ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 272–276 (2004) 1292–1294 Large positive entropy change in several charge-ordering perovskites N Chaua,*, D.H Cuonga, N.D Thoa, H.N Nhata, N.H Luonga, B T Congb a b Center for Materials Science, Hanoi Vietnam National University, 334 Nguyen Trai, Hanoi 844, Viet Nam Department of Solid State Physics, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Abstract The Nd0.5Sr0.5Mn1ÀxCuxO3 (x ¼ 0:00; 0.02; 0.10) and Nd0.25Pr0.25Sr0.5MnO3 perovskites have been prepared by solid reaction technology The manganites are of single phase with orthorhombic structure The FC and ZFC measurements show an evident existence of charge-ordering effect at low temperatures At the charge-ordering temperature the magnetic entropy change, DS; is positive and the jDSmax j is larger than that determined at ferromagnetic-paramagnetic phase transition temperature r 2003 Elsevier B.V All rights reserved PACS: 75.47.Lx Manganites Keywords: Magnetocaloric effect; Magnetic oxides; Perovskite structure; Magnetization; Charge ordering Charge-ordered phases of rare earth manganites are novel effect arising from interactions between the charge carriers and phonons, giving rise to the localization of carriers at specific sites in the lattice below a certain temperature Charge-ordering (CO) compounds have a large variation of resistivity and magnetization as a function of temperature and magnetic field [1–3] In this work we report our study of magnetocaloric effect in several charge-ordering perovskites The four compositions: No: : Nd0:5 Sr0:5 MnO3 ; No: : Nd0:5 Sr0:5 Mn0:98 Cu0:02 O3 ; No: : Nd0:5 Sr0:5 Mn0:9 Cu0:1 O3 ; No:4 : Nd0:25 Pr0:25 Sr0:5 MnO3 are prepared by solid reaction technique *Corresponding author Tel./fax: +84-4-8589496 E-mail address: chau@cms.edu.vn (N Chau) Fig shows the X-ray diffraction patterns of studied samples The analysis showed that the crystal structure of the samples is orthorhombic The Rietveld refinement was done for determination of bond angle and bond length in investigated samples Compared to the structure of Nd0.5Sr0.5MnO3 published in Ref [4], our sample No showed a good coincidence Fig shows the thermomagnetic field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves measured at 20 and 50 Oe for all samples studied We can see that the existence of charge-ordering effect is clearly evident in sample Nos 1, and (Fig 2a,b and d) but a larger amount of Cu subtituted for Mn destroys CO (Fig 2c) Two magnetic transitions are clear from these curves: the paramagnetic (PM) to ferromagnetic (FM) at TC ¼ 265 K for the sample Nos and 4, TC ¼ 230 K for the sample No and TC ¼ 260 K for sample No as well as FM-AFM transition TCO ¼ 175 K for sample No 1, 170 K for sample Nos and Note that the small amounts of Cu substituted for Mn lead to the strong reduction of magnetization and TC (Fig 2b and c) The entropy change resulting from the spin ordering induced by the applied magnetic field can be obtained 0304-8853/$ - see front matter r 2003 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2003.12.074 ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1292–1294 (a) S (J/Kg.K) Intensity (a.u) o N N o o 20 30 40 50 60 2-Theta ∇ N o N 70 80 M (emu/g) 120 (b) 0.05 FC ZFC H= 20 Oe (c) FC ZFC 0.06 0.02 H = 50 Oe FC ZFC (d) 1.5 0.5 H=20 Oe 100 150 200 250 1.3T 1T 0.5T 300 350 Temperature (K) Fig Thermomagnetic field-cooled (FC) and zero-fieldcooled (ZFC) curves for the samples: (a) No 1, (b) No 2, (c) No and (d) No according to the thermodynamic relation [5]: Z H max DSðT; Hị ẳ fqMT; Hị=qTgH dH: 160 200 240 280 Tem perature (K) 320 Fig The magnetic-entropy change as a function of temperature for the samples No (a) and No (b) FC ZFC H=20 Oe 0.15 (b) -1 (a) 0.5 1.35T 1T 0.5T -1 90 Fig X-ray diffraction patterns of studied samples 1.5 1293 ð1Þ The evaluation of DSðTÞ was presented in Ref [6] Fig shows the magnetic entropy changes, DS; over large temperature interval for the sample Nos and From these curves we can see that a sharp positive peak in DSðTÞ at TCO is occurred together with the second negative peak in DSðTÞ around TC and jDSj at TCO is nearly two times larger than that determined around TC : The doping of Cu in sample Nos and not only leads to the decreasing of magnetization and Curie temperature of the samples but also strongly reduces jDSmax j by nearly one order of magnitude Note that CO transition is of first order in nature [7], whereas at TC there is the second-order transition We suggest that when Cu is partially substituted for Mn, the Mn3+/Mn4+ ratio decreases, because Cu ions are in the state of Cu3+ or even Cu2+ This factor leads to weakening the double exchange in both sample Nos and That is the main reason of the decrease of magnetization, Curie temperature as well as jDSmax j: In summary we conclude that the four compositions Nd0.5Sr0.5Mn1ÀxCuxO3 (x ¼ 0:00; 0.02, and 0.10) and Nd0.25Pr0.25Sr0.5MnO3 were prepared with single phase and exhibited orthorhombic structure The FC and ZFC thermomagnetic measurements show the evident existence of charge-ordering effect in samples Nd0.5Sr0.5MnO3, Nd0.5Sr0.5Mn0.98Cu0.02O3, and Nd0.25Pr0.25Sr0.5MnO3 The maximum value of magnetic entropy change, DSmax measured at first-order phase transition is positive and absolute value of DSmax is about two times larger than that measured at TC : In the sample with x ¼ 0:1 the charge ordering is destroyed, moreover magnetization is strongly reduced References [1] G Xiao, G.Q Gong, C.L Canedy, E.J McNiff Jr., A Gupta, J Appl Phys 81 (1997) 5324 [2] Y Moritomo, Phys Rev B 60 (1999) 10374 ! [3] J Lopez, P.N Lisboa-Filho, O.F Delima, F.M AranjoMoreira, J Magn Magn Mater 242–245 (2002) 683 ARTICLE IN PRESS 1294 N Chau et al / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1292–1294 [4] V Caignaert, F Millange, M Hervieu, E Suard, B Raveau, Solid State Commun 99 (1996) 173 [5] A.M Tishin, J Magn Magn Mater 184 (1998) 62 [6] N Chau, H.N Nhat, N.H Luong, D.L Minh, N.D Tho, N.N Chau, Physica B 327 (2003) 270 [7] H Kuwahara, Y Tomioka, A Asamitsu, Y Morytomo, Y Tokura, Science 270 (1995) 961; H Kuwahara, Y Tokura, in: C.N.R Rao, B Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific Pub., Singapore, 1998, p 217 ... positive peak in DSðTÞ at TCO is occurred together with the second negative peak in DSðTÞ around TC and jDSj at TCO is nearly two times larger than that determined around TC : The doping of Cu in sample... evaluation of DSðTÞ was presented in Ref [6] Fig shows the magnetic entropy changes, DS; over large temperature interval for the sample Nos and From these curves we can see that a sharp positive. .. value of magnetic entropy change, DSmax measured at first-order phase transition is positive and absolute value of DSmax is about two times larger than that measured at TC : In the sample with