DSpace at VNU: Preparation and magneto-caloric effect of La1-xAgxMnO3 (x=0.10-0.30) perovskite compounds

Physica B 319 (2002) 168–173 Preparation and magneto-caloric effect of La1ÀxAgxMnO3 (x=0.10–0.30) perovskite compounds Nguyen The Hiena,b,*, Nguyen Phu Thuya,b a Cryogenic Laboratory, College of Natural Science, Faculty of Physics, Vietnam National University, 334 Nguyen Trai Road, Thanh Xuan, Thoung Dinh, Hanoi, Viet Nam b International Training Institute for Materials Science (ITIMS), The ITIMS Building, DHBK, Dai Co Viet Road, Hanoi, Viet Nam Received 16 February 2002; received in revised form 16 March 2002 Abstract Polycrystalline La1ÀxAgxMnO3 perovskite compounds (with x=0.10–0.30) have been synthesised by both the conventional solid-state reaction and the sol–gel method While all samples with Ag concentrations up to 0.20 consist of single-phase perovskites with rhombohedral structure, unreacted Ag was found in the samples with Ag concentrations of 0.22 and higher Magnetic properties of the as-prepared materials have been investigated The magneto-caloric effect in these compounds has been found to be considerably large and higher than that in other perovskite compounds in which La is substituted by divalent alkali-earth elements r 2002 Elsevier Science B.V All rights reserved Keywords: Magneto-caloric effect; La1ÀxAgxMnO3; Perovskite compounds Introduction The lanthanum-based manganite and cobaltate perovskite compounds, such as La1ÀxAxMnO3, La1ÀxAxCoO3 with A=Ca, Sr and Ba, etc have shown a variety of interesting electrical, electronic and magnetic properties that have great potentials for application Due to their colossal magnetoresistance (CMR) effect, these materials have been considered as promising candidates for magnetic sensor, magneto-resistive memory and recording applications, etc They have attracted, therefore, much research work in the last few years [1–3] Recent investigations have revealed that replacing *Corresponding author Fax: +84-4-858-4438 E-mail address: thehien@cryolab-hu.edu.vn (N.T Hien) the divalent alkali-earth metals by monovalent elements, such as Na, K and Li, also leads to similar phenomena in these compounds [4–6] Since the success in the fabrication of a continuously working demonstration magnetic refrigerator [7] and the discovery of the giant magneto-caloric effect (MCE) in the Gd5(Ge1ÀxSix)4 compounds (with 0pxp0.5) [8], there is a growing interest concerning the MCE and magnetic refrigeration Investigations are now focussed on new materials with high MCE at high (close to room) temperatures to be used as magnetic refrigerants [9] It has been shown that the perovskite compounds with lanthanum and divalent alkali-earth elements also exhibit a large MCE and, therefore, they can be considered as potential candidates for application as refrigerants in 0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) 1 - N.T Hien, N.P Thuy / Physica B 319 (2002) 168–173 Samples of La1ÀxAgxMnO3 were prepared by both the conventional solid-state reaction (with x ¼ 0:10; 0.13, 0.15, 0.17, 0.20, 0.22, 0.25, 0.27 and 0.30) and the sol–gel method (with x ¼ 0:10; 0.15, 0.20, 0.25 and 0.30) For the conventional solid-state reaction samples, powders of La2O3 of 3N purity, AgNO3 or Ag2O (4N) and MnO2 (4N) were two times manually ground, mixed, pelletised and fired at 8501C for 10–20 h Finally, the prefired pellets were reground, pressed again and sintered at 9501C for 48 h For the sol– gel samples, solutions of lanthanum nitrate hydrate La(NO3)2 Á 6H2O (4N), AgNO3 (4N), Mn(CH3COO)2 (4N), C6H8O7 Á 2H2O (4N), CH3COOH (4N) and NH4OH (4N) as starting chemicals were mixed in the nominal compositional ratio of the cations The xerogels obtained from the procedure were dried at 801C and heated at 650–7001C for about 2–5 h As-prepared samples were examined by X-ray diffraction (XRD) and by electron microscopy as well Magnetisation as a function of the temperature was measured in a vibrating sample magnetometer (VSM) for the temperature range from 100 to 350 K in applied fields up to mT From the data, the Curie temperatures of the paramagnetic to ferromagnetic phase transition were deduced Magnetisation curves were measured in applied fields up to T, at various temperatures around the Curie point, in the pulsed-field magnetometer (PFM) at the International Training Institute for Materials Science (ITIMS) [14] From these magnetisation curves, the MCE, i.e., the magnetic-entropy change –DSmag due to the change of Results and discussion In Fig 1, we show the XRD patterns for some La1ÀxAgxMnO3 samples as representatives for the two series of compounds prepared by the sol–gel (a) and by the solid-state reaction method (b) As can be noted in Fig 1a, the patterns for the three sol–gel samples with xp0:20 consist of reflections typical for single-phase La1ÀxAgxMnO3 perovskite compounds with rhombohedral structure, in agreement with the results reported by Tang et al [13] for their solid-state reaction (sintered) samples of similar compositions In the patterns for sintered samples with xX0:22; however, additional peaks occur at 2y ¼ 381; 44.51 and 64.51, which can be identified as due to the presence of metallic silver in the samples As it is clearly seen in Fig 2b, Intensity (arb.units) Experimental the applied fields DB; have been determined for all samples investigated, using the same procedure described previously [15] x =0.20 x =0.15 x =0.10 (a) 20 Intensity (arb.units) magnetic refrigeration, especially with respect to the material costs [10–12] Recent magneto-caloric investigations have also revealed considerable MCE in the lanthanum manganites where monovalent elements instead of divalent alkali-earth metals are substituted for La [4,5,13] In the present paper, we report on the solid-state reaction and sol–gel preparation, and the MCE in La1ÀxAgxMnO3 perovskite compounds with x ¼ 0:1020:30: 169 30 40 50 70 60 80 x =0.13 x=0.20 x =0.22 x =0.27 20 (b) 30 40 50 (degree) 60 70 Fig XRD patterns for some La1ÀxAgxMnO3 samples prepared by the sol–gel (a) and the solid-state reaction method (b) N.T Hien, N.P Thuy / Physica B 319 (2002) 168–173 170 the intensity of these additional peak increases in our corresponding sintered samples with increasing Ag concentration We note that the solid-state reaction samples were sintered at 9501C for 48 h whereas the sol–gel samples were heated at 7001C for h only We have found that even in the low Ag-concentration region, heating the xerogels at lower temperature and shorter time, and/or sintering the solid-state reaction samples at temperatures above 9501C for even longer time, both leads to inclusions of pure Ag in the samples Scanning and transmission electron microscopy (SEM and TEM) experiments (not shown here) were carried out to check the grain structure of the as-prepared samples The SEM photographs showed that the grains in the sintered samples reach sizes in the order of microns, whereas TEM experiments on a sol–gel sample of La0.90Ag0.10MnO3 revealed grain sizes of about Magnetisation (A.m2/kg) 1.2 1.0 0.8 0.6 La 0.80Ag 0.20MnO 0.4 FC 0.2 0.0 ZFC 100 (a) 200 300 Temperature (K) 400 Magnetisation (A.m2/kg) 1.2 (b) FC 1.0 0.8 0.6 ZF C La 0.83Ag 0.17MnO 0.4 0.2 100 130 160 190 220 250 280 310 340 Temperature (K) Fig (a) Magnetisation of the La0.80Ag0.20MnO3 sol–gel sample in the temperature range from 100 to 360 K in an applied field of 2.5 mT (b) Magnetisation of the La0.83Ag0.17MnO3 sintered sample in the temperature range from 100 to 350 K in an applied field of 10 mT 50 nm The presence of small amounts of pure metallic silver in the high Ag-concentration sintered samples is also indicated by another SEM analysis [16] Using both aforementioned preparation methods, however, we have not succeeded to fabricate a single-phase sample of the compound with x ¼ 0:30: We note that Tang et al [13] have reported the presence of unreacted metallic Ag, other precursor oxides and LaMnO3 in sintered samples with xX0:25: As an example for the compounds prepared by the sol–gel method, Fig 2a shows the magnetisation of the La0.80Ag0.20MnO3 sample as a function of temperature, measured on a VSM in a field of 2.5 mT for the temperature region from 100 to 360 K, in both field-cooled and zero fieldcooled modes The curves obtained are denoted by MðTÞFC and MðTÞZFC ; respectively Both MðTÞFC and MðTÞZFC curves show a sharp phase transition of the sample at about 305 K from the paramagnetic to the ferromagnetic state There is another phase transition at about 160 K obvious, which is probably related to the so-called reentrant magnetic phase transition The distinct separation between the MðTÞFC and MðTÞZFC curves in the temperature range below the Curie point suggests a spin-glass- or cluster-class-like behaviour often observed in this type of compounds [2] For comparison, we show in Fig 2b, the magnetisation of the La0.83Ag0.17MnO3 sample, prepared by the solid-state reaction method, in the temperature range from 100 to 350 K measured on a VSM in 10 mT in both the field-cooled and the zero fieldcooled mode Also here, the compound shows a spin-glass-like behaviour at low temperatures and a sharp phase transition at about 290 K from the paramagnetic to the ferromagnetic state Such a significant reduction of the magnetisation at low temperature as observed in the sol–gel sample, however, does not appear in the sintered type of samples Moreover, the MðTÞZFC curve reveals a lower transition temperature than the MðTÞFC one Further investigations are under way to elaborate the origin of the above-mentioned phenomena In Figs 3a and b, we show the magnetisation as a function of the applied field measured in the N.T Hien, N.P Thuy / Physica B 319 (2002) 168–173 PFM at various temperatures around the Curie point, on two La0.80Ag0.20MnO3 samples prepared by the two different methods, again as representatives for the series of compounds investigated While the sintered sample clearly exhibits a saturated ferromagnetic state just below the Curie point, which can be observed in the behaviour of the MðBÞ curves in Fig 3b, the magnetisation MðBÞ curves for the La0.80Ag0.20MnO3 sol–gel sample in Fig 3a at temperatures far below the Curie point not show any tendency of saturation even in applied fields as high as T This can be due to the competition between the antiferroand the ferromagnetic phases and/or a superparamagnetic behaviour of the nanosized particles in this sample Actually, as mentioned above, we have observed grain sizes in the order of about 50 nm in a La0.90Ag0.10MnO3 sol–gel sample, which was prepared by the same procedure as the one used in this measurement 230 K 240 K 250 K 260 K 50 270 K 40 280 K 290 K 30 300 K 20 Fig Entropy change as a function of the temperature at different field variations for the La0.78Ag0.22MnO3 sintered sample La 0.80 Ag0.20 MnO3 Sol-gel sample 10 0 (a) Field (T) 80 Magnetisation (A.m2/kg) 210K 220K 235K 245K 255K 260K 265K 270K 275K 280K 285K 290K 295K 300K 305K 310K La 0.80 Ag0.20 MnO3 70 60 50 40 30 20 10 0 (b) From these magnetisation curves we derived the magnetic-entropy change ÀDSmag caused by the variation of the applied field as the MCE for the samples Results shown in Fig present the magnitude of the MCE for the La0.78Ag0.22MnO3 sintered sample at different field variations (from zero field up to the indicated value DB) It is clearly seen that for DB ¼ T; the magnetic-entropy change at the Curie temperature in this sample reaches a value of about 2.9 J/kg K, and about 7.8 J/kg K for DB ¼ T: In Fig 5, we show the entropy change as a function of temperature, at a field variation of DB ¼ T for three sintered samples with x ¼ 0:17; 0.20 and 0.22 We note 0.5 1.5 2.5 3.5 La0.83Ag0.17MnO3 2.5 -∆ ∆ Smag (J/kg K) Magnetisation (A.m2/kg) 60 171 La0.80Ag0.20MnO3 La0.78Ag0.22MnO3 1.5 0.5 Field (T) Fig (a) Isothermal magnetisation curves for the La0.80Ag0.20MnO3 sol–gel sample measured at different temperatures from 230 to 300 K (b) Isothermal magnetisation curves for the La0.80Ag0.20MnO3 sintered sample measured at different temperatures from 210 to 310 K 260 270 290 280 T (K) 300 310 Fig Entropy change at a field variation of T, as a function of temperature, for La1ÀxAgxMnO3 sintered samples 172 N.T Hien, N.P Thuy / Physica B 319 (2002) 168–173 that our sample with x ¼ 0:22 shows the highest MCE The Curie temperature TC and the maximum magnetic-entropy change ÀDSmag at DB ¼ T as a function of the Ag content is summarised in Fig As we can see, the Curie temperature of the sol–gel samples increases rapidly from 150 (for x=0.10) to around 300 K for x ¼ 0:15; and becomes almost saturated at about 310 K for Ag concentrations of xX0:20: For the sintered samples, TC increases more gradually from 250 K for x ¼ 0:10; via 280 K for x ¼ 0:13; 290 K for x ¼ 0:17; to 300 K for x ¼ 0:20 and finally to a saturated value of about 306 K for x > 0:20: These values are compared with those reported by Tang et al [13] for their solid-state reaction samples of corresponding composition, i.e 214, 278, 306 and 306 K for x ¼ 0:05; 0.20, 0.25 and 0.30, respectively The values of TC for the sintered solid-state reaction samples are somewhat lower than those for the sol–gel samples of the corresponding compositions This might be caused by the lower actual Ag concentration, as can be inferred from the XRD experiments, due to the presence of small amounts of unreacted metallic Ag in the samples In this figure, we can also see that the MCE in the sol–gel samples is somewhat lower than that in the sintered ones This can be ascribed to the fact that the sol–gel samples were heated at 7001C for h only Under these conditions, the single-phase La1ÀxAgxMnO3 compounds have been fully formed, but the grains have not been so far developed as in the sintered samples The MCE in our sintered samples reaches a maximum value of 2.9 J/kg K at x ¼ 0:22: This value is, however, somewhat lower than a maximum value of 3.4 J/ kg K reported by Tang et al [13] for their x ¼ 0:20 sample Even so, this is significantly higher than, for instance, 2.4 J/kg K in the perovskite compound of La0.60Ca0.40MnO3 [11] It is thus, worth to note that for the same field variation, the MCE at the Curie temperature in the La1ÀxAgxMnO3 system is remarkably higher than in the perovskite compounds with lanthanum and divalent alkaliearth metals Hysteresis loop measurements (not shown here) yielded coercivities as low as mT, similar to the values reported by Tang et al [13] This revealed the materials to be of soft ferromagnetic type and, in this respect, also suitable for room temperature magnetic-refrigeration application In conclusion, we have prepared samples of La1ÀxAgxMnO3 perovskite compounds (with x¼ 0:1020:30) by both the sol–gel and solid-state reaction methods While single-phase materials of the rhombohedral perovskite compounds have been obtained for Ag concentrations up to x ¼ 0:20; in the samples with xX0:25 small amounts of metallic Ag are still present The materials as obtained show significant MCE at Curie temperatures as high as 310 K, along with other interesting magnetic properties, and can be considered as a promising potential candidate for the application as magnetic refrigerants in room temperature magnetic refrigeration Acknowledgements Fig The Curie temperature (right scale) and the entropy change (left scale) as a function of the silver concentrations in the La1ÀxAgxMnO3 system This paper is dedicated to Prof Dr J.J.M Franse from the University of Amsterdam who will celebrate his 65th anniversary these days The authors are grateful to him who, in his long standing scientific co-operation with Vietnam, has given a lot of stimulation and inspiration on the development of the high-pulsed-field magnetometer at ITIMS, and of the research activities on superconducting cuprates at the Cryogenic N.T Hien, N.P Thuy / Physica B 319 (2002) 168–173 Laboratory The work described here is actually a follow up of such developments This work is part of the research project QGTD00-01 granted by the Vietnam National University (VNU), Hanoi, and partly supported by the State Programme in Fundamental Research of Vietnam Furthermore, the authors would like to express their sincere thank to their colleagues: Mr Pham Van Tong from the Cryogenic Laboratory, Dr Le Van Vu and Mr Phung Quoc Thanh from the Centre for Materials Science (CMS), Faculty of Physics, College of Natural Science, VNU Hanoi; 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