Journal of Alloys and Compounds 478 (2009) 566–571
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Near room temperature magneto caloric effect in V doped La0.67 Ca0.33 MnO3 ceramics P. Nisha a , P.N. Santhosh b , K.G. Suresh c , C. Pavithran a , Manoj Raama Varma a,∗ a b c
Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology [NIIST], Industrial Estate Road, Trivandrum 695019, Kerala, India Department of Physics, Indian Institute of Technology Madras-600036, Chennai, India Department of Physics, Indian Institute of Technology Bombay-400076 Mumbai, India
a r t i c l e
i n f o
Article history: Received 1 September 2008 Received in revised form 17 November 2008 Accepted 21 November 2008 Available online 30 November 2008 Keywords: Magnetic measurements X-ray diffraction Magnetically ordered materials:crystal structure and symmetry
a b s t r a c t La0.67 Ca0.33 MnO3 doped with different amounts of V, resulting in the series La0.67 Ca0.33 Mn1−x Vx O3 [x = 0.03, 0.06, 0.1, 0.15, 0.25, 0.5] was synthesized by conventional solid-state ceramics route. Heat treatment conditions during synthesis were optimized to have small changes in the Curie temperature (TC ). The compounds with x > 0.06 were found to have the additional LaVO4 phase. A quantitative analysis of the extra phase with increase in V doping was investigated by the Rietveld refinement of the X-ray diffraction patterns. Variation of magnetic and magnetocaloric properties at different temperatures and magnetic fields was studied. Magnetocaloric effect has been studied in terms of isothermal magnetic entropy change (SM ). Magnetization was found to change drastically at around 275 K and the isothermal magnetic entropy change was found to peak in the temperature range 265–275 K. With increase in V content, SM initially shows a considerable increase and then a decrease at higher V concentrations. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Mixed-valent perovskite manganites of the formula A1−x A x MnO3 (A = trivalent rare earth ion, A = divalent alkaline earth ion like Ca, Sr, Ba, etc.) have recently attracted a great attention of researchers due to their interesting electrical, magnetic and magneto-transport properties [1–6]. Among them, La0.67 Ca0.33 MnO3 is a well-known magneto resistive material showing considerable magnetoresistance (MR). Transport and magnetic properties of manganites can be explained by means of double exchange (DE) and super exchange (SE) mechanisms and it is known that the competition between DE and SE mechanisms is trivial in determining the magnetic properties of the system [7–10]. DE and SE are known to be sensitive to the variation of Mn–O bond length and Mn–O–Mn bond angle, both controlled by the average ionic radius of A or B site ions, whereas the density of charge carriers is controlled by the Mn3+ /Mn4+ ratio [7–10]. Recent studies have shown that La0.67 Ca0.33 MnO3 exhibits significant magnetocaloric effect (MCE) as well [11–15]. These materials are now being probed in the search for novel giant magnetocaloric materials, especially for the near room temperature applications.
∗ Corresponding author. Tel.: +91 471 2515377; fax: +91 471 2491712. E-mail addresses:
[email protected] (P. Nisha),
[email protected] (P.N. Santhosh),
[email protected] (K.G. Suresh),
[email protected] (C. Pavithran),
[email protected] (M.R. Varma). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.11.091
To improve the magnetic properties of perovskite manganites, doping of different elemental ions to replace A site or B site ions, at least partially or forming a second phase, has been done successfully. Doping can alter the competition between DE and SE by modifying the mismatch between Mn3+ –O2− –Mn4+ network [16–18]. Several elements from the transition metal group have been used as dopants and they have been reported to destroy long-range ferromagnetic order, decrease the transition temperature, and increase the magnetoresistance [19–28]. However the effects of dopants and the second phase formation as a result of doping on the magnetocaloric effect of La0.67 Ca0.33 MnO3 has not been investigated in great detail, as in the case of magnetoresistance. Recently Gencer et al. [29] have reported the microstructure and magnetoresistance in La0.67 Ca0.33 Mn1−x Vx O3 . They showed the existence of a second phase in the material and the temperature dependence of resistance of the material showed a maximum in the V doped sample for x ≤ 0.15. They also found that the peak value of the magnetoresistance around the insulator-metal transition temperature increases by V doping. This study showed a sharp decrease or increase of magnetoresistance in the temperature range of 250–275 K. Zhao et al. [30] have studied the phase structure, transport and microstructural properties of the La0.7 Ca0.3 Mn1−x Vx O3 (0 ≤ x ≤ 0.2). They observed the existence of a multi-phase compound when sintered in air and found that V does not substitute for Mn easily to form La0.67 Ca0.33 Mn1−x Vx O3 compound. Both Gencer et al. [29] and Zhao et al. [30] have observed a reduction in the Curie temperature with increase in V content. In order to understand the magnetocaloric effect in these compounds,
P. Nisha et al. / Journal of Alloys and Compounds 478 (2009) 566–571
we have studied the magnetic and magnetocaloric properties of La0.67 Ca0.33 Mn1−x Vx O3 system. The main motivation behind this work is to identify novel magnetic refrigerant materials suitable for room temperature applications. Since V is a pentavalent cation and is known to contribute cation vacancies when substituted in Mn4+ or Mn3+ sites which are known to produce a sharp decrease in the electrical resistance (not magnetoresistance). This is a desirable property for magnetic refrigerant materials. In the light of the above facts, the authors felt that the magneto caloric effect of V substituted La0.7 Ca0.3 MnO3 may be interesting and will have an added advantage over undoped La0.7 Ca0.3 MnO3 . Another objective was to study the fractions of different phases present in the material when the doping level and the heat treatment conditions were changed. The work was focused on to study the effect of these phases on the magnetocaloric effect and to fine tune the phase compositions through heat treatments to maintain the transition temperature at the same level as in the case of undoped La0.7 Ca0.3 MnO3 . 2. Experimental procedures La0.67 Ca0.33 Mn1−x Vx O3 , where x = 0.03, 0.06, 0.1, 0.15, 0.25, 0.5 was synthesized by conventional solid-state ceramics route. Stoichiometric amounts of high purity powders of La2 O3 (IRE, India), V2 O3 , CaCO3 and MnO (Sigma–Aldrich) were mixed in distilled water medium and were ball milled for 24 h in a PVC container using zirconia balls. Resulting slurry was dried in a hot air oven and the powder was well ground in an agate mortar. The dried powder was calcined at different temperatures namely, 600, 900 and 1200 ◦ C for 4 h in air and powder X-ray diffraction (XRD) patterns were recorded to check the phase purity of the material. Crystal structure refinements of calcined powders were carried out using the Rietveld method with the help of GSAS software [31], which also yielded the structural parameters and the fractions of different phases. Powders of the La0.67 Ca0.33 Mn1−x Vx O3 compounds were ground well in an agate mortar and mixed with 4% PVA solution and dried. Dried powder was pelletized in to thin pellets of 14 mm dia and 1 mm thick and sintered in air at the optimized sintering condition (for attaining maximum density) was 1250 ◦ C for 4–6 h in air. The sintered samples were thermally etched and their surface morphology and average grain size were studied using a scanning electron microscope (JEOL-JSM 5600 LV, Japan). Average grain area and average grain diameter were determined by intercept method described in the ASTM standard E 112-96 (ASTM International; Designation: E 112-96 (Reapproved 2004)), standard for determining grain size. Actual grain size determination as per the above procedure was done with the help of image analysis software. Procedure described in ASTM E 112-96 standard is applicable for the grain size, crystal size or cell size determination of grains with well-defined grain boundaries for metals and non metallic materials, the method was applied characterization of SEM microstructures. Magnetization measurements were carried out on as calcined powder using a vibrating sample magnetometer (PPMS, Quantum Design) in the temperature range of 5–300 K, up to a maximum field of 5 T. The magnetocaloric effect was calculated in terms of isothermal magnetic entropy change, using the magnetization isotherms collected at various temperatures close to TC . MCE in terms of isothermal magnetic entropy change (SM ), has been calculated using the relation [11,32]
H2 SM (T, H) =
ıM(T, H) ıT
dH
(1)
H
H1
In the calculation, H1 is taken as zero and H2 is the applied field. The magnetization isotherms taken in a temperature interval of 4 K have been used in the calculation. The relative cooling power (RCP) have been calculated using the relation [33]
T2 SM (T ) dT
RCP =
(2)
T1
where T1 and T2 are the temperatures of hot and cold sinks and correspond to the full-width at half-maximum (FWHM) points.
3. Results and discussion The typical XRD pattern refined using GSAS software is shown in Fig. 1. Table 1 shows structural parameters of La0.67 Ca0.33 Mn1−x Vx O3 compounds. It can be seen that the structure of La0.67 Ca0.33 Mn0.97 V0.03 O3 is similar to that of La0.67 Ca0.33 MnO3 reported by Huang et al. [8] with small changes in lattice parameters. Substitution of pentavalent V for the tetravalent Mn ions
567
Fig. 1. Refined XRD pattern for La0.67 Ca0.33 Mn0.97 V0.03 O3 . The plot at the bottom is the difference between the calculated and observed intensities.
produces anion vacancies and small changes in lattice parameters. With increase in the concentration of V (up to x = 0.6), lattice parameters (a and c) are found to increase. This trend ceases when the second phase LaVO4 is formed. With further increase in the amount of V, the amount of second phase increases. Basic data for the refinement of LaVO4 was reported by Bashir and Khan [34]. For La0.67 Ca0.33 Mn0.5 V0.5 O3 , a minor quantity of a third phase with composition LaMnO3±ı is found to be present [35]. Presence of multi-phase structure is seen to reflect in properties such as density. Detailed micro-structural and composition analysis of the different phases observed in this system has been reported by Gencer et al. [29] and Zhao et al. [30]. They have used SEM and EDAX to determine different phase compositions. The observations about the phase compositions and their fractions in the present work are in reasonably good agreement with these reports. The small differences mainly arise due to the differences in the analytical techniques as well as the synthesis conditions. SEM micrographs for the sintered La0.67 Ca0.33 Mn1−x Vx O3 compounds are shown in Fig. 2a–f. Sintering at 1250 ◦ C for 4–6 h in air can alter the phase compositions from that of calcined powder for which the phase compositions were determined by XRD refinement and given in Table 1. For low V concentrations (x = 0.03 and 0.06), no second phase could be detected. However, on increasing the amount of V to 0.1, second phase can be seen, as observed in the XRD data. Grain growth also increases considerably up to vanadium concentration of X = 0.15. From there onwards amount of second phases increases with V concentration, as is evident from Table 2 and Fig. 2e and f. For vanadium concentration of x = 0.5, a third phase also appear as seen from XRD analysis. It can also be seen (from Fig. 2 and Table 2) that the grain growth also increases with vanadium concentration up to x = 0.15. Thereafter grain size drastically reduces. Therefore, SEM micrographs also indicate the same trends on phase proportions as obtained from the XRD analysis. Variation of magnetization with temperature for La0.67 Ca0.33 Mn1−x Vx O3 compounds is shown in Fig. 3. It can be seen that, irrespective of the composition, the magnetization decreases to a very low value at around 265 K. It is to be noted that the Curie temperature of La0.67 Ca0.33 MnO3 is close to 260 K [11]. The Curie temperature remains almost unaltered even for the highest V concentration. This implies that in our method of preparation, V would not have gone completely to the substitutional site and therefore, the majority phase remains close to La0.67 Ca0.33 Mn03 composition with partial substitution of V for Mn. This is the difference from the earlier reports wherein considerable lowering of the Curie temperature was observed even with small amounts of V doping. However, there is a significant change in the magnetization values
568
P. Nisha et al. / Journal of Alloys and Compounds 478 (2009) 566–571
Table 1 Structural parameters of La0.67 Ca0.33 Mn1−x Vx O3 compounds. Composition
Major phases
Space group
a (Å)
b (Å)
c (Å)
Fraction of the phase
La0.67 Ca0.33 Mn0.97 V0.03 O3 La0.67 Ca0.33 Mn0.94 V0.06 O3
La0.67 Ca0.33 MnO3 La0.67 Ca0.33 MnO3
Pnma Pnma
5.4975 5.5293
7.7838 7.7780
5.4809 5.5164
100 100
La0.67 Ca0.33 Mn0.9 V0.1 O3
La0.67 Ca0.33 MnO3 LaVO4
Pnma P21 /n
5.4445 7.0152
7.7074 7.2592
5.4700 6.7040
93.0 7.0
La0.67 Ca0.33 Mn0.85 V0.15 O3
La0.67 Ca0.33 MnO3 LaVO4
Pnma P21 /n
5.4429 7.0043
7.7106 7.2379
5.4510 6.6958
89.2 10.8
La0.67 Ca0.33 Mn0.75 V0.25 O3
La0.67 Ca0.33 MnO3 LaVO4
Pnma P21 /n
5.4518 7.0188
7.7028 7.2538
5.4773 6.7064
83.4 16.6
La0.67 Ca0.33 Mn0.5 V0.5 O3
La0.67 Ca0.33 MnO3 LaVO4 LaMnO3±␦
Pnma P21 /n ¯ R 3c
5.4450 7.0188 5.4712
7.6937 7.2538 5.4712
5.4639 6.7064 5.4712
56.8 42.1 0.9
as a function of V concentration. For small amount of V, the magnetization value initially decreases and then reverses the trend. This might be directly related to the concentration of V that has really gone into the major phase. At higher concentration, more amount
of V goes into the second phase than in the majority phase and hence it shows the magnetization value similar to the undoped or dilutely doped La0.67 Ca0.33 MnO3 . It can be seen from Fig. 3b, d and e that around 50 K a small step is found in the M–T curves of
Fig. 2. SEM micrographs for sintered La0.67 Ca0.33 Mn1−x Vx O3 compounds (a) La0.67 Ca0.33 Mn0.97 V0.03 O3 , (b) La0.67 Ca0.33 Mn0.94 V0.06 O3 , (c) La0.67 Ca0.33 Mn0.9 V0.1 O3 , (d) La0.67 Ca0.33 Mn0.85 V.15 O3 , (e) La0.67 Ca0.33 Mn0.75 V0.25 O3 , and (f) La0.67 Ca0.33 Mn0.5 V0.5 O3 .
P. Nisha et al. / Journal of Alloys and Compounds 478 (2009) 566–571
569
Table 2 Phase fractions and average grain diameters of different phases in La0.67 Ca0.33 Mn1−x Vx O3 compounds. Composition
No. of phases observed
Phase-I fraction % and average grain diameter (m)
Phase-II fraction % and average grain diameter (m)
Phase-III fraction % and average grain diameter (m)
La0.67 Ca0.33 Mn0.97 V0.03 O3
1
100 0.31
–
–
La0.67 Ca0.33 Mn0.94 V0.06 O3
1
100 1.86
–
–
La0.67 Ca0.33 Mn0.9 V0.1 O3
2
89.47 1.56
10.53 0.71
–
La0.67 Ca0.33 Mn0.85 V0.15 O3
2
90.30 0.33
9.69 0.16
–
La0.67 Ca0.33 Mn0.75 V0.25 O3
2
91.75 0.36
8.24 0.18
–
La0.67 Ca0.33 Mn0.5 V0.5 O3
3
80.78 0.48
17.17 1.12
2.03 0.20
La0.67 Ca0.33 Mn1−x Vx O3 for x values 0.06, 0.15 and 0.25. This could be due to the presence of the minor impurity phases present in the material. The quantities of these phases are so small that no additional peaks could be detected in the XRD pattern and no additional phases were visible from the SEM/EDAX studies. Several authors have studied the variation of MCE under various applied fields in undoped La0.67 Ca0.33 MnO3 . Sun et al. [32] have synthesized La0.67 Ca0.33 MnO3 using conventional solid-state ceramics route and has reported the highest entropy change of 6.4 J/kg K, Curie temperature of 267 K and RCP of 134 J/kg for a magnetic field change of 3 T. Many other authors like Zhang [36], Mira [37] and Hueso [10] had also studied the MCE of La0.67 Ca0.33 MnO3 and obtained values ranging from 2 to 5 J/kg K. Guo et al. [38] studied the MCE of La0.67 Ca0.33 MnO3 synthesized by sol–gel method and obtained a maximum entropy change of 4.3 J/kg K and an RCP of 47 J/kg when the magnetic field was 1.5 T. Phan et al. [9,11] have reported an entropy change of 2 J/kg K for a field of 5 T in La0.67 Ca0.33 MnO3 thin films. Fig. 4 shows variation of SM at various temperatures, for different fields from 10 kOe to 50 kOe, for La0.67 Ca0.33 Mn1−x Vx O3 compounds with x = 0.03 and 0.50. With the max increases marginally to about 6.7 J/kg K for addition of V, SM x ≤ 0.1 and then decreases monotonically with V concentration. The
Fig. 3. Temperature variation of magnetization of La0.67 Ca0.33 Mn1−x Vx O3 compounds at 500 Oe (a) La0.67 Ca0.33 Mn0.97 V0.03 O3 , (b) La0.67 Ca0.33 Mn0.96 V0.04 O3 , (d) La0.67 Ca0.33 Mn0.85 V.15 O3 , (e) (c) La0.67 Ca0.33 Mn0.9 V0.1 O3 , La0.67 Ca0.33 Mn0.75 V0.25 O3 , and (f) La0.67 Ca0.33 Mn0.5 V0.5 O3 .
Fig. 4. Variation of isothermal magnetic entropy change with temperature for (a) La0.67 Ca0.33 Mn0.9 V0.1 O3 and (b) La0.67 Ca0.33 Mn0.97 V0.03 O3 .
570
P. Nisha et al. / Journal of Alloys and Compounds 478 (2009) 566–571
Table 3 Comparison of Curie temperature (TC ), maximum entropy change (SM ) and RCP for La0.67 Ca0.33 MnO3 -based materials. Composition
TC (K)
H (T)
−SM (J/kg K)
RCP (J/kg)
References
La0.67 Ca0.33 MnO3 (solid-state ceramics route) La0.67 Ca0.33 MnO3 (by sol–gel method) La0.67 Ca0.33 MnO3 (by sol–gel method) La0.67 Ca0.33 MnO3 (thin film deposited by MOD technique) La0.67 Ca0.33 Mn0.97 V0.03 O3 (solid-state ceramics route) La0.67 Ca0.33 Mn0.94 V0.06 O3 (solid-state ceramics route) La0.67 Ca0.33 Mn0.9 V0.1 O3 (solid-state ceramics route) La0.67 Ca0.33 Mn0.85 V0.15 O3 (solid-state ceramics route) La0.67 Ca0.33 Mn0.75 V0.25 O3 (solid-state ceramics route) La0.67 Ca0.33 Mn0.5 V0.5 O3 (solid-state ceramics route)
267 260 260 252 277.5 277.5 277.5 287.5 287.5 287.5
3 1.5 1 5 5 5 5 5 5 5
6.4 4.3 5.0 2.06 6.7 6.29 6.13 4.44 3.13 2.94
134 47 35 175 135.5 151.7 218.5 114.6 83.8 76.1
Sun et al. [32] Guo et al. [38] Hueso et al. [10] Morelli et al. [9] Present work Present work Present work Present work Present work Present work
route. Variation of the structure with the increase in V doping was investigated by refinement of powder X-ray diffraction patterns using Rietveld method. It was observed that more than one phase is formed when the V concentration is increased to 0.1. Amount of secondary phases as function of V content was also determined from the Rietveld refinement. SEM studies are found to be in agreement with the XRD data. Variation of magnetization and magnetocaloric effect in terms of entropy change for different temperatures and magnetic fields were studied. The magnetic entropy change is found to improve considerably for the initial concentrations of V. The composition range of 0.03 ≤ x ≤ 0.1 is found to be very useful for magnetic refrigeration applications. Acknowledgements
Fig. 5. Variation of maximum isothermal magnetic entropy change and the relative cooling power in La0.67 Ca0.33 Mn1−x Vx O3 compounds as a function of V concentration.
increase in the case of x = 0.03 is significant in view of the fact that this material is completely free from the additional phases. The fact that the MCE in x = 0.1 is nearly equal to that for x = 0.06 suggests that the small amount of second phase formation is not detrimental to the MCE. These materials with the Curie temperatures near the room temperature and possessing considerable values of SM are promising in the search for room temperature magnetic refrigerants. Fig. 5 shows the variation of the relative cooling power of these compounds, along with the maximum entropy change. As can be seen, the relative cooling power increases from the reported value [32] of 134 J/kg for La0.67 Ca0.33 MnO3 and shows a peak at around x = 0.1 and then decreases with increase in x, whereas the SM shows a small peak near x = 0.1 and then decreases with x. The initial increase in the RCP is indicative of the broadening of the SM peak. These observations suggest that the presence of second phase to the extent present in x = 0.1 in La0.67 Ca0.33 Mn1−x Vx O3 series improves the magnetocaloric behavior, thereby enabling it to be a potential candidate for magnetic refrigeration applications. Table 3 summarizes the reported values of magnetic and magnetocaloric parameters obtained by different groups, along with the present data. It is to be mentioned here that the synthesis condition used by Sun et al. was almost similar to that used in the present investigation and therefore, we have analyzed the effect of V by comparing the value reported by Sun et al. for the undoped compound. 4. Conclusions La0.67 Ca0.33 Mn1−x Vx O3 compounds with x = 0.03, 0.06, 0.1, 0.15, 0.25, 0.5 were synthesized by conventional solid-state ceramics
Authors would like to thank Defense Research and Development Organization, DRDO, Government of India, for the financial support in the form of a sponsored project. References [1] R.V. Helmolt, J. Wecker, V. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331–2333. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413–415. [3] Y. Tokura, Y. Tomioka, J. Magn. Magn. Mater. 200 (1999) 1. [4] J.M.D. Coey, M. Viret, S. von Molnar, Adv. Phys. 48 (1999) 167. [5] E. Dagotto, T. Hotta, A. Moreo, Phys. Rep. 344 (2001) 1. [6] M. Ziese, Rep. Prog. Phys. 65 (2002) 143. [7] L.M. Roudriguiz-Martinez, J. Paul Attfield, Phys. Rev. B. 54 (1996) R15622. [8] Q. Huang, A. Santoro, J.W. Lymn, R.W. Erwin, J.A. Borchers, J.L. Peng, K. Ghosh, R.L. Greene, Phys. Rev. B 58 (1998) 2684. [9] D.T. Morelli, A.M. Mance, J.V. Mantese, A.L. Micheli, J. Appl. Phys. 79 (1996) 373. [10] L.E. Hueso, P. Sande, D.R. Miguens, J. Rivas, F. Rivadulla, M.A. Lopez-Quintela, J. Appl. Phys. 91 (2002) 9943. [11] M.H. Phan, S.C. Yu, J. Magn. Magn. Mater. 308 (2007) 325. [12] M.H. Phan, S.C. Yu, N.H. Hur, Appl. Phys. Lett. 86 (2005) 072504. [13] M.H. Phan, S.C. Yu, N.H. Hur, J. Magn. Magn. Mater. 290–291 (2005) 665. [14] X. Bohigas, J. tejada, E. Del Barco, X.X. Zhang, M. Sales, Appl. Phys. Lett. 73 (1998) 390. [15] J. Barclay, J. Alloys. Compd. 207–208 (1994) 355. [16] K. Ghosh, S.B. Ogale, R. Ramesh, R.L. Greene, T. Venkatesan, K.M. Gopchup, R. Bathe, S.I. Patil, Phys. Rev. B. 59 (1999) 533. [17] J.R. Sun, G.H. Rao, B.G. Shen, H.K. Wong, Appl. Phys. Lett. 73 (1996) 2998. [18] J. Cho, M. Gomi, M. Abe, Jpn. J. Appl. Phys. 29 (1990) 1686. [19] J.M. de Teresa, M.R. Ibarra, J. Gracia, C. Marquina, P.A. Algrabel, C. Marquina, C. Ritter, R. van Helmolt, J. Phys. Rev. B 54 (1993) 1187. [20] J. Blasca, J. Gracia, J.M. de Teresa, M.R. Ibarra, J. Perez, P.A. Algrabel, C. Marquina, C. Ritter, Phys. Rev. B 55 (1997) 8905. [21] Y. Sun, X. Xu, L. Zheng, Y. Zhang, Phys. Rev. B 60 (1999) 12317. ˝ K. Nenkov, V. Sridharan, V. Sankara Sastry, K.H. Muller, ˝ [22] L.S. Lakshmi, K. Dorr, J. Magn. Magn. Mater. 290–291 (2005) 924. [23] C. Martin, A. Maignan, H. Herview, C. Autret, B. Reveau, Phys. Rev. B 63 (2001) 174402. [24] A. Pena, J. Gutierrez, J.M. Barandiaran, J.L. Pixario, L.M. Rodrigiguiz-Martinez, M. Insausti, T. Rojo, J. Alloys Compd. 323–324 (2001) 524. [25] C.M. Srivastava, S. Banorjee, T.K. Gundu Rao, A.K. Nigam, D. Bhadur, J. Phys.: Condens Matter 15 (2003) 2375. [26] L. Morales, R. Zysler, A. Canaeiro, Physica B 320 (2002) 100. [27] R. Mahendiran, M. Hervieu, A. Maignan, C. Martin, B. Raveau, Solid State Commun. 114 (2000) 429.
P. Nisha et al. / Journal of Alloys and Compounds 478 (2009) 566–571 [28] X.H. Li, Y.H. Huang, C.H. Yan, Z.M. Wang, C.S. Liao, J. Phys.: Condens. Matter 14 (2002) L 177. [29] H. Gencer, B.S. Kolat, S. Atalay, J. Alloys Compd. 422 (2006) 40. [30] T.S. Zhao, B.H. Li, G. Han, J. Magn. Magn. Mater. 320 (2008) 924. [31] A.C. Larson, R.B. von Dreele, GSAS, General Structure Analysis System, University of California, 2000, p. 1. [32] Y. Sun, X. Xu, Y.H. Zhang, J. Magn. Magn. Mater. 219 (2000) 183. [33] A.M. Tishin, Y.I. Spichkin, The Magneto Caloric Effect and Its Applications, IOP, New York, 2003, p. 1.
571
[34] J. Bashir, M. Nasir Khan, Mater. Lett. 60 (2006) 470. [35] J.A.M. van Roosmalen, E.H.P. Cordfunke, R.B. Helmholdt, H.W. Zandbergen, J. Solid State Chem. 110 (1994) 100. [36] X.X. Zhang, J. Tejada, Y. Xin, G.F. Sun, K.W. Wong, X. Bohigas, Appl. Phys. Lett. 69 (1996) 3596. [37] J. Mira, J. Rivas, L.E. Hueso, F. Rivadulla, M.A. Lopez Quintela, J. Appl. Phys. 91 (2002) 8903. [38] Z.B. Guo, Y.W. Du, J.S. Zhu, H. Huang, W.P. Ding, D. Feng, Phys. Rev. Lett. 78 (1997) 1142.
