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ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 310 (2007) 2826–2828 www.elsevier.com/locate/jmmm Large magnetocaloric effect around room temperature in La0.7Ca0.3ÀxPbxMnO3 perovskites D.T Hanha, M.S Islamb, F.A Khanc, D.L Minhd, N Chaua,Ã a Center for Materials Science, University of Science, Vietnam National University, 334 Nguyen Trai Road, Hanoi, Vietnam b Department of Physics, Government Bangla College, Mirpur, Dhaka, Bangladesh c Department of Physics, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh d Department of Physics, University of Science, Vietnam National University, 334 Nguyen Trai Road, Hanoi, Vietnam Available online 21 November 2006 Abstract Among perovskite manganites, a series La1ÀxCaxMnO3 has the largest magnetocaloric effect (MCE) (|DSm|max ¼ 3.2–6.7 J/kg K in DH ¼ 1.35 T) but their Curie temperatures, TC, are quite low (165–270 K) A system La1ÀxPbxMnO3 has quite high TC (235–360 K) but exhibits not so large MCE (0.65–1.53 J/kg K) The manganites La0.7Ca0.3ÀxPbxMnO3 (x ¼ 0.05, 0.10, 0.15, and 0.20) have been fabricated by solid-state reaction technique with the expectation that they could establish large MCE at room temperature region The prepared samples are of single phase with orthorhombic structure Lattice parameters as well as volume of unit cell are continuously increased with increasing x due to large Pb2+ ions substituted for smaller Ca2+ ions There is spin-glass (or cluster glass)-like state occurring in studied samples TC increases continuously from 270 K (for x ¼ 0.05) to 338 K (for x ¼ 0.20) Large MCE has been obtained in all samples and |DSm|max reached the highest value of 3.72 J/kg K (with magnetic field variation DH ¼ 1.35 T) for the sample x ¼ 0.05 and the studied samples could be considered as giant magnetocaloric materials working for magnetic refrigeration The magnetoresistance of samples exhibited colossal value r 2006 Elsevier B.V All rights reserved PACS: 75.30.Sg; 75.47.Gk; 75.47.Lx; 75.50.Lk Keywords: Perovskite structure; Magnetic oxides; Spin-glass behavior; Magnetocaloric effect One of recent researches aims to study the interplay between structure, magnetic, and transport properties in perovskite-type manganites Ln1ÀxA0 xMnO3 (where Ln and A0 are rare earth and alkaline element, respectively, which locate at A-sites in the perovskite ABO3) [1,2] Their magnetotransport and structural properties are functions of ionic radius at A-site, applied magnetic field, doping concentration, and temperature In previous papers, the authors have paid attention to hole-doped manganites with A ¼ Pb [3–5] Among manganites, La1ÀxCaxMnO3 has the largest magnetocaloric effect (MCE) (|DSm|max ¼ 3.2–6.7 J/ kg K in DH ¼ 1.35 T) but their Curie temperatures, TC, are quite low (165–270 K) [6] A system La1ÀxPbxMnO3 has quite high TC (235–360 K) but exhibits not so large MCE ÃCorresponding author Tel.: +84 5582216; fax: +84 8589496 E-mail address: chau@cms.edu.vn (N Chau) 0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2006.10.1061 (0.65–1.53 J/kg K) [4] The manganites La0.7Ca0.3ÀxPbx MnO3 (x ¼ 0.05, 0.10, 0.15, and 0.20) have been fabricated by solid-state reaction technique with the expectation that they could establish large MCE at room temperature region The structure of samples was examined in a Bruker D5005 X-ray diffractometer The microstructure was studied in a scanning electron microscope (SEM) 5410 LV Jeol Magnetic measurements were performed in a vibrating sample magnetometer (VSM) Digital Measurement System DMS 880 in magnetic field up to 1.35 T The SEM pictures of the surface of samples show that the studied samples are quite homogeneous and particle size is approximately 0.8–1.2 mm and almost remains unchanged with increasing Pb content The X-ray diffraction patterns of samples show that all samples are of single phase with orthorhombic symmetry Reasonably, lattice ARTICLE IN PRESS D.T Hanh et al / Journal of Magnetism and Magnetic Materials 310 (2007) 2826–2828 parameters as well as volume of unit cell are continuously enhanced with increasing x due to the large Pb2+ ions ˚ substituted for smaller Ca2+ ions (r2ỵ Pb ẳ 1:35 A) (r2ỵ Ca ẳ 1:18 A) Lattice parameters and volume of unit cell are listed in Table The FC and ZFC magnetization measurements were performed in low applied magnetic field of mT and the results are presented in Fig From this figure we can see that at low temperatures, the FC and ZFC magnetization curves are separated from each other and the samples can be regarded as spin-glass-like manganites The ferromagnetic-to-paramagnetic transition temperature, TC, and the spin-glass transition temperature, Tf, were estimated for all samples As a result of increasing /rAS, the values of TC were shifted to higher temperature with doping Pb content, from 270 K (x ¼ 0.05) to 338 K (x ¼ 0.20) The increasing of TC (or magnetic coupling versus double exchange interaction) can be due to the increasing of /rAS and the increasing of Mn4+/Mn3+ ratio In this case, the increasing of TC is due to the first term The variation of magnetic entropy change, |DSm|, on temperature was calculated and shown in Fig All samples exhibit large MCE and they could be considered as giant MCE materials The maximum value of |DSm| was 3.72 J/kg K for composition x ¼ 0.05 This decreasing of |DSm|max in comparison with that of the system Table Lattice parameters of samples La0.7Ca0.3ÀxPbxMnO3 Sample a (A˚) b (A˚) c (A˚) Vunit x ¼ 0.05 x ¼ 0.10 x ¼ 0.15 x ¼ 0.20 5.4695 5.4728 5.4817 5.4898 5.4703 5.4783 5.4914 5.5107 7.7417 7.7474 7.7583 7.7657 231.0 232.3 233.1 234.9 cell (A˚3) 2827 Fig Magnetic entropy change as a function of temperature of the samples La0.7Ca0.3ÀxPbxMnO3 La1ÀxCaxMnO3 (|DSm|max ¼ 6.7 J/kg K) is attributed to the inhomogeneity of the structure Another interesting fact is that TC values are in the vicinity of room temperature Therefore, our investigated samples could be used as giant MCE materials working around room temperature For all studied samples, we also estimated the relative cooling power (RCP), which is defined by the expression RCP ¼ |DSm|max Á dTFWHM, where dTFWHM means the full-width at half-maximum of the magnetic entropy change versus temperature [7] The maximum value of RCP (sample x ¼ 0.05) is nearly two times larger than that of sample La0.67Ca0.33MnO3 (RCP ¼ 47 J/kg) [8] The obtained results indicated that the sample La0.7Ca0.2Pb0.1MnO3 with TC ¼ 300 K and RCP ¼ 76 J/ kg (DH ¼ 1.35 T) is a good material for use in magnetic refrigeration working at room temperature region To study more deeply the properties of samples, the measurements of magnetoresistance (MR) versus temperature at applied magnetic field of 1.35 T were performed The compositions show a peak of MR at near respective TC and the maximum of MR has reached 23% for composition x ¼ 0.10 This work was supported by the Vietnam National Fundamental Research Program for Natural Sciences References Fig The FC and ZFC thermomagnetic curves of samples La0.7 Ca0.3ÀxPbxMnO3, H ¼ mT [1] H.Y Hwang, S.-W Cheong, P.G Radaelli, M Marezio, B Batlogg, Phys Rev Lett 75 (1995) 914 [2] P.G Radaelli, D.E Cox, M Marezio, S.-W Cheong, P.E Schiffer, A.P Ramirez, Phys Rev Lett 75 (1995) 4488 [3] N Chau, D.T Hanh, N.D Tho, N.H Luong, J Magn Magn Mater 303 (2006) e335 ARTICLE IN PRESS 2828 D.T Hanh et al / Journal of Magnetism and Magnetic Materials 310 (2007) 2826–2828 [4] N Chau, H.N Nhat, N.H Luong, D.L Minh, N.D Tho, N.N Chau, Physica B 327 (2003) 270 [5] T.S Wang, C.H Chen, M.F Tai, Mater Res Soc Symp Proc 674 (2001) U341 [6] T.D Hiep, N Chau, N.D Tho, N.H Luong, in: Proceedings of the Ninth Vietnam Biennial Conference on Radio and Electronics (REV’04), November 26–27, 2004, Hanoi, Vietnam, p 339 [7] K.A Gschneidner, V.K Pecharsky, Annu Rev Mater Sci 30 (2000) 387 [8] Z.B Guo, Y.W Du, J.S Zhu, H Huang, W.P Ding, D Feng, Phys Rev Lett 78 (1997) 1142 ... the vicinity of room temperature Therefore, our investigated samples could be used as giant MCE materials working around room temperature For all studied samples, we also estimated the relative... 0.20) The increasing of TC (or magnetic coupling versus double exchange interaction) can be due to the increasing of /rAS and the increasing of Mn4+/Mn3+ ratio In this case, the increasing of TC... transition temperature, TC, and the spin-glass transition temperature, Tf, were estimated for all samples As a result of increasing /rAS, the values of TC were shifted to higher temperature with doping
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