effect

All the alloys in this work show the first order structural/magnetic transition confirmed by magneti- zation measurement and XRD analysis at different temperatures. Fig.3 shows the temperature dependence of the (M—T curves) measured during heating in an applied field of 0.01 T, and temperature from 140 K up to

Fig.3 Magnetization dependence of temperature (M—T curves) for alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 with x = 0, 0.15, 0.20 and 0.25, measured in applied field of 0.01 T

Fig.4 XRD patterns for alloy (Gd_0.80Tb_0.20)₅Si_1.72Ge_2.28 taken at 260 and 180 K, showing completion from monoclinic to orthorhombic transformation during cooling: (a) T=180 K, orthorhombic Gd₅Si₄-type; (b) T=260 K, monoclinic Gd₅Si₂Ge₂- type

The Curie temperature obtained from the M—T curves varied with different Tb concentrations x are given in Table 2. The variation of T_C with Tb concentration x from 0.15 to 0.25 is linear. The T_C for the pure Gd₅Si_1.72Ge_2.28 compound in this work is 247 K, which is approximately in agreement with that reported in Refs.[4, 7]. From Fig.3 and Table 2, it can be seen that the T_C in alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 decreases with increasing Tb content, showing good agreement with phenomenological expectations. The transition temperature() of the most intra-rare-earth alloys obeys the empirical formula= 46G^2/3 (G is the average de Gennes factor for rare earth)^[21]. For heavy rare earth elements Gd and Tb, G of Gd is larger than that of Tb. So Tb substitution decreases the Curie temperature of alloys. The similar explanation has been illuminated in Ref.[15]. This common phenomenon has also been found in heavy rare earth Gd_5-xDy_xSi₂Ge₂ compounds^[22].

Table 2 Curie temperature, maximum magnetic entropy change |?S_max| and cooling capacity (q) in (Gd_1-xTb_x)₅Si_1.72Ge_2.28 alloys with x = 0, 0.15, 0.20 and 0.25

Magnetization data were collected with a 2-10 K step in temperature and 0.05 T step in the magnetic field from the lowest selected temperature. The measurement sequence at each temperature was carried out during a field increasing from zero magnetic field, after sample temperature was stabilized and held constant for 5 min. After completion of the field dependent measurements at a specific temperature, the alloy was slowly warmed (about 1 K/min) to the next temperature in zero magnetic field. The isothermal magnetization curves (M—B curves) are shown in Fig.5 for alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28(x = 0 and 0.20). Using Maxwell relation, the MEC, |?S_max|, calculated from the isothermal magnetization curves are presented in Table 2 and Fig.6. The MECs reach the maximum near alloys’ Curie temperatures. All the studied alloys show a relative large magnetocaloric effect. The maximum MEC, |?S_max|, for the pure alloy Gd₅Si_1.72Ge_2.28 in the magnetic field change of 0-1.0 T is 5.7 J/(kg?K) at 247 K in this work. For alloy (Gd_0.80- Tb_0.20)₅Si_1.72Ge_2.28, the maximum magnetic entropy change is nearly 4 times as large as that of compound (Gd_0.3Dy_0.7)₅Si₄ (|?S_max| = 2.24 J/(kg?K), T_C = 198 K) in Ref.[14], and reaches 8.7 J/(kg?K). This indicates that appropriate partial substitution of Tb can increase themagnetocaloric effect of Gd₅Si_1.72Ge_2.28 alloy. To combine with previous work in Refs.[18-19], alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 exhibit large |?S_max| in low magnetic fields, and the Curie temperature T_C for this series of alloys can be easily adjusted by changing the Gd and Tb contents.

Fig.5 Isothermal magnetization curves (M—B curves) for alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 with x = 0 (a) and x=0.20 (b), measured at various temperatures around T_C in magnetic field range of 0-1.0 T

Fig.6 Maximum magnetic entropy change, |?S_max|, as function of temperature in magnetic field change of 0-1.0 T for alloys (Gd_1-xTb_x)₅Si_1.72Ge_2.28 with x = 0, 0.15, 0.20 and 0.25

For applications, however, it is interesting to consider the cooling capacity based on the MEC. The cooling capacity is defined as

q =                                (1)

where  T_cold and T_hot are the temperatures of the cold and hot sinks near the temperature corresponding to the magnetic entropy change |?S_max|, respectively^[23]. In order to compare, ?T (i.e. T_hot–T_cold) equals 50 K in all the alloys. The cooling capacities in all alloys are listed in Table 2. For alloys Gd₅Si_1.72Ge_2.28 and (Gd_0.80Tb_0.20)₅- Si_1.72Ge_2.28, the cooling capacities equal 88.9 and 90.2 J/kg, respectively, which are larger than that of Gd metal (q = 77.6 J/kg, ?T = 60 K)^[24].

4 Conclusions

1) Suitable partial substitution of Tb in Gd₅Si_1.72- Ge_2.28 compound can keep their first order magnetic- crystallographic transition and enhance their magnetic entropy change significantly.

2) The as-cast (Gd_1-xTb_x)₅Si_1.72Ge_2.28 alloys consist of (Gd, Tb)₅Si_1.72Ge_2.28 phase with Gd₅Si₂Ge₂-type structure and (Gd, Tb)₅Si₄ phase with Gd₅Si₄-type structure.

3) The Curie temperature of (Gd_1-xTb_x)₅Si_1.72Ge_2.28 alloys decreases by Tb substitution, which is directly proportion to the average de Gennes factor for rare earth.

4) The cooling capacity in the (Gd_1-xTb_x)₅Si_1.72- Ge_2.28 alloys retains a larger value in low magnetic field. It equals 90.2 J/kg in alloy (Gd_0.80Tb_0.20)₅Si_1.72Ge_2.28 in the applied field of 1.0 T.

Acknowledgements

The authors would like to thank Professor G.H. Rao of Institute of Physics (Chinese Academy of Science) for his guidance and help in this work. The X-ray diffraction experiments at low temperature were performed by Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Science.

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Foundation item: Project (50371058) supported by the National Natural Science Foundation of China

Received date: 2007-11-05; Accepted date: 2007-12-18

Corresponding author: WANG Ri-chu, Professor, PhD; Tel: +86-13973121940; E-mail: wrc@mail.csu.edu.cn

(Edited by CHEN Wei-ping)