Magnetocaloric effect of La0.8Sr0.2MnO3 compound under pressure
Transcrição
Magnetocaloric effect of La0.8Sr0.2MnO3 compound under pressure
JOURNAL OF APPLIED PHYSICS 97, 10M317 共2005兲 Magnetocaloric effect of La0.8Sr0.2MnO3 compound under pressure Daniel L. Rocco Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP, Caixa Postal 6165, 13083-970 Campinas, SP, Brazil R. Almeida Silva Departamento de Física e Ciência dos Materiais, Instituto de Física de São Carlos, Universidade de São Paulo-USP, CP 369, 13560-590 São Carlos, SP, Brazil A. Magnus G. Carvalho and Adelino A. Coelho Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP, Caixa Postal 6165, 13083-970 Campinas, SP, Brazil José P. Andreeta Departamento de Física e Ciência dos Materiais, Instituto de Física de São Carlos, Universidade de São Paulo-USP, CP 369, 13560-590 São Carlos, SP, Brazil Sergio Gama Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP, Caixa Postal 6165, 13083-970 Campinas, SP, Brazil 共Presented on 9 November 2004; published online 16 May 2005兲 The La0.8Sr0.2MnO3 compound presents a ferromagnetic paramagnetic transition around room temperature to which a reasonably high magnetocaloric effect is associated, turning this material of interest for application in magnetic refrigeration. We synthesized this compound in fiber single crystalline form by the Laser Heated Pedestal Growth method. The sample was characterized by x-ray diffraction and magnetic measurements as a single phase and with the required magnetic properties. We measured the magnetic properties and the magnetocaloric effect under hydrostatic pressure for pressures up to 6 kbar as a function of temperature. Our results indicate that the Curie temperature increases with pressure while the low temperature transition from the orthorhombic to the rhombhoedral structures decreases as pressure increases. This is in close agreement with the literature. Measurement of the magnetocaloric effect at the high temperature transition indicates that the peak of the effect follows the trend of the Curie temperature, but its maximum value remains almost constant as a function of pressure. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1856891兴 I. INTRODUCTION The manganites are ceramic compounds with great interest due to the colossal magnetoresistance effect and also because they present a rich set of physical phenomena, such as spin, charge, and orbital orderings, which impart them interest for applications as well as theoretical studies.1 The system La1−xSrxMnO3 is of great importance because it can show a high temperature ferromagnetic transition and also presents a rich electrical and magnetic phase diagram.2 For low Sr concentration x ⬍ 0.1, the material presents a metallic antiferromagnetic state, for 0.1⬍ x ⬍ 0.15 it shows an insulating ferromagnetic state and for x ⬎ 0.15 it shows a metallic ferromagnetic state.2 Besides this, the magnetic transition temperature varies strongly with x, from 170 K for x = 0.11 up to 310 K for x = 0.2, and this allows one to easily tune the transition to a desired value.3 Recently, the manganites have been considered as potential candidates for active magnetic regenerators in magnetic refrigeration systems because they can show considerable magnetocaloric effects 共MCE兲, are easily fabricated and can have the Curie temperatures tuned in a wide temperature range.4,5 The system La1−xSrxMnO3 shows a great potential in relation to the MCE because of the properties already 0021-8979/2005/97共10兲/10M317/3/$22.50 described and because for x = 0.13 and x = 0.16 it shows MCE comparable to the ones observed for Gd and Ca doped manganites and for metallic alloys.4 Also, its properties under pressure have been studied for x = 0.10, which corresponds to the lower edge of the concentration region in which the low temperature state is a ferromagnetic insulator.6 The compound with x = 0.2 is of particular interest in relation to the MCE because it presents a TC of ⬃305 K, and a structural transition from rhombohedral 共R兲 to orthorhombic 共O*兲 structure at 100 K. Here we present our results on the determination of its MCE under hydrostatic pressure. II. EXPERIMENT The compound with nominal composition La0.8Sr0.2 MnO3 has been prepared directly from the stoichiometric mixture of oxides and carbonates by the Laser Heated Pedestal Growth 共LHPG兲 method.7 The resulting sample in single crystalline fiber form has been characterized by x-ray diffraction and Laue back reflection diffraction, metallographic analysis, electron microprobe analysis, and magnetic measurements. The MCE has been measured through the magnetic method using the numerically integrated Maxwell relation 97, 10M317-1 © 2005 American Institute of Physics Downloaded 21 Jun 2005 to 143.106.128.168. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp 10M317-2 J. Appl. Phys. 97, 10M317 共2005兲 Rocco et al. FIG. 1. Effect of pressure on the low temperature structural transition of the La0.8Sr0.2MnO3 compound. ⌬S共T,H兲 = 冕冉 冊 H2 H1 M T dH. 共1兲 H Magnetization measurements were done using a commercial SQUID magnetometer. For the pressure measurements, we used a Cu–Be clamp type cell, able to work up to 12 kbar at 300 K. The sample was inserted in a Teflon container filled with mineral oil. Our pressure scale has been obtained using a MnAs sample as a manometer, measuring TC’s determined increasing the temperature, and comparing with data from Menyuk et al..8 For the MCE determination, M ⫻ H curves 共up to H = 5 T兲 at several fixed pressures were taken with both field and temperature always increasing. III. RESULTS AND DISCUSSIONS The results of x-ray Laue back reflection diffraction, metallographic and electron microprobe analyses confirmed the single phase nature of our sample, presenting the expected perovskite structure. The magnetic analysis showed that the sample presents the low temperature transition at 110 K from the R to O* structure, with thermal hysteresis, and the ferromagnetic paramagnetic transition at 302 K, of second order character. Application of pressure displaces the low temperature transition to even lower temperatures, and has the effect of decrease its magnetic amplitude, as if the pressure diminishes the difference between the two structures. This effect is shown in Fig. 1, and the curves shown are the ones for increasing temperature. This transition shifts to lower temperatures at a rate of −6.9 K / kbar, or, in terms of d共ln共TS兲 / dp, −0.72 GPa−1. Pressure increases the TC of the high temperature transition, as shown in Fig. 2, at a rate of 1.4 K / kbar, or, in terms of d共ln共TC兲 / dp, 0.045 GPa−1. This last value compares favorably with the same quantity observed for the compound with x = 0.15.9 Figure 3 shows the variation of the transition temperatures as a function of pressure, as well as the respective linear fittings of the curves. Under pressure, our measurements at 4 K applying magnetic fields up to 7 T show that the saturation magnetization FIG. 2. Effect of pressure on the Curie temperature of the La0.8Sr0.2MnO3 compound. of this compound does not change with pressure up to 6 kbar. We measured the MCE at the ferroparamagnetic transition for a field variation of 5 T always increasing field and temperature. Figure 4 shows the results obtained. For ambient pressure the MCE has a value similar to the one measured for the compounds with x = 0.13 and x = 0.16.4 Under pressure, the MCE shows a slight decrease in the peak value with pressure and the peak is displaced to higher temperatures, following the same behavior as the Curie temperature. This behavior is in contrast with the behavior of the Gd5Ge2Si2 compound under pressure, whose Curie temperature also increases with pressure, and whose MCE shows a markedly decrease with pressure.10,11 This behavior is also in great contrast with the one observed for the MnAs compound, for which the Curie temperature decreases and the MCE becomes colossal.12 IV. CONCLUSIONS Under pressure, the compound La0.8Sr0.2MnO3 presents a displacement of the low temperature structural transition to FIG. 3. Transition temperatures for the ferromagnetic–paramagnetic and structural transitions as a function of pressure for the La0.8Sr0.2MnO3 compound. Downloaded 21 Jun 2005 to 143.106.128.168. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp 10M317-3 J. Appl. Phys. 97, 10M317 共2005兲 Rocco et al. ACKNOWLEDGMENTS The authors thank financial support from Fundação de Amparo à Pesquisa do Estado de S. Paulo—Fapesp, from Coordenação de Aperfeiçoamento do Pessoal de Nível Superior—Capes and from Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq. E. Dagotto, T. Hotta, and A. Moreo, Phys. Rep. 344, 1 共2001兲. A. Dutta, N. Gayathri, and R. Ranganathan, Phys. Rev. B 68, 054432 共2003兲. 3 B. Dabrowski, X. Xiong, Z. Bukowski, R. Dybzinski, P. W. Klamut, J. E. Siewenie, O. Chmaissem, J. Shaffer, C. W. Kimball, J. J. Jorgensen, and S. Short, Phys. Rev. B 60, 7006 共1999兲. 4 A. Szewczyk, H. Szymczak, A. Wisniewski, K. Piotrowski, R. Kartaszynski, B. Dabrowski, S. Kolesnik, and Z. Bukowski, Appl. Phys. Lett. 77, 1026 共2000兲. 5 W. Zhong, W. Chen, C. T. Au, and Y. W. Du, J. Magn. Magn. Mater. 261, 238 共2003兲. 6 E. S. Itskevich, V. F. Kraidenov, A. E. Petrova, V. A. Ventcel’, and A. V. Rudnev, Low Temp. Phys. 29, 30 共2003兲. 7 R. de Almeida Silva, A. S. S. de Camargo, C. Cusatis, L. A. O. Nunes, and J. P. Andreeta, J. Cryst. Growth 262, 246 共2004兲. 8 N. Menyuk, J. A. Kafalas, K. Dwight, and J. B. Goodenough, Phys. Rev. 177, 942 共1969兲. 9 M. Itoh, K. Nishi, J. Ding Yu, and Y. Inaguma, Phys. Rev. B 55, 14408 共1997兲. 10 A. Magnus G. Carvalho, A. de Campos, A. A. Coelho, S. Gama, F. C. G. Gandra, P. J. von Ranke, and N. A. de Oliveira, 49th Annual Conference on Magnetism and Magnetic Materials, Jacksonville, FL, November 2004. 11 S. Gama, A. Magnus, G. Carvalho, A. de Campos, A. A. Coelho, F. C. G. Gandra, P. J. von Ranke, and N. A. de Oliveira, 20th General Conference of the Condensed Matter Division of the European Physical Society, Prague, July 19–23, 2004, Prague, July 2004. 12 S. Gama, A. A. Coelho, A. de Campos, A. Magnus, G. Carvalho, F. C. G. Gandra, P. J. von Ranke, and N. A. de Oliveira, Phys. Rev. Lett. 93, 237202 共2004兲. 1 2 FIG. 4. Behavior of the magnetocaloric effect for a field variation of 5 T for the La0.8Sr0.2MnO3 compound. lower temperatures, at a rate of −6.9 K / kbar, and at the same time the transition is becoming less pronounced. The temperature of the ferroparamagnetic phase transition increases with pressure at a rate of 1.4 K / kbar, or, equivalently, d共ln共TC兲 / dp = 0.045 GPa−1, comparable with the value observed for the x = 0.15 compound. The saturation magnetization is not altered by the pressure up to 6 kbar. The magnetocaloric effect decreases very slightly with pressure, and the peak value is also displaced to higher temperatures, following the Curie temperature. Downloaded 21 Jun 2005 to 143.106.128.168. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
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