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Magnetic transitions and thermomagnetic properties of GdCu6
Journal of Magnetism and Magnetic Materials 322 (2010) 3142–3147
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic transitions and thermomagnetic properties of GdCu6 M.K. Chattopadhyay, P. Arora, P. Mondal, S.B. Roy Magnetic and Superconducting Materials Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 9 November 2009 Received in revised form 11 May 2010 Available online 1 June 2010
We report results of dc magnetization and specific heat studies focusing on the paramagnetic to antiferromagnetic transition in GdCu6. These results clearly reveal the evidences of multiple magnetic transitions in GdCu6. In addition, a marked thermomagnetic irreversibility is observed in the temperature dependence of magnetization in low ð o 10 kOeÞ applied magnetic fields. Nature of the magnetic response changes with the increase in applied magnetic field in the temperature regime around the paramagnetic–antiferromagnetic transition temperature and also well inside the antiferromagnetic state. Experimentally measured specific heat in GdCu6 is quite large in the temperature regime below 20 K, which indicates to the potential of GdCu6 as a magnetic regenerator material for cryocooler related applications. Isothermal magnetic entropy change estimated from the results of magnetization and specific heat measurements shows a change in sign at the antiferromagnetic ordering temperature. & 2010 Elsevier B.V. All rights reserved.
Keywords: Antiferromagnetic transition Specific heat Magnetic entropy Magnetocaloric effect Magnetic regenerator material
1. Introduction In cryogenic engineering a large specific heat is the most important property of the materials used as heat generators [1]. As a result, magnetic materials having large specific heat associated with a magnetic transition in the temperature region 2–20 K are of considerable interest as magnetic regenerator materials for applications in low temperature cryocoolers [2,3]. These materials can help to overcome the lack of efficiency of the conventional regenerator materials like Pb below 20 K, which is one of the main obstacles for the cryocoolers to reach low temperatures. Finding a new potential regenerator material, however, still remains a matter of intuition and trial and error [2,3]. There exist certain guidelines for the desirable magnetic properties, which include high spin unquenched by crystal fieldinteractions and large resultant magnetic moment, so that the magnetic entropy and also the change in magnetic entropy by applied magnetic field are large [2]. The Gd-based compounds are promising here because of relatively high spin ðJ ¼ 72Þ and lack of crystal-field quenching in Gd [2]. There exist now considerable amount of studies of magnetic properties and specific heat in various Gd-based dielectric [2] and intermetallic compounds [4]. Continuing the search of such Gd-based potential magnetic regenerator materials, we have studied the magnetic properties and specific heat of the intermetallic compound GdCu6 in detail. With large concentration of Cu in the material, GdCu6 would also
Corresponding author.
E-mail address: [email protected] (S.B. Roy). 0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.05.049
be attractive from economy points of view. This compound forms in the orthorhombic CeCu6 structure and orders antiferromagnetically around 16 K [5]. This magnetic transition is clearly reflected in magnetization, resistivity and heat capacity measurements [5]. Further in the temperature region well below the ordering temperature there is an indication of a field induced metamagnetic transition [5]. However, since this early work [5] on a single crystal sample of GdCu6, to the best of our knowledge no further study on the magnetic and thermal properties of GdCu6 have appeared in the literature. From large scale application points of view it is more likely that polycrystalline samples of GdCu6 rather than single crystals will be of considerable interest. In this respect it is worth exploring the magnetic and thermal properties of polycrystalline samples of GdCu6. We shall present here a detailed study of field-temperature dependence of magnetization and specific heat in a polycrystalline sample of GdCu6. While confirming some of the results of the earlier study on the single crystal sample of GdCu6 [5], the present study has revealed newer and interesting aspects of the magnetic and thermomagnetic properties of GdCu6. Application of magnetic field causes distinct changes in the magnetic response in the magnetically ordered as well as the paramagnetic regime. These changes are reflected both in the magnetization and specific heat measurements. The experimentally measured specific heat in and around the temperature region of antiferromagnetic transition is considerably higher than that of Pb in the same temperature regime [3] and comparable to various Gd-based compounds [4] and other potential magnetic regenerator materials like HoCu2 and ErNi0.9Co0.1 [3]. Furthermore we have estimated the isothermal magnetic field induced change in entropy (i.e. magnetocaloric
M.K. Chattopadhyay et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3142–3147
effect) in GdCu6 at various temperatures both below and above the paramagnetic to antiferromagnetic transition region.
2. Experimental A polycrystalline sample GdCu6 was prepared by arc melting the pure elements (Gadolinium with purity of 99.9% and Copper with purity of 99.999%) under argon atmosphere. The sample was remelted several times to ensure homogeneity and then annealed in vacuum at 600 1C for 48 h. X-ray diffraction study with Philips powder diffractometer using Cu-Ka radiation revealed the orthorhombic structure of the compound, without any trace of impurity phases. However, such X-ray diffraction study would not be sensitive enough to reveal impurity phase of quantity o 5%. For this reason the present GdCu6 sample was subjected to a detailed metallographic examination using a high resolution optical microscope (LEICA-DMI5000M), which ruled out the presence of any second phase 4 1%. The average grain size in our GdCu6 sample was found to be around 50 mm. The most likely second phase in a GdCu6 sample is the sister compound GdCu5, which orders antiferromagnetically around 30 K [4]. High resolution magnetic measurements are often useful to determine very small amount of magnetic second phase in a sample, which sometimes evade detection in X-ray diffraction and metallographic studies. The magnetization measurements (described below) performed on the present GdCu6 sample could not detect any trace of GdCu5. The actual composition of the sample has been determined by energy dispersive X-ray (EDX) analysis performed using a Philips XL-30pc machine. The compositions at five different points chosen randomly in the sample were
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found to be Gd14.63Cu85.37, Gd15.03Cu84.97, Gd14.81Cu85.19, Gd15.2Cu84.8 and Gd14.78Cu85.22. A vibrating sample magnetometer (VSM; Quantum Design, USA) and a physical properties measurements system (PPMS; Quantum Design, USA) have been used for dc magnetization and specific heat measurements. We use three experimental protocols for temperature dependent magnetization measurements: zero field cooled and warming (ZFC), field cooled cooling (FCC) and field cooled warming (FCW) . In ZFC mode the sample is cooled to the lowest temperature of measurement before the measuring magnetic field is switched on and the measurement is made while warming up the sample. In the FCC mode the applied field is switched on in the temperature regime well above the paramagnetic to antiferromagnetic transition temperature (TN) of GdCu6 and the measurement is made while cooling across TN to the lowest temperature of measurement, namely, 2 K. After going down to 2 K in the FCC mode, the data points are taken again in the presence of same applied field while warming up the sample across the TN. This is called FCW mode. All the isothermal field dependent magnetization measurements are performed in the ZFC mode. In addition, a polycrystalline sample of isostructural LaCu6 has been prepared, which was used for subtracting out the electronic and lattice contribution of specific heat from the measured specific heat of GdCu6.
3. Results and discussion In Figs. 1 and 2, we present magnetization (M) versus temperature (T) plots of GdCu6 in various applied magnetic fields (H) obtained under ZFC, FCC and FCW protocols. A sharp peak at TN ¼16 K in the
Fig. 1. Magnetization (M) versus temperature (T) plots for GdCu6 obtained under ZFC, FCC and FCW mode in various external magnetic fields between 100 and 10 kOe.
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Fig. 2. Magnetization (M) versus temperature (T) plots for GdCu6 obtained in various external magnetic fields between 20 and 50 kOe.
M–T curves obtained with H¼100 Oe (see Fig. 1(a)) indicates the onset of antiferromagnetic transition. This temperature matches well with the value of TN reported earlier in the literature [2]. While MFCC(T) overlaps with MFCW(T) at all temperatures for H¼100 Oe, a distinct thermomagnetic irreversibility, i.e. MZFC ðTÞ a MFCC ðTÞ (MFCW(T)) is observed starting at a temperature Tirrv almost comparable to TN. With the increase in applied H, Tirrv decreases and within the present experimental resolution no thermomagnetic irreversibility is observed with an applied H¼10 kOe at least down to 2 K (see Fig. 1(d)). There is also a distinct upturn in magnetization both in the MZFC(T) and in the MFCC(T) (MFCW(T)) below about 4 K (see Fig. 1(a)–(d)), which tends to get suppressed with the increase in H, and ceases to exist (at least down to 2 K) for H¼20 kOe (see Fig. 2(a)). With further increase in applied H above 20 kOe, there is a distinct rise in magnetization at low temperatures giving rise to a new minimum in the temperature dependence of M(T) in the intermediate temperature regime below TN. This feature is highlighted in Fig. 2(b)– (d), which show the M–T curves obtained with H¼30, 35 and 50 kOe. In applied fields H 4 20 kOe, the sharp peak structure in M(T) at TN tends to get diffused. To elaborate this in Fig. 3 we plot magnetization normalized with respect to the magnetization measured at 2 K in an expanded temperature scale near TN. This normalized M(T) curve reveals two distinct changes in slope near TN with an almost flat region in between. The span of this flat region in the M(T) curve increases with the increase in applied H (see Fig. 3). This field dependence of the nature of the peak structure in M(T) around TN in GdCu6 and the low temperature and low field thermomagnetic irreversibility (i.e. MZFC ðTÞ aMFCC ðTÞ) were not reported in the earlier study of GdCu6
Fig. 3. (Colour online) Normalized magnetization (M(T)/M(2 K)) versus temperature (T) plots for GdCu6 obtained in various external magnetic fields between 30 and 50 kOe and highlighting the magnetic field dependence of the nature of the magnetization peak structure associated with the antiferromagnetic transition.
[5]. All these results suggest that the antiferromagnetic state in GdCu6 is far from a simple one. It is apparent that there are more than one magnetic transition involved in GdCu6 around 16 K, and these transitions tend to get resolved with the application of
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Fig. 4. Isothermal magnetization (M) versus field (H) plots for GdCu6 at various temperatures.
magnetic field. The appearance of a minimum in the M(T) curve further down the temperature around 4 K at low applied fields ðH r 10 kOeÞ gives indication of another magnetic transition, which gets suppressed with the application of relatively high magnetic field ðH 4 10 kOeÞ. In addition, the observed thermomagnetic irreversibility, i.e. MZFC ðTÞ aMFCC ðTÞ at temperatures below TN is suggestive of spin-glass like behaviour [6] and hence some intrinsic source of frustration in the magnetic interactions between the Gd-spins in GdCu6. However, it should also be noted here that the same thermomagnetic irreversibility can arise in an antiferromagnetic system due to the hindrance in the motion of domains/domain walls [7]. In Fig. 4, we present isothermal M versus H curves obtained at various temperatures. At the measurement temperature T¼ 2 K, there is a distinct field induced phase transition or metamagnetic transition at 30 kOe. This result is in consonance with an earlier report of fieldinduced transition along the b- and c-axes of single crystal sample of GdCu6 [5]. This kind of metamagnetic transition is common in various classes of magnetic compounds with AFM ordering and is often accompanied with thermomagnetic hysteresis [8]. This latter effect is attributed to the first order nature of the metamagnetic transition [8]. In contrast, there is no field hysteresis observed across the metamagnetic transition in GdCu6 in the field increasing and decreasing cycle, suggesting the second order nature of the transition. There is no sign of this field induced metamagnetic transition in the temperature regime 10 K and above (see Fig. 4). In Fig. 5, we present total specific heat (CP) versus temperature (T) plots for GdCu6 in zero magnetic field. A distinct ltype
Fig. 5. Total specific heat CP versus T plots for GdCu6 in zero magnetic field. The lower inset shows the CP versus T plots for GdCu6 in an expanded temperature scale near the antiferromagnetic transition. Upper inset shows the magnetic contribution to specific heat CMag versus T plots for GdCu6 in zero magnetic field.
anomaly is observed around 16 K. In single crystal samples of GdCu6 a small second peak in specific heat is also seen at T¼16.25 K [5]. In contrast, only a distinct shoulder is visible at
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T¼16.25 K in the present polycrystalline sample with the main peak at 15.8 K (see lower inset of Fig. 5). In addition, a broad hump in Cp(T) is visible around 6 K, the possible origin of which will be discussed below. To investigate this ltype anomaly further, we have estimated the magnetic contribution to the specific heat CMag of GdCu6 by subtracting out the electronic and lattice contributions to the specific heat of GdCu6. These latter contributions are determined from the experimentally measured Cp versus T curves of the non-magnetic isostructural compound LaCu6 and using the scaling relation proposed by Bouvier et al. [4]. In the upper inset of Fig. 5, we plot CMag versus T plots for GdCu6. Normally antiferromagnetic transition temperature TN is estimated in such cases from the inflection point of CMag versus T. However, the estimated inflection point at 16.4 K is distinctly higher than the temperatures 16.25 and 15.82 K where, respectively, a shoulder and a peak are observed in the CMag versus T. This observation clearly suggests that one is dealing with more than one magnetic transition. The maximum value of CMag ðTÞ 40 mJ=g K is perceptibly higher than those of many other Gd-based compounds with magnetic transition temperature in the range 10–30 K [4]. The value CP(T) is also higher than that of Pb in the temperature region T r 16 K and comparable with the specific heat of other potential magnetic regenerator materials like HoCu2 and ErNi0.9Co0.1 [3]. CMag(T) persists in the temperature region well above TN, up to at least 35 K, suggesting the existence of magnetic fluctuations even in the temperature regime well above TN. Application of magnetic fields produces interesting effects on the CP(T) in the temperature regime around TN. These results are shown in Fig. 6 in the form of CP versus T plots obtained in various H. The CP(T) curve does not change appreciably with the application of magnetic fields H o20 kOe. With the increase in
the applied magnetic field in the range H¼20–50 kOe, the specific heat peak at TN initially flattens out and then with further increase in H evolves into a distinct two peak structure in CP(T). These observations are in consonance with the results of magnetization measurements where a marked change in the temperature dependence of M(T) was observed with the applied fields H Z 20 kOe, and together they emphasize further the complex nature of the antiferromagnetic transition in GdCu6. The magnetic entropy SMag is estimated at various temperatures by integrating CMag(T)/T starting from T¼0 K to the T of measurement. SMag thus obtained is plotted as a function T in Fig. 7 in various applied H, which shows a distinct change in slope around 16 K. The estimated value of SMag at 16 K is 16 J/mol K, which is quite close to the theoretical value R ln 8¼ 17.3 J/mol K consistent with the expected ground state of 8 S7=2 in the Gd3 + ion.
Fig. 7. (Colour online) Magnetic entropy SMag versus T plots for GdCu6 in various applied magnetic fields. Inset gives the magnified view around 16 K highlighting the fact that the change in magnetic entropy ðDSM Þ changes sign at T 16 K.
Fig. 6. (Colour online) Total specific heat CP versus T plots for GdCu6 obtained in various applied magnetic fields.
Fig. 8. (Colour online) Isothermal change in entropy DSM versus T plots for GdCu6 for application of magnetic fields HMax ¼ 20, 30 and 50 kOe. Closed symbols represent results obtained from isothermal field dependence of magnetization. Open triangles represent results obtained from specific heat measurements obtained with HMax ¼50 kOe.
M.K. Chattopadhyay et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3142–3147
Field dependence of the change in magnetic entropy ðDSM Þ in GdCu6 is also quite interesting in nature. DSM changes sign at TN, going from negative to positive with decreasing temperature (see inset of Fig. 7). The broad hump in the CP(T) around 5 K (see Figs. 5 and 6) cannot be associated with crystal field effect, because Gd3 + ion has a spherical symmetric charge distribution with ground state 8 S7=2 . In analogy with similar behaviour observed in GdB6 [9], Takyanagi et al. [5] suggested the possibility of a subtle phase transition as a cause of this hump in CP(T). The results of present magnetization measurements also indicate the possibility of such a phase transition. The results of specific heat study, however, clearly show that within the present experimental resolution the nature and the position of this hump in CP(T) are not affected by the applied magnetic field. Hence this feature cannot be correlated with a magnetic transition in a straight forward manner. The change of magnetic entropy in GdCu6 due to an isothermal change of magnetic field is estimated from the experimentally obtained isothermal M versus H curves using the following equation: @SM ðT,HÞ @MðT,HÞ ¼ ð1Þ @H @T T H For magnetization measured isothermally at discrete field intervals, Eq. (1) can be approximately written as [10] Z H Z H 1 DSM ¼ MðT þ DT,HÞ dH MðT,HÞ dH ð2Þ DT 0 0 Thus the area under the isothermal M–H curves obtained at close temperature intervals is obtained through numerical integration of each curve from 0 to various HMax. The area obtained for the M–H curves at a temperature T is subtracted from that for the adjacent M–H curves corresponding to the next higher temperature T þ DT. The remaining area is divided by DT to get DSM corresponding to the temperature ðT þ 12DTÞ. The magnitude of DSM as a function of temperature estimated for different values of HMax ¼ 10, 20 and 50 kOe is shown in Fig. 8. We have also estimated the change in magnetic entropy ðDSM Þ due to an isothermal change of applied magnetic field to HMax ¼50 kOe from the CP versus T curves using the following relationship [11,12]: Z T CðTÞH1 CðTÞH2 dT ð3Þ DSM ðTÞDH ¼ T 0 Fig. 8 shows that the DSM is enhanced with HMax, with the maximum value of ðDSM Þmax obtained for HMax ¼50 kOe is 0.51 J/ kg K at T 14:2 K. With an increase in temperature DSM changes sign around 16 K and then reaches a value of 0.63 J/kg K at T 21 K. Further, an appreciable value of DSM remains even at temperatures well above the TN. DSM estimated from specific heat
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measurement in the temperature regime above TN is found to be slightly higher than that estimated from isothermal field dependence of magnetization (see Fig. 8).
4. Summary and conclusion Summarizing the results of magnetization and specific heat measurements we can say that the paramagnetic to antiferromagnetic transition in GdCu6 is quite complex in nature. There are clearly two consecutive transitions around 16 K, which get resolved further with the application of external magnetic field. There are indications of a further magnetic transition in the results of magnetization measurements with low applied fields. This, however, is not supported by the results of specific heat measurements. In addition the results of magnetization measurements reveal the presence of interesting thermomagnetic irreversibilities usually seen in spin-glass systems. The magnitude of specific heat in GdCu6 is found to be larger than Pb in the temperature regime below 20 K. Further the estimated isothermal magnetic field induced entropy change is found to be quite significant in the temperature regime in and around the antiferromagnetic transition. All these observations suggest that GdCu6 has certain potential as a magnetic regenerator material. Future studies of ESR, mSR, magnetic X-ray scattering and thermal conductivity in GdCu6 will be quite useful and informative in elucidating the both microscopic and applied magnetic properties of this interesting Gd-based compound.
Acknowledgement We thank Mrs. P. Tiwary for the EDX measurements and analysis. References [1] K.H.J. Buschow, J.F. Olijhoek, A.R. Miedema, Cryogenics 18 (1975) 262. [2] J.A. Barclay, W.A. Steyert, Cryogenics 18 (1982) 73. [3] I.A. Tanaeva, H. Ikeda, L.J.A. van Bokhaven, Y. Matsubara, A.T.A.M. de Waele, Cryogenics 43 (2003) 441. [4] M. Bouvier, P. Lethuillier, D. Schmitt, Phys. Rev. 43 (1991) 13137. [5] S. Takayanagi, et al., J. Phys. Soc. Jpn 58 (1989) 1031. [6] J.A. Mydosh, Spin Glasses, Taylor and Francis, 1992. [7] S.B. Roy, A.K. Pradhan, P. Chaddah, J. Phys. Condens. Matter 6 (1994) 5155. [8] S.B. Roy, P. Chaddah, V. Pecharsky, K.A. Gschneidenr Jr., Acta Mater. 56 (2008) 5895. [9] S. Kunii, et al., J. Magn. Magn. Mater. 52 (1985) 275. [10] R.D. McMichael, J.D. Ritter, R.D. Shull, J. Appl. Phys. 73 (1993) 6946. [11] A.M. Tishin, Y.I. Spichkin, The Magnetocaloric Effect and its Applications, IOP Publishing Ltd., London, 2003. [12] K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Tsokol, Rep. Prog. Phys. 68 (2005) 1479.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic transitions and thermomagnetic properties of GdCu6 M.K. Chattopadhyay, P. Arora, P. Mondal, S.B. Roy Magnetic and Superconducting Materials Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 9 November 2009 Received in revised form 11 May 2010 Available online 1 June 2010
We report results of dc magnetization and specific heat studies focusing on the paramagnetic to antiferromagnetic transition in GdCu6. These results clearly reveal the evidences of multiple magnetic transitions in GdCu6. In addition, a marked thermomagnetic irreversibility is observed in the temperature dependence of magnetization in low ð o 10 kOeÞ applied magnetic fields. Nature of the magnetic response changes with the increase in applied magnetic field in the temperature regime around the paramagnetic–antiferromagnetic transition temperature and also well inside the antiferromagnetic state. Experimentally measured specific heat in GdCu6 is quite large in the temperature regime below 20 K, which indicates to the potential of GdCu6 as a magnetic regenerator material for cryocooler related applications. Isothermal magnetic entropy change estimated from the results of magnetization and specific heat measurements shows a change in sign at the antiferromagnetic ordering temperature. & 2010 Elsevier B.V. All rights reserved.
Keywords: Antiferromagnetic transition Specific heat Magnetic entropy Magnetocaloric effect Magnetic regenerator material
1. Introduction In cryogenic engineering a large specific heat is the most important property of the materials used as heat generators [1]. As a result, magnetic materials having large specific heat associated with a magnetic transition in the temperature region 2–20 K are of considerable interest as magnetic regenerator materials for applications in low temperature cryocoolers [2,3]. These materials can help to overcome the lack of efficiency of the conventional regenerator materials like Pb below 20 K, which is one of the main obstacles for the cryocoolers to reach low temperatures. Finding a new potential regenerator material, however, still remains a matter of intuition and trial and error [2,3]. There exist certain guidelines for the desirable magnetic properties, which include high spin unquenched by crystal fieldinteractions and large resultant magnetic moment, so that the magnetic entropy and also the change in magnetic entropy by applied magnetic field are large [2]. The Gd-based compounds are promising here because of relatively high spin ðJ ¼ 72Þ and lack of crystal-field quenching in Gd [2]. There exist now considerable amount of studies of magnetic properties and specific heat in various Gd-based dielectric [2] and intermetallic compounds [4]. Continuing the search of such Gd-based potential magnetic regenerator materials, we have studied the magnetic properties and specific heat of the intermetallic compound GdCu6 in detail. With large concentration of Cu in the material, GdCu6 would also
Corresponding author.
E-mail address: [email protected] (S.B. Roy). 0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.05.049
be attractive from economy points of view. This compound forms in the orthorhombic CeCu6 structure and orders antiferromagnetically around 16 K [5]. This magnetic transition is clearly reflected in magnetization, resistivity and heat capacity measurements [5]. Further in the temperature region well below the ordering temperature there is an indication of a field induced metamagnetic transition [5]. However, since this early work [5] on a single crystal sample of GdCu6, to the best of our knowledge no further study on the magnetic and thermal properties of GdCu6 have appeared in the literature. From large scale application points of view it is more likely that polycrystalline samples of GdCu6 rather than single crystals will be of considerable interest. In this respect it is worth exploring the magnetic and thermal properties of polycrystalline samples of GdCu6. We shall present here a detailed study of field-temperature dependence of magnetization and specific heat in a polycrystalline sample of GdCu6. While confirming some of the results of the earlier study on the single crystal sample of GdCu6 [5], the present study has revealed newer and interesting aspects of the magnetic and thermomagnetic properties of GdCu6. Application of magnetic field causes distinct changes in the magnetic response in the magnetically ordered as well as the paramagnetic regime. These changes are reflected both in the magnetization and specific heat measurements. The experimentally measured specific heat in and around the temperature region of antiferromagnetic transition is considerably higher than that of Pb in the same temperature regime [3] and comparable to various Gd-based compounds [4] and other potential magnetic regenerator materials like HoCu2 and ErNi0.9Co0.1 [3]. Furthermore we have estimated the isothermal magnetic field induced change in entropy (i.e. magnetocaloric
M.K. Chattopadhyay et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3142–3147
effect) in GdCu6 at various temperatures both below and above the paramagnetic to antiferromagnetic transition region.
2. Experimental A polycrystalline sample GdCu6 was prepared by arc melting the pure elements (Gadolinium with purity of 99.9% and Copper with purity of 99.999%) under argon atmosphere. The sample was remelted several times to ensure homogeneity and then annealed in vacuum at 600 1C for 48 h. X-ray diffraction study with Philips powder diffractometer using Cu-Ka radiation revealed the orthorhombic structure of the compound, without any trace of impurity phases. However, such X-ray diffraction study would not be sensitive enough to reveal impurity phase of quantity o 5%. For this reason the present GdCu6 sample was subjected to a detailed metallographic examination using a high resolution optical microscope (LEICA-DMI5000M), which ruled out the presence of any second phase 4 1%. The average grain size in our GdCu6 sample was found to be around 50 mm. The most likely second phase in a GdCu6 sample is the sister compound GdCu5, which orders antiferromagnetically around 30 K [4]. High resolution magnetic measurements are often useful to determine very small amount of magnetic second phase in a sample, which sometimes evade detection in X-ray diffraction and metallographic studies. The magnetization measurements (described below) performed on the present GdCu6 sample could not detect any trace of GdCu5. The actual composition of the sample has been determined by energy dispersive X-ray (EDX) analysis performed using a Philips XL-30pc machine. The compositions at five different points chosen randomly in the sample were
3143
found to be Gd14.63Cu85.37, Gd15.03Cu84.97, Gd14.81Cu85.19, Gd15.2Cu84.8 and Gd14.78Cu85.22. A vibrating sample magnetometer (VSM; Quantum Design, USA) and a physical properties measurements system (PPMS; Quantum Design, USA) have been used for dc magnetization and specific heat measurements. We use three experimental protocols for temperature dependent magnetization measurements: zero field cooled and warming (ZFC), field cooled cooling (FCC) and field cooled warming (FCW) . In ZFC mode the sample is cooled to the lowest temperature of measurement before the measuring magnetic field is switched on and the measurement is made while warming up the sample. In the FCC mode the applied field is switched on in the temperature regime well above the paramagnetic to antiferromagnetic transition temperature (TN) of GdCu6 and the measurement is made while cooling across TN to the lowest temperature of measurement, namely, 2 K. After going down to 2 K in the FCC mode, the data points are taken again in the presence of same applied field while warming up the sample across the TN. This is called FCW mode. All the isothermal field dependent magnetization measurements are performed in the ZFC mode. In addition, a polycrystalline sample of isostructural LaCu6 has been prepared, which was used for subtracting out the electronic and lattice contribution of specific heat from the measured specific heat of GdCu6.
3. Results and discussion In Figs. 1 and 2, we present magnetization (M) versus temperature (T) plots of GdCu6 in various applied magnetic fields (H) obtained under ZFC, FCC and FCW protocols. A sharp peak at TN ¼16 K in the
Fig. 1. Magnetization (M) versus temperature (T) plots for GdCu6 obtained under ZFC, FCC and FCW mode in various external magnetic fields between 100 and 10 kOe.
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Fig. 2. Magnetization (M) versus temperature (T) plots for GdCu6 obtained in various external magnetic fields between 20 and 50 kOe.
M–T curves obtained with H¼100 Oe (see Fig. 1(a)) indicates the onset of antiferromagnetic transition. This temperature matches well with the value of TN reported earlier in the literature [2]. While MFCC(T) overlaps with MFCW(T) at all temperatures for H¼100 Oe, a distinct thermomagnetic irreversibility, i.e. MZFC ðTÞ a MFCC ðTÞ (MFCW(T)) is observed starting at a temperature Tirrv almost comparable to TN. With the increase in applied H, Tirrv decreases and within the present experimental resolution no thermomagnetic irreversibility is observed with an applied H¼10 kOe at least down to 2 K (see Fig. 1(d)). There is also a distinct upturn in magnetization both in the MZFC(T) and in the MFCC(T) (MFCW(T)) below about 4 K (see Fig. 1(a)–(d)), which tends to get suppressed with the increase in H, and ceases to exist (at least down to 2 K) for H¼20 kOe (see Fig. 2(a)). With further increase in applied H above 20 kOe, there is a distinct rise in magnetization at low temperatures giving rise to a new minimum in the temperature dependence of M(T) in the intermediate temperature regime below TN. This feature is highlighted in Fig. 2(b)– (d), which show the M–T curves obtained with H¼30, 35 and 50 kOe. In applied fields H 4 20 kOe, the sharp peak structure in M(T) at TN tends to get diffused. To elaborate this in Fig. 3 we plot magnetization normalized with respect to the magnetization measured at 2 K in an expanded temperature scale near TN. This normalized M(T) curve reveals two distinct changes in slope near TN with an almost flat region in between. The span of this flat region in the M(T) curve increases with the increase in applied H (see Fig. 3). This field dependence of the nature of the peak structure in M(T) around TN in GdCu6 and the low temperature and low field thermomagnetic irreversibility (i.e. MZFC ðTÞ aMFCC ðTÞ) were not reported in the earlier study of GdCu6
Fig. 3. (Colour online) Normalized magnetization (M(T)/M(2 K)) versus temperature (T) plots for GdCu6 obtained in various external magnetic fields between 30 and 50 kOe and highlighting the magnetic field dependence of the nature of the magnetization peak structure associated with the antiferromagnetic transition.
[5]. All these results suggest that the antiferromagnetic state in GdCu6 is far from a simple one. It is apparent that there are more than one magnetic transition involved in GdCu6 around 16 K, and these transitions tend to get resolved with the application of
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Fig. 4. Isothermal magnetization (M) versus field (H) plots for GdCu6 at various temperatures.
magnetic field. The appearance of a minimum in the M(T) curve further down the temperature around 4 K at low applied fields ðH r 10 kOeÞ gives indication of another magnetic transition, which gets suppressed with the application of relatively high magnetic field ðH 4 10 kOeÞ. In addition, the observed thermomagnetic irreversibility, i.e. MZFC ðTÞ aMFCC ðTÞ at temperatures below TN is suggestive of spin-glass like behaviour [6] and hence some intrinsic source of frustration in the magnetic interactions between the Gd-spins in GdCu6. However, it should also be noted here that the same thermomagnetic irreversibility can arise in an antiferromagnetic system due to the hindrance in the motion of domains/domain walls [7]. In Fig. 4, we present isothermal M versus H curves obtained at various temperatures. At the measurement temperature T¼ 2 K, there is a distinct field induced phase transition or metamagnetic transition at 30 kOe. This result is in consonance with an earlier report of fieldinduced transition along the b- and c-axes of single crystal sample of GdCu6 [5]. This kind of metamagnetic transition is common in various classes of magnetic compounds with AFM ordering and is often accompanied with thermomagnetic hysteresis [8]. This latter effect is attributed to the first order nature of the metamagnetic transition [8]. In contrast, there is no field hysteresis observed across the metamagnetic transition in GdCu6 in the field increasing and decreasing cycle, suggesting the second order nature of the transition. There is no sign of this field induced metamagnetic transition in the temperature regime 10 K and above (see Fig. 4). In Fig. 5, we present total specific heat (CP) versus temperature (T) plots for GdCu6 in zero magnetic field. A distinct ltype
Fig. 5. Total specific heat CP versus T plots for GdCu6 in zero magnetic field. The lower inset shows the CP versus T plots for GdCu6 in an expanded temperature scale near the antiferromagnetic transition. Upper inset shows the magnetic contribution to specific heat CMag versus T plots for GdCu6 in zero magnetic field.
anomaly is observed around 16 K. In single crystal samples of GdCu6 a small second peak in specific heat is also seen at T¼16.25 K [5]. In contrast, only a distinct shoulder is visible at
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T¼16.25 K in the present polycrystalline sample with the main peak at 15.8 K (see lower inset of Fig. 5). In addition, a broad hump in Cp(T) is visible around 6 K, the possible origin of which will be discussed below. To investigate this ltype anomaly further, we have estimated the magnetic contribution to the specific heat CMag of GdCu6 by subtracting out the electronic and lattice contributions to the specific heat of GdCu6. These latter contributions are determined from the experimentally measured Cp versus T curves of the non-magnetic isostructural compound LaCu6 and using the scaling relation proposed by Bouvier et al. [4]. In the upper inset of Fig. 5, we plot CMag versus T plots for GdCu6. Normally antiferromagnetic transition temperature TN is estimated in such cases from the inflection point of CMag versus T. However, the estimated inflection point at 16.4 K is distinctly higher than the temperatures 16.25 and 15.82 K where, respectively, a shoulder and a peak are observed in the CMag versus T. This observation clearly suggests that one is dealing with more than one magnetic transition. The maximum value of CMag ðTÞ 40 mJ=g K is perceptibly higher than those of many other Gd-based compounds with magnetic transition temperature in the range 10–30 K [4]. The value CP(T) is also higher than that of Pb in the temperature region T r 16 K and comparable with the specific heat of other potential magnetic regenerator materials like HoCu2 and ErNi0.9Co0.1 [3]. CMag(T) persists in the temperature region well above TN, up to at least 35 K, suggesting the existence of magnetic fluctuations even in the temperature regime well above TN. Application of magnetic fields produces interesting effects on the CP(T) in the temperature regime around TN. These results are shown in Fig. 6 in the form of CP versus T plots obtained in various H. The CP(T) curve does not change appreciably with the application of magnetic fields H o20 kOe. With the increase in
the applied magnetic field in the range H¼20–50 kOe, the specific heat peak at TN initially flattens out and then with further increase in H evolves into a distinct two peak structure in CP(T). These observations are in consonance with the results of magnetization measurements where a marked change in the temperature dependence of M(T) was observed with the applied fields H Z 20 kOe, and together they emphasize further the complex nature of the antiferromagnetic transition in GdCu6. The magnetic entropy SMag is estimated at various temperatures by integrating CMag(T)/T starting from T¼0 K to the T of measurement. SMag thus obtained is plotted as a function T in Fig. 7 in various applied H, which shows a distinct change in slope around 16 K. The estimated value of SMag at 16 K is 16 J/mol K, which is quite close to the theoretical value R ln 8¼ 17.3 J/mol K consistent with the expected ground state of 8 S7=2 in the Gd3 + ion.
Fig. 7. (Colour online) Magnetic entropy SMag versus T plots for GdCu6 in various applied magnetic fields. Inset gives the magnified view around 16 K highlighting the fact that the change in magnetic entropy ðDSM Þ changes sign at T 16 K.
Fig. 6. (Colour online) Total specific heat CP versus T plots for GdCu6 obtained in various applied magnetic fields.
Fig. 8. (Colour online) Isothermal change in entropy DSM versus T plots for GdCu6 for application of magnetic fields HMax ¼ 20, 30 and 50 kOe. Closed symbols represent results obtained from isothermal field dependence of magnetization. Open triangles represent results obtained from specific heat measurements obtained with HMax ¼50 kOe.
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Field dependence of the change in magnetic entropy ðDSM Þ in GdCu6 is also quite interesting in nature. DSM changes sign at TN, going from negative to positive with decreasing temperature (see inset of Fig. 7). The broad hump in the CP(T) around 5 K (see Figs. 5 and 6) cannot be associated with crystal field effect, because Gd3 + ion has a spherical symmetric charge distribution with ground state 8 S7=2 . In analogy with similar behaviour observed in GdB6 [9], Takyanagi et al. [5] suggested the possibility of a subtle phase transition as a cause of this hump in CP(T). The results of present magnetization measurements also indicate the possibility of such a phase transition. The results of specific heat study, however, clearly show that within the present experimental resolution the nature and the position of this hump in CP(T) are not affected by the applied magnetic field. Hence this feature cannot be correlated with a magnetic transition in a straight forward manner. The change of magnetic entropy in GdCu6 due to an isothermal change of magnetic field is estimated from the experimentally obtained isothermal M versus H curves using the following equation: @SM ðT,HÞ @MðT,HÞ ¼ ð1Þ @H @T T H For magnetization measured isothermally at discrete field intervals, Eq. (1) can be approximately written as [10] Z H Z H 1 DSM ¼ MðT þ DT,HÞ dH MðT,HÞ dH ð2Þ DT 0 0 Thus the area under the isothermal M–H curves obtained at close temperature intervals is obtained through numerical integration of each curve from 0 to various HMax. The area obtained for the M–H curves at a temperature T is subtracted from that for the adjacent M–H curves corresponding to the next higher temperature T þ DT. The remaining area is divided by DT to get DSM corresponding to the temperature ðT þ 12DTÞ. The magnitude of DSM as a function of temperature estimated for different values of HMax ¼ 10, 20 and 50 kOe is shown in Fig. 8. We have also estimated the change in magnetic entropy ðDSM Þ due to an isothermal change of applied magnetic field to HMax ¼50 kOe from the CP versus T curves using the following relationship [11,12]: Z T CðTÞH1 CðTÞH2 dT ð3Þ DSM ðTÞDH ¼ T 0 Fig. 8 shows that the DSM is enhanced with HMax, with the maximum value of ðDSM Þmax obtained for HMax ¼50 kOe is 0.51 J/ kg K at T 14:2 K. With an increase in temperature DSM changes sign around 16 K and then reaches a value of 0.63 J/kg K at T 21 K. Further, an appreciable value of DSM remains even at temperatures well above the TN. DSM estimated from specific heat
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measurement in the temperature regime above TN is found to be slightly higher than that estimated from isothermal field dependence of magnetization (see Fig. 8).
4. Summary and conclusion Summarizing the results of magnetization and specific heat measurements we can say that the paramagnetic to antiferromagnetic transition in GdCu6 is quite complex in nature. There are clearly two consecutive transitions around 16 K, which get resolved further with the application of external magnetic field. There are indications of a further magnetic transition in the results of magnetization measurements with low applied fields. This, however, is not supported by the results of specific heat measurements. In addition the results of magnetization measurements reveal the presence of interesting thermomagnetic irreversibilities usually seen in spin-glass systems. The magnitude of specific heat in GdCu6 is found to be larger than Pb in the temperature regime below 20 K. Further the estimated isothermal magnetic field induced entropy change is found to be quite significant in the temperature regime in and around the antiferromagnetic transition. All these observations suggest that GdCu6 has certain potential as a magnetic regenerator material. Future studies of ESR, mSR, magnetic X-ray scattering and thermal conductivity in GdCu6 will be quite useful and informative in elucidating the both microscopic and applied magnetic properties of this interesting Gd-based compound.
Acknowledgement We thank Mrs. P. Tiwary for the EDX measurements and analysis. References [1] K.H.J. Buschow, J.F. Olijhoek, A.R. Miedema, Cryogenics 18 (1975) 262. [2] J.A. Barclay, W.A. Steyert, Cryogenics 18 (1982) 73. [3] I.A. Tanaeva, H. Ikeda, L.J.A. van Bokhaven, Y. Matsubara, A.T.A.M. de Waele, Cryogenics 43 (2003) 441. [4] M. Bouvier, P. Lethuillier, D. Schmitt, Phys. Rev. 43 (1991) 13137. [5] S. Takayanagi, et al., J. Phys. Soc. Jpn 58 (1989) 1031. [6] J.A. Mydosh, Spin Glasses, Taylor and Francis, 1992. [7] S.B. Roy, A.K. Pradhan, P. Chaddah, J. Phys. Condens. Matter 6 (1994) 5155. [8] S.B. Roy, P. Chaddah, V. Pecharsky, K.A. Gschneidenr Jr., Acta Mater. 56 (2008) 5895. [9] S. Kunii, et al., J. Magn. Magn. Mater. 52 (1985) 275. [10] R.D. McMichael, J.D. Ritter, R.D. Shull, J. Appl. Phys. 73 (1993) 6946. [11] A.M. Tishin, Y.I. Spichkin, The Magnetocaloric Effect and its Applications, IOP Publishing Ltd., London, 2003. [12] K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Tsokol, Rep. Prog. Phys. 68 (2005) 1479.