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Structure, magnetic properties and giant magnetocaloric effect of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy
Intermetallics 57 (2015) 68e72
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Structure, magnetic properties and giant magnetocaloric effect of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy J.X. Min a, X.C. Zhong a, *, V. Franco b, H.C. Tian a, Z.W. Liu a, Z.G. Zheng a, D.C. Zeng a a b
School of Materials Science & Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China Departamento Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 July 2014 Received in revised form 27 September 2014 Accepted 2 October 2014 Available online
Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was prepared by arc melting followed by annealing at 1193 K for 168 h. Structural characterizations reveal that monoclinic (Tb, Gd)5Si2Ge2-type phase, secondary phase with orthorhombic 5-4 type structure and hexagonal 5-3 type structure coexist in the alloy. The paramagnetic Curie temperature (qp) is 120 K, indicating that the dominant exchange interaction is ferromagnetic or ferrimagnetic. That the thermal hysteresis of 13 K between heating and cooling and the negative slopes of Arrott plots derived from MeH curves between 116 K and 170 K confirm a typical first-order magnetic transition from ferromagnetism to paramagnetism occurs. The maximum magnetic entropy changes of the Tb4Gd1Si2.05Ge1.95Mn0.03 alloy for magnetic field changes of 0e1 T, 0e2 T, 0e3 T, 0e4 T and 0e5 T are about 3.3, 8.6, 14.0, 18.9 and 22.4 J/kg K, respectively. And the effective refrigeration capacity (RCeff) value is 231 J/kg with a subtracted magnetic hysteresis loss of 30 J/kg for a magnetic field change from 0 to 5 T. Large DSM and RCeff suggest that Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is an attractive potential magnetocaloric material working in the vicinity of 143 K. © 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Rare-earth intermetallics B. Magnetic properties B. Phase identification G. Magnetic application
1. Introduction Compared with traditional gas compression technology, magnetic refrigeration based on magnetocaloric effect (MCE) possesses some advantages such as energy-saving, efficiency and ecofriendliness. The exploration of potential magnetic refrigeration materials has been accelerated in the recent years after the discovery of the giant magnetocaloric effect [1], with large efforts to increase the magnetic refrigeration efficiency under a relatively low external field [2e4]. Gd5(SixGe1x)4 alloys have been extensively investigated because of their giant magnetocaloric effect [1,5e9]. It is reported that the primacy magnetic transitions are undergone when varying the Si/Ge ratios [6e8]: (i) The second-order magnetic transition from ferromagnetic (FM) to paramagnetic (PM) between 300 K and 335 K when 0.575 < x < 1. (ii) The first-order transformation from the monoclinic Gd5Si2Ge2-type structure (PM) to the orthorhombic Gd5Si4-type structure (FM) between 270 K and 320 K when 0.40
* Corresponding author. Tel.: þ86 2087111312. E-mail address: [email protected] (X.C. Zhong). http://dx.doi.org/10.1016/j.intermet.2014.10.002 0966-9795/© 2014 Elsevier Ltd. All rights reserved.
magnetocaloric effect of Tb5SixGe4x alloys [10e14], as well as other element doping into Gd5Si2Ge2 [15e17]. Tb is next to Gd in the periodic table, and its magnetic moment (9.77 mB) is higher than that of Gd atom (7.94 mB). In Thuy's work [10], the transition temperature of Tb5Si2Ge2 is found to be 116 K, and for Tb5(SixGe1x)4 with x ¼ 0.75 and 1.0, the Curie temperature increases to 215 and 223.2 K respectively. The peak magnetic entropy changes peak DSM for Tb5Si2Ge2 and Tb5Si3Ge1 are 12.8 and 6.7 J/kg K in an applied field change of 5 T, respectively. They also found that the transition from orthorhombic phase with 5-4 type structure to monoclinic phase with 5-2-2 type structure could occur with much smaller Ge content. The Influences of transition metal Fe, Mn additions on magnetocaloric effect of Tb5(SixGe1x)4 alloys were also investigated [13,14]. With Fe alloying [13], unit cell volumes of Tb5Si2xGe2xFe2x alloys decrease, but the transition temperatures remain unchanged. The field-induced first-order phase transition is suppressed, which results in a low magnetic hysteresis loss. Substitution of Mn atom [14] increases the Curie temperatures and magnetic entropy changes of Tb5Si2xGe2xMn2x (2x ¼ 0, 0.08, 0.1) value of Tb5Si1.95Ge1.95Mn0.1 is about 20.84 J/ alloys. The DSpeak M kg around 123 K for a magnetic field change of 0e5 T. In our previous work [18], the results showed that Gd5Si2.05xGe1.95xNi2x alloys with 2x ¼ 0.1 obtained a nearly single 5-2-2 phase, but when decreasing Ni content to 2x ¼ 0.08, the formation of 5-2-2 phase
J.X. Min et al. / Intermetallics 57 (2015) 68e72
was suppressed, thus the hysteresis effect can be reduced evidently. Likewise, Shull's work [17] showed the Gd5Si2Ge2 alloy remained the single phase with monoclinic 5-2-2 type when Bi and Sn replacing Ge, and the doping had little effect on hysteresis effect. However, a small amount of Cu, Ga, Mn, and Al substituting for Ge could completely eliminate the large hysteresis effect, which was similar to the result of the Fe-doped alloy [16]. In this work, the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was prepared, the work about the structure, magnetic property and magnetocaloric effect of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was initiated. 2. Experimental procedures The bulk Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was prepared by arcmelting pure elements (Tb, Gd > 99.9 wt.%; Si, Ge > 99.999 wt.%) in a water-cooled copper crucible with a non-consumable tungsten electrode under argon atmosphere. The ingot was turned over and re-melted several times to ensure the compositional homogeneity. The as-cast alloy was sealed in a quartz tube under purified argon, annealed at 1193 K for 168 h and subsequently quenched in ice water. X-ray diffraction (XRD) spectra were taken at room temperature by using a PANalytical diffractometer with Cu-Ka radiation (1.54056 Å) to identify the phase structure. The microstructure characterization was performed using an optical microscope (OM) (model DMIRM) and a FEI scanning electron microscope (SEM) (model Nova Nano SEM 430) with an Oxford energy dispersive Xray spectrometer (EDS) (model Inca Energy 350). The magnetic properties were measured in a Quantum Design physical property measurement system (model PPMS-9) with a Vibrating Sample Magnetometer option.
69
In order to identify the phases of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy clearly, examination of the sample was performed using optical microscopy (OM) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The faceted surfaces of the sample are shown in Fig. 2. The light matrix regions present the 5:4 structure, which is related to the monoclinic 5-2-2 phase. The orthorhombic 5-4 phase lies in the linear dark grain boundary. A set of dark linear features grew in specific directions in the light €tten structure [19,20]. In matrix region are known as Widmansta general, the linear features all maintain a high ratio of its length to width with lengths up to 100 mm and thicknesses less than 1 mm. The phase composition of the matrix, the dark boundary phase and the thin dark linear feature in Fig. 2(b) determined by EDS are listed in Table 1. Because of the minor quantity of Mn addition and the accuracy limit of the EDS employed, no Mn can be detected. Although they are too small to obtain reliable composition data using EDS in SEM, the thin linear dark features (regions presented as C and D) have been confirmed to be hexagonal (Tb, Gd)5(Si, Ge)3 [21,22]. The temperature dependence of magnetization (MeT) measured under the field of 0.05 T during the cooling (FC) and
3. Results and discussion From the XRD pattern of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy (Fig. 1), it can be seen that the 5-2-2 type main phase with monoclinic structure (space group, P1121/a), the secondary phase as 5-4 type (Tb, Gd)5(Si, Ge, Mn)4 phase with orthorhombic structure and 5-3 type (Tb, Gd)5(Si, Ge, Mn)3 phase with hexagonal structure coexist in the alloy. According to the Joint Committee on Powder Diffraction Standards (JCPDS) tables, the XRD peak at ~31.3 corresponds to ð231Þ plane of monoclinic phase, and the peaks at ~68.0 and 73.4 correspond to (0102) and (392) planes of orthorhombic phase. The overlapping peaks at ~35.3 and 36.4 are the signature peaks of hexagonal 5-3 phase corresponding to (211) and (300) planes.
Fig. 1. XRD pattern of the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy.
Fig. 2. Optical micrograph (a), and backscattered electron micrograph (b) of the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy.
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J.X. Min et al. / Intermetallics 57 (2015) 68e72
Table 1 Phase compositional analysis of different regions in Tb4GdSi2.035Ge1.935Mn0.03 alloy using energy disperse spectroscopy. Region
A B C D E a
Compositiona (at.%) Tb
Gd
Si
Ge
Mn
44.67 41.97 45.84 47.34 47.18
11.81 12.21 13.11 12.53 13.76
23.61 23.16 20.38 16.55 13.94
19.91 22.66 20.67 23.58 25.12
e e e e e
Phase type
Scanning method
e 5-2-2 5-3 5-3 5-4
Area Area Point Point Point
Excluding oxygen.
heating (FH) processes is shown in Fig. 3(a). As shown in the inset of the Fig. 3(a), the transition temperature is determined as the minima of the dM/dT curve. The Curie temperature (TC) is 143 K for FH process, corresponding to the monoclinic 5-2-2 type phase. As reported earlier, the TC of Tb5Si2Ge2 is ~105 K [11] (76 K for Ref. [12]), and the transition temperature of Gd5Si2Ge2 is 276 K [1]. Hence, the substitution of Gd for Tb increases the transition temperature of the alloy. According to the FC and FH curves, a thermal hysteresis of 13 K can be observed, indicating that the alloy undergoes a first-order magnetic transition (FOMT) which is the origin of giant magnetocaloric effect. There are two steps of the M vs. T curve (two minimum of dM/ dT vs. T curve) above 150 K. Judging from the Gd5Si4eGd5Ge4 pseudo binary phase diagram [6], Gd5SixGe4x (0 < x < 0.3) alloys undergo a first-order transition from Gd5Si4-type structure to
Fig. 3. (a) Temperature dependence of magnetization under a magnetic field of 0.05 T for the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy, the inset displays the curve of dM vs. dT. (b) Inverse susceptibility versus temperature derived from FH curve.
Sm5Ge4-type structure at low temperature region, thus the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy may exhibit a similar transition at 173 K due to Ge-rich Sm5Ge4-type structure. The later transition around 214 K is related to a second order magnetic transition due to (Tb, Gd)5(Si, Ge, Mn)4 phase with a negligible thermal hysteresis of 5 K, comparable to the thermal inertia during heating and cooling. However, owing to the fact that magnetization measurements are insensitive even for 10e20 vol.% 5-3 phase contained in 5-2-2 alloys [22], the antiferromagnetic transition of (Tb, Gd)5(Si, Ge, Mn)3 is not detected. It can be observed that there is a peak related to a spin reorientation transition in Fig. 3(a) around 33 K, the transition can be known as the canted ferromagnetic to ferromagnetic ordering of the Tb magnetic moments [13,23,24]. Moreover, the FH data at high temperature follows the CurieeWeiss law, as shown in Fig. 3(b). The paramagnetic Curie temperature (qp) is derived as the intercept by extrapolating the linear part of inverse susceptibility at high temperature. The positive qp (~120 K) means that the dominant exchange interaction of the alloy is ferromagnetic or ferrimagnetic, with a slope giving a paramagnetic moment of 9.13 mB for Tb4Gd1Si2.035Ge1.935Mn0.03, which is close to 9.1 mB for Tb5Si2Ge2 compound [10]. Fig. 4(a) displays the magnetization as a function of magnetic field of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy measured around the TC. The alloy was firstly cooled down to 116 K and remained for 10 min at zero field, and then the plots of M vs. H curves were recorded when increasing and decreasing a magnetic field of 5 T. Using a Landau type expansion of the thermodynamic potential near the Curie temperature [25]:
Fig. 4. Magnetization isotherms (a) and Arrott plots (b) derived from M(H) data for Tb4Gd1Si2.035Ge1.935Mn0.03 alloy.
J.X. Min et al. / Intermetallics 57 (2015) 68e72
G T; M ¼ G0 þ AM2 þ BM 4 þ / MH
(1)
where A, B are the Landau coefficients, Arrott plots can be obtained after transformation:
M2 ¼
1 H A $ þ 4B M 2B
(2)
According to Banerjee criterion [26], a material undergoes a first-order phase transition if B < 0. Fig. 4(b) shows the Arrott plots derived from MeH curves of the alloy. The curves with negative slope or S-shaped suggest a typical first-order magnetic transition from ferromagnetic state to paramagnetic state around the TC. Although it has been recently demonstrated that this criterion could lead to erroneous results [27], the failure of the criterion goes in the sense of identifying a first order phase transition as a second order one, which is not the case here. Moreover, the thermal hysteresis supports this identification as a first order phase transition. According to Maxwell relation, the value of DSM can be calculated from the isothermal magnetization curves around TC. However, care has to be taken when using Maxwell relation for the determination of the magnetic entropy changes of first order phase transition materials [28]. Recent studies [29] indicate that if the field is large enough to induce the magneto-structural transition, the magnetic entropy change spikes appear at different temperatures when increasing and decreasing field, being more relevant for materials with larger thermal hysteresis. Therefore, the calculation of the DSM for both directions of the field change can give an idea of the reliability of the results. Due to their reduced thermal hysteresis when compared to other relevant giant magnetocaloric materials, GdSiGe-type compounds do not exhibit too large spurious spikes, unless the temperature difference between isotherms is well below the thermal hysteresis, which is not the case in this study. The DSM value of the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy as a function of temperature for increasing and decreasing magnetic field is shown in Fig. 5. It is observed that a field-induced first-order crystallographic phase change [30] results in the peak of (DSM) shifting tardily from 138.5 K under low field to 142 K under high field (0e5 T). The DSpeak values for the Tb4Gd1Si2.035M Ge1.935Mn0.03 alloy for a magnetic field changes of 0e1 T, 0e2 T,
71
0e3 T, 0e4 T and 0e5 T are about 3.3, 8.6, 14.0, 18.9 and 22.4 J/kg K, peak respectively. Compared with the DSM of Gd (9.7 J/kg K, 0e5 T) [31], Gd5Si2Ge2 (20 J/kg K, 0e5 T) [16], and Tb5Si2Ge2 (12.91 J/kg K [13], 21.8 J/kg K [11], 0e5 T), the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is more competitive for application in magnetic refrigeration. Besides (DSM), refrigeration capacity (RC) is another effective criterion for characterizing the refrigerant efficiency. The value could be calculated [32] as equation (3):
RC ¼
ZT2 max ΔSM dT
(3)
T1
Where T1 and T2 are the initial and terminal temperatures at halfmaximum of the (DSM) T curves. The RC value for Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is 261 J/kg for a magnetic field of 0e5 T. The hysteresis loss was found as the area between the M vs. H curves during increasing and decreasing field [33,34] and plotted in the inset of Fig. 5. The average hysteretic loss determined by integrating the area under the curves of hysteresis losses versus temperature using the same temperature range for the calculation of RC as the integration limits [33,35] is found to be 30 J/kg, which is lower than that of the Gd5Si2Ge2 [16] (average value is about 65 J/ kg), and the effective RC (RCeff) is 231 J/kg. 4. Conclusions The 5-2-2 type main phase with monoclinic structure, as well as secondary phase with 5-4 type orthorhombic structure and 5-3 type hexagonal structure are obtained in Tb4Gd1Si2.035Ge1.935Mn0.03 alloy. An obvious first-order magnetic transition undergoes around 143 K with a thermal hysteresis of 13 K. The inverse susceptibility as a function of temperature at high temperature obeys the CurieeWeiss law, and the paramagnetic Curie temperature and effective paramagnetic moment are 120 K and 9.13 mB, respectively. A spin reorientation transition corresponding to canted ferromagnetic to ferromagnetic ordering of the Tb magnetic moments occurs at about 33 K. Under an applied field change from 0 to 5 T, the DSpeak and RCeff of Tb4Gd1Si2.035Ge1.935Mn0.03 M compound are 22.4 J/kg K and 231 J/kg, respectively, and the average magnetic hysteresis loss of the alloy is 30 J/kg (already subtracted from RC), which is about 11% of the total RC value. Lower hysteresis loss, large (DSM) and RCeff suggest that Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is an attractive potential magnetocaloric material working in the vicinity of 143 K. Acknowledgments This work was partly supported by the Guangzhou Municipal Science and Technology Program (Grant No. 12F582080022) and the Fundamental Research Funds for the Central Universities, (Grant Nos. 2014ZZ0005 and 2012ZZ0013), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Grant No. x2clB7120290), and the Guangdong Provincial Science and Technology Program (Grant Nos. 2010B050300008 and 2009B090300273). References
Fig. 5. Magnetic entropy change as a function of temperature for Tb4Gd1Si2.035Ge1.935Mn0.03 alloy under a magnetic field change of 0e1 T, 0e2 T, 0e3 T, 0e4 T, and 0e5 T, for increasing and decreasing applied magnetic fields, respectively. The inset is the hysteresis loss for the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy under a magnetic field change from 0 to 5 T.
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[5] Pecharsky VK, Gschneidner Jr KA. Appl Phys Lett 1997;70(24):3299e301. [6] Pecharsky AO, Gschneidner Jr KA, Pecharsky VK, Schindler CE. J Alloys Compd 2002;338:126e35. [7] Gschneidner Jr KA. J Alloys Compd 2002;344:356e61. [8] Gschneidner Jr KA, Pecharsky VK, Tsokol AO. Rep Prog Phys 2005;68: 1479e539. [9] Gschneidner Jr KA, Mudryk Y, Pecharsky VK. Scr Mater 2012;67:572e7. [10] Thuy NP, Nong NV, Hien NT, Tai LT, Vinh TQ, Thang PD, et al. J Magn Magn Mater 2002;242e245:841e3. [11] Morellon L, Magen C, Algarabel PA, Ibarra MR, Ritter C. Appl Phys Lett 2001;79:1318e20. [12] Tegus O, Dagula O, Brück E, Zhang L, de Boer FR, Buschow KHJ. J Appl Phys 2002;91(10):8534e6. [13] Yüzüak E, Emre B, Yücel A, Elerman Y. J Alloys Compd 2009;476:929e34. [14] Yüzüak E, Emre B, Elerman Y, Yücel A. Chin Phys B 2010;19(5):057501. [15] Pecharsky VK, Gschneidner Jr KA. J Magn Magn Mater 1997;167:L179e84. [16] Provenzano V, Shapiro AJ, Shull RD. Nature 2004;429:853e7. [17] Shull RD, Provenzano V, Shapiro AJ, Fu A, Lufaso MW, Karapetrova J, et al. J Appl Phys 2006;99:08K908. [18] Zhong XC, Min JX, Liu ZW, Zheng ZG, Zeng DC, Franco V, et al. J Appl Phys 2013;113:17A916. [19] Raj Kumar DM, Raja Manivel M, Gopalan R, Singh AK, Chandrasekaran V, Suresh KG. Intermet 2010;18:518e22. [20] Raj Kumar DM, Raja Manivel M, Prabahar K, Chandrasekaran V, Poddar A, Ranganathan R, et al. J Magn Magn Mater 2011;323:1750e4. [21] Ugurlu O, Chumbley LS, Schlagel DL, Lograsso TA, Tsokol AO. Scr Mater 2005;53:373e7.
[22] Chumbley LS, Ugurlu O, McCallum RW, Dennis KW, Mudryk Y, Gschneidner Jr KA, et al. Acta Mater 2008;56:527e36. [23] Morellon L, Ritter C, Magen C, Algarabel PA, Ibarra MR. Phys Rev B 2003;68: 024417. [24] Ritter C, Morellon L, Algarabel PA, Magen C, Ibarra MR. Phys Rev B 2002;65: 094405. [25] Amaral JS, Amaral VS. J Magn Magn Mater 2010;322:1552e7. [26] Banerjee BK. Phys Lett 1964;12:16e7. F, García LM, Parra-Borderías M, [27] Bonilla CM, Herrero-Albillos J, Bartolome Franco V. Phys Rev B 2010;81:224424. [28] Tocado L, Palacios E, Burriel R. J Appl Phys 2009;105:093918. [29] Wang GF. Magnetic and calorimetric study of the magnetocaloric effect in intermetallics exhibiting first-order magnetostructural transitions. Spain: Universidad de Zaragoza; 2012. p. 63e99 [PhD thesis]. [30] Choe W, Pecharsky VK, Pecharsky AO, Gschneidner Jr KA, Young Jr VG, Miller GJ. Phys Rev Lett 2000;84:4617e20. [31] Shen J, Wu JF, Sun JR. J Appl Phys 2009;106:083902. [32] Gschneidner Jr KA, Pecharsky VK, Pecharsky AO, Zimm CB. Mater Sci Forum 1999;315e317:69e76. [33] Pathak AK, Dubenko I, Karaca HE, Stadler S, Ali N. Appl Phys Lett 2010;97: 062505. [34] Ma SC, Zheng YX, Xuan HC, Shen LJ, Cao QQ, Wang DH, et al. J Magn Magn Mater 2012;324:135e9. [35] Ma SC, Cao QQ, Xuan HC, Zhang CL, Shen LJ, Wang DH, et al. J Alloys Compd 2011;509:1111e4.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Structure, magnetic properties and giant magnetocaloric effect of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy J.X. Min a, X.C. Zhong a, *, V. Franco b, H.C. Tian a, Z.W. Liu a, Z.G. Zheng a, D.C. Zeng a a b
School of Materials Science & Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China Departamento Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 July 2014 Received in revised form 27 September 2014 Accepted 2 October 2014 Available online
Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was prepared by arc melting followed by annealing at 1193 K for 168 h. Structural characterizations reveal that monoclinic (Tb, Gd)5Si2Ge2-type phase, secondary phase with orthorhombic 5-4 type structure and hexagonal 5-3 type structure coexist in the alloy. The paramagnetic Curie temperature (qp) is 120 K, indicating that the dominant exchange interaction is ferromagnetic or ferrimagnetic. That the thermal hysteresis of 13 K between heating and cooling and the negative slopes of Arrott plots derived from MeH curves between 116 K and 170 K confirm a typical first-order magnetic transition from ferromagnetism to paramagnetism occurs. The maximum magnetic entropy changes of the Tb4Gd1Si2.05Ge1.95Mn0.03 alloy for magnetic field changes of 0e1 T, 0e2 T, 0e3 T, 0e4 T and 0e5 T are about 3.3, 8.6, 14.0, 18.9 and 22.4 J/kg K, respectively. And the effective refrigeration capacity (RCeff) value is 231 J/kg with a subtracted magnetic hysteresis loss of 30 J/kg for a magnetic field change from 0 to 5 T. Large DSM and RCeff suggest that Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is an attractive potential magnetocaloric material working in the vicinity of 143 K. © 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Rare-earth intermetallics B. Magnetic properties B. Phase identification G. Magnetic application
1. Introduction Compared with traditional gas compression technology, magnetic refrigeration based on magnetocaloric effect (MCE) possesses some advantages such as energy-saving, efficiency and ecofriendliness. The exploration of potential magnetic refrigeration materials has been accelerated in the recent years after the discovery of the giant magnetocaloric effect [1], with large efforts to increase the magnetic refrigeration efficiency under a relatively low external field [2e4]. Gd5(SixGe1x)4 alloys have been extensively investigated because of their giant magnetocaloric effect [1,5e9]. It is reported that the primacy magnetic transitions are undergone when varying the Si/Ge ratios [6e8]: (i) The second-order magnetic transition from ferromagnetic (FM) to paramagnetic (PM) between 300 K and 335 K when 0.575 < x < 1. (ii) The first-order transformation from the monoclinic Gd5Si2Ge2-type structure (PM) to the orthorhombic Gd5Si4-type structure (FM) between 270 K and 320 K when 0.40
* Corresponding author. Tel.: þ86 2087111312. E-mail address: [email protected] (X.C. Zhong). http://dx.doi.org/10.1016/j.intermet.2014.10.002 0966-9795/© 2014 Elsevier Ltd. All rights reserved.
magnetocaloric effect of Tb5SixGe4x alloys [10e14], as well as other element doping into Gd5Si2Ge2 [15e17]. Tb is next to Gd in the periodic table, and its magnetic moment (9.77 mB) is higher than that of Gd atom (7.94 mB). In Thuy's work [10], the transition temperature of Tb5Si2Ge2 is found to be 116 K, and for Tb5(SixGe1x)4 with x ¼ 0.75 and 1.0, the Curie temperature increases to 215 and 223.2 K respectively. The peak magnetic entropy changes peak DSM for Tb5Si2Ge2 and Tb5Si3Ge1 are 12.8 and 6.7 J/kg K in an applied field change of 5 T, respectively. They also found that the transition from orthorhombic phase with 5-4 type structure to monoclinic phase with 5-2-2 type structure could occur with much smaller Ge content. The Influences of transition metal Fe, Mn additions on magnetocaloric effect of Tb5(SixGe1x)4 alloys were also investigated [13,14]. With Fe alloying [13], unit cell volumes of Tb5Si2xGe2xFe2x alloys decrease, but the transition temperatures remain unchanged. The field-induced first-order phase transition is suppressed, which results in a low magnetic hysteresis loss. Substitution of Mn atom [14] increases the Curie temperatures and magnetic entropy changes of Tb5Si2xGe2xMn2x (2x ¼ 0, 0.08, 0.1) value of Tb5Si1.95Ge1.95Mn0.1 is about 20.84 J/ alloys. The DSpeak M kg around 123 K for a magnetic field change of 0e5 T. In our previous work [18], the results showed that Gd5Si2.05xGe1.95xNi2x alloys with 2x ¼ 0.1 obtained a nearly single 5-2-2 phase, but when decreasing Ni content to 2x ¼ 0.08, the formation of 5-2-2 phase
J.X. Min et al. / Intermetallics 57 (2015) 68e72
was suppressed, thus the hysteresis effect can be reduced evidently. Likewise, Shull's work [17] showed the Gd5Si2Ge2 alloy remained the single phase with monoclinic 5-2-2 type when Bi and Sn replacing Ge, and the doping had little effect on hysteresis effect. However, a small amount of Cu, Ga, Mn, and Al substituting for Ge could completely eliminate the large hysteresis effect, which was similar to the result of the Fe-doped alloy [16]. In this work, the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was prepared, the work about the structure, magnetic property and magnetocaloric effect of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was initiated. 2. Experimental procedures The bulk Tb4Gd1Si2.035Ge1.935Mn0.03 alloy was prepared by arcmelting pure elements (Tb, Gd > 99.9 wt.%; Si, Ge > 99.999 wt.%) in a water-cooled copper crucible with a non-consumable tungsten electrode under argon atmosphere. The ingot was turned over and re-melted several times to ensure the compositional homogeneity. The as-cast alloy was sealed in a quartz tube under purified argon, annealed at 1193 K for 168 h and subsequently quenched in ice water. X-ray diffraction (XRD) spectra were taken at room temperature by using a PANalytical diffractometer with Cu-Ka radiation (1.54056 Å) to identify the phase structure. The microstructure characterization was performed using an optical microscope (OM) (model DMIRM) and a FEI scanning electron microscope (SEM) (model Nova Nano SEM 430) with an Oxford energy dispersive Xray spectrometer (EDS) (model Inca Energy 350). The magnetic properties were measured in a Quantum Design physical property measurement system (model PPMS-9) with a Vibrating Sample Magnetometer option.
69
In order to identify the phases of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy clearly, examination of the sample was performed using optical microscopy (OM) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The faceted surfaces of the sample are shown in Fig. 2. The light matrix regions present the 5:4 structure, which is related to the monoclinic 5-2-2 phase. The orthorhombic 5-4 phase lies in the linear dark grain boundary. A set of dark linear features grew in specific directions in the light €tten structure [19,20]. In matrix region are known as Widmansta general, the linear features all maintain a high ratio of its length to width with lengths up to 100 mm and thicknesses less than 1 mm. The phase composition of the matrix, the dark boundary phase and the thin dark linear feature in Fig. 2(b) determined by EDS are listed in Table 1. Because of the minor quantity of Mn addition and the accuracy limit of the EDS employed, no Mn can be detected. Although they are too small to obtain reliable composition data using EDS in SEM, the thin linear dark features (regions presented as C and D) have been confirmed to be hexagonal (Tb, Gd)5(Si, Ge)3 [21,22]. The temperature dependence of magnetization (MeT) measured under the field of 0.05 T during the cooling (FC) and
3. Results and discussion From the XRD pattern of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy (Fig. 1), it can be seen that the 5-2-2 type main phase with monoclinic structure (space group, P1121/a), the secondary phase as 5-4 type (Tb, Gd)5(Si, Ge, Mn)4 phase with orthorhombic structure and 5-3 type (Tb, Gd)5(Si, Ge, Mn)3 phase with hexagonal structure coexist in the alloy. According to the Joint Committee on Powder Diffraction Standards (JCPDS) tables, the XRD peak at ~31.3 corresponds to ð231Þ plane of monoclinic phase, and the peaks at ~68.0 and 73.4 correspond to (0102) and (392) planes of orthorhombic phase. The overlapping peaks at ~35.3 and 36.4 are the signature peaks of hexagonal 5-3 phase corresponding to (211) and (300) planes.
Fig. 1. XRD pattern of the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy.
Fig. 2. Optical micrograph (a), and backscattered electron micrograph (b) of the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy.
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J.X. Min et al. / Intermetallics 57 (2015) 68e72
Table 1 Phase compositional analysis of different regions in Tb4GdSi2.035Ge1.935Mn0.03 alloy using energy disperse spectroscopy. Region
A B C D E a
Compositiona (at.%) Tb
Gd
Si
Ge
Mn
44.67 41.97 45.84 47.34 47.18
11.81 12.21 13.11 12.53 13.76
23.61 23.16 20.38 16.55 13.94
19.91 22.66 20.67 23.58 25.12
e e e e e
Phase type
Scanning method
e 5-2-2 5-3 5-3 5-4
Area Area Point Point Point
Excluding oxygen.
heating (FH) processes is shown in Fig. 3(a). As shown in the inset of the Fig. 3(a), the transition temperature is determined as the minima of the dM/dT curve. The Curie temperature (TC) is 143 K for FH process, corresponding to the monoclinic 5-2-2 type phase. As reported earlier, the TC of Tb5Si2Ge2 is ~105 K [11] (76 K for Ref. [12]), and the transition temperature of Gd5Si2Ge2 is 276 K [1]. Hence, the substitution of Gd for Tb increases the transition temperature of the alloy. According to the FC and FH curves, a thermal hysteresis of 13 K can be observed, indicating that the alloy undergoes a first-order magnetic transition (FOMT) which is the origin of giant magnetocaloric effect. There are two steps of the M vs. T curve (two minimum of dM/ dT vs. T curve) above 150 K. Judging from the Gd5Si4eGd5Ge4 pseudo binary phase diagram [6], Gd5SixGe4x (0 < x < 0.3) alloys undergo a first-order transition from Gd5Si4-type structure to
Fig. 3. (a) Temperature dependence of magnetization under a magnetic field of 0.05 T for the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy, the inset displays the curve of dM vs. dT. (b) Inverse susceptibility versus temperature derived from FH curve.
Sm5Ge4-type structure at low temperature region, thus the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy may exhibit a similar transition at 173 K due to Ge-rich Sm5Ge4-type structure. The later transition around 214 K is related to a second order magnetic transition due to (Tb, Gd)5(Si, Ge, Mn)4 phase with a negligible thermal hysteresis of 5 K, comparable to the thermal inertia during heating and cooling. However, owing to the fact that magnetization measurements are insensitive even for 10e20 vol.% 5-3 phase contained in 5-2-2 alloys [22], the antiferromagnetic transition of (Tb, Gd)5(Si, Ge, Mn)3 is not detected. It can be observed that there is a peak related to a spin reorientation transition in Fig. 3(a) around 33 K, the transition can be known as the canted ferromagnetic to ferromagnetic ordering of the Tb magnetic moments [13,23,24]. Moreover, the FH data at high temperature follows the CurieeWeiss law, as shown in Fig. 3(b). The paramagnetic Curie temperature (qp) is derived as the intercept by extrapolating the linear part of inverse susceptibility at high temperature. The positive qp (~120 K) means that the dominant exchange interaction of the alloy is ferromagnetic or ferrimagnetic, with a slope giving a paramagnetic moment of 9.13 mB for Tb4Gd1Si2.035Ge1.935Mn0.03, which is close to 9.1 mB for Tb5Si2Ge2 compound [10]. Fig. 4(a) displays the magnetization as a function of magnetic field of Tb4Gd1Si2.035Ge1.935Mn0.03 alloy measured around the TC. The alloy was firstly cooled down to 116 K and remained for 10 min at zero field, and then the plots of M vs. H curves were recorded when increasing and decreasing a magnetic field of 5 T. Using a Landau type expansion of the thermodynamic potential near the Curie temperature [25]:
Fig. 4. Magnetization isotherms (a) and Arrott plots (b) derived from M(H) data for Tb4Gd1Si2.035Ge1.935Mn0.03 alloy.
J.X. Min et al. / Intermetallics 57 (2015) 68e72
G T; M ¼ G0 þ AM2 þ BM 4 þ / MH
(1)
where A, B are the Landau coefficients, Arrott plots can be obtained after transformation:
M2 ¼
1 H A $ þ 4B M 2B
(2)
According to Banerjee criterion [26], a material undergoes a first-order phase transition if B < 0. Fig. 4(b) shows the Arrott plots derived from MeH curves of the alloy. The curves with negative slope or S-shaped suggest a typical first-order magnetic transition from ferromagnetic state to paramagnetic state around the TC. Although it has been recently demonstrated that this criterion could lead to erroneous results [27], the failure of the criterion goes in the sense of identifying a first order phase transition as a second order one, which is not the case here. Moreover, the thermal hysteresis supports this identification as a first order phase transition. According to Maxwell relation, the value of DSM can be calculated from the isothermal magnetization curves around TC. However, care has to be taken when using Maxwell relation for the determination of the magnetic entropy changes of first order phase transition materials [28]. Recent studies [29] indicate that if the field is large enough to induce the magneto-structural transition, the magnetic entropy change spikes appear at different temperatures when increasing and decreasing field, being more relevant for materials with larger thermal hysteresis. Therefore, the calculation of the DSM for both directions of the field change can give an idea of the reliability of the results. Due to their reduced thermal hysteresis when compared to other relevant giant magnetocaloric materials, GdSiGe-type compounds do not exhibit too large spurious spikes, unless the temperature difference between isotherms is well below the thermal hysteresis, which is not the case in this study. The DSM value of the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy as a function of temperature for increasing and decreasing magnetic field is shown in Fig. 5. It is observed that a field-induced first-order crystallographic phase change [30] results in the peak of (DSM) shifting tardily from 138.5 K under low field to 142 K under high field (0e5 T). The DSpeak values for the Tb4Gd1Si2.035M Ge1.935Mn0.03 alloy for a magnetic field changes of 0e1 T, 0e2 T,
71
0e3 T, 0e4 T and 0e5 T are about 3.3, 8.6, 14.0, 18.9 and 22.4 J/kg K, peak respectively. Compared with the DSM of Gd (9.7 J/kg K, 0e5 T) [31], Gd5Si2Ge2 (20 J/kg K, 0e5 T) [16], and Tb5Si2Ge2 (12.91 J/kg K [13], 21.8 J/kg K [11], 0e5 T), the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is more competitive for application in magnetic refrigeration. Besides (DSM), refrigeration capacity (RC) is another effective criterion for characterizing the refrigerant efficiency. The value could be calculated [32] as equation (3):
RC ¼
ZT2 max ΔSM dT
(3)
T1
Where T1 and T2 are the initial and terminal temperatures at halfmaximum of the (DSM) T curves. The RC value for Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is 261 J/kg for a magnetic field of 0e5 T. The hysteresis loss was found as the area between the M vs. H curves during increasing and decreasing field [33,34] and plotted in the inset of Fig. 5. The average hysteretic loss determined by integrating the area under the curves of hysteresis losses versus temperature using the same temperature range for the calculation of RC as the integration limits [33,35] is found to be 30 J/kg, which is lower than that of the Gd5Si2Ge2 [16] (average value is about 65 J/ kg), and the effective RC (RCeff) is 231 J/kg. 4. Conclusions The 5-2-2 type main phase with monoclinic structure, as well as secondary phase with 5-4 type orthorhombic structure and 5-3 type hexagonal structure are obtained in Tb4Gd1Si2.035Ge1.935Mn0.03 alloy. An obvious first-order magnetic transition undergoes around 143 K with a thermal hysteresis of 13 K. The inverse susceptibility as a function of temperature at high temperature obeys the CurieeWeiss law, and the paramagnetic Curie temperature and effective paramagnetic moment are 120 K and 9.13 mB, respectively. A spin reorientation transition corresponding to canted ferromagnetic to ferromagnetic ordering of the Tb magnetic moments occurs at about 33 K. Under an applied field change from 0 to 5 T, the DSpeak and RCeff of Tb4Gd1Si2.035Ge1.935Mn0.03 M compound are 22.4 J/kg K and 231 J/kg, respectively, and the average magnetic hysteresis loss of the alloy is 30 J/kg (already subtracted from RC), which is about 11% of the total RC value. Lower hysteresis loss, large (DSM) and RCeff suggest that Tb4Gd1Si2.035Ge1.935Mn0.03 alloy is an attractive potential magnetocaloric material working in the vicinity of 143 K. Acknowledgments This work was partly supported by the Guangzhou Municipal Science and Technology Program (Grant No. 12F582080022) and the Fundamental Research Funds for the Central Universities, (Grant Nos. 2014ZZ0005 and 2012ZZ0013), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Grant No. x2clB7120290), and the Guangdong Provincial Science and Technology Program (Grant Nos. 2010B050300008 and 2009B090300273). References
Fig. 5. Magnetic entropy change as a function of temperature for Tb4Gd1Si2.035Ge1.935Mn0.03 alloy under a magnetic field change of 0e1 T, 0e2 T, 0e3 T, 0e4 T, and 0e5 T, for increasing and decreasing applied magnetic fields, respectively. The inset is the hysteresis loss for the Tb4Gd1Si2.035Ge1.935Mn0.03 alloy under a magnetic field change from 0 to 5 T.
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