ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 290–291 (2005) 690–693 www.elsevier.com/locate/jmmm Properties of perovskites La1ÀxCdxMnO3 N.H LuongÃ, D.T Hanh, N Chau, N.D Tho, T.D Hiep Center for Materials Science, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam Available online 14 December 2004 Abstract The La1ÀxCdxMnO3 (x ¼ 0:1; 0.2, 0.3) perovskites have been prepared by solid reaction technology with sintering temperature of 1050 1C The samples are of single phase with rhombohedral structure of R-3c symmetry The fieldcooled (FC) and zero-field-cooled (ZFC) thermomagnetic measurements at low field indicate the spin glass-like state (or cluster glass) at low temperatures and a sharp change of magnetization around the phase-transition point of composition La0.7Cd0.3MnO3 This sample exhibits large value for maximum magnetic-entropy change |DSm|max of 2.88 J/kg K in the field of 13.5 kOe and reveals giant magnetocaloric effect This value for |DSm|max is larger than that of La0.7Sr0.3MnO3 and La0.7Pb0.3MnO3 perovskites The resistance measurements show that the conductivity of La1ÀxCdxMnO3 perovskites is metallic at low temperatures and semiconducting at high temperatures but the metal–semiconductor transition temperatures are not coinciding with paramagnetic–ferromagnetic transition ones The results can not be explained by using double-exchange (DE) model only In addition to the DE Jahn–Teller lattice distortion plays an important role r 2004 Elsevier B.V All rights reserved PACS: 75.30.Sg Keywords: Perovskite structure; Magnetic oxides; Spin-glass behavior; Magnetocaloric effect The Ln1ÀxA0 xBO3 perovskites (Ln ¼ rare earth element, A0 ¼ alkaline earth element, B ¼ Mn or Co) are attracting considerable interest because they exhibit many interesting physical effects and have potential for applications due to the complex relationship between crystal structure, electrical, magnetic and magnetocaloric properties [1–3] Troyanchuk et al [4,5] have reported structure, magnetic and resistivity measurements in Ln1ÀxCdxMnO3 (Ln ¼ La, Nd) compounds They found a paramagnetic to ferromagnetic transition, but not an insulator–metal transition in sample with x between 0.25 and 0.5 On the other hand, Sahana et al ÃCorresponding author Tel.: +84 5582216; fax: +84 8589496 E-mail address: luongnh@vnu.edu.vn (N.H Luong) [6] studied polycrystalline and thin film samples of La0.67Cd0.33MnO3 have found an insulator–metal transition at Curie temperature, TC In the present work, we report our study of structure, magnetocaloric effect and electric properties in La1ÀxCdxMnO3 perovskites The manganites with the following compositions have been prepared by conventional solid reaction technique: La1ÀxCdxMnO3 (No (x ¼ 0.1), No (x ¼ 0.2), No (x ¼ 0.3)) In order to prevent the evaporation of Cd during the synthesis, the samples were sintered at low as 1050 1C for 15 h The structure of samples was examined by Bruker X-ray diffractometer D5005 The microstructure and chemical composition were studied in a 5410 LV Jeol scanning electron microscope (SEM) which includes an energy dispersion spectrometer (EDS) Magnetic 0304-8853/$ - see front matter r 2004 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2004.11.338 ARTICLE IN PRESS N.H Luong et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 690–693 691 measurements were performed by using vibrating sample magnetometer (VSM) DMS 880 in magnetic field up to 13.5 kOe Resistance of samples was measured by the conventional DC-current four-probe method The SEM pictures showed that the samples are homogeneous, with the higher Cd amount substituted for La, the grain size increases from about 0.6 mm (x ¼ 0:1) to about 0.8 mm (x ¼ 0:3) due to low melting temperature of CdO EDS spectra of the samples show no strange elements and the sample compositions are similar to the nominal ones, i.e there has been no evaporation of Cd X-ray diffraction patterns of the samples are displayed in Fig No impurity peak was observed in the powder diffraction patterns Cd causes the deformation of geometric structure of samples because divalent ion Cd2+ has smaller radius than that of La3+ The structure analysis showed that the samples are of single phase with rhombohedral structure of R-3c symmetry Table presents the lattice parameters of the studied samples The ZFC and FC magnetization measurements were carried out in the applied field of 50 Oe and Fig shows these results It can be seen from this figure that the FC and ZFC curves are separated from each other at low temperatures and the higher the Cd content the stronger separation and the higher the magnetization Obviously, the larger Cd content substituted for La, the larger Mn4+ amount is formed and this leads to the strengthening of the ferromagnetic interaction In our Fig Thermomagnetic field-cooled (FC) and zero-fieldcooled (ZFC) curves of the studied samples Fig X-ray diffraction patterns of studied samples Table Lattice parameters of studied samples Sample a, b (A˚) c (A˚) c=a V (A˚3) x ¼ 0:1 x ¼ 0:2 x ¼ 0:3 5.501 5.503 5.490 13.331 13.333 13.327 2.433 2.423 2.428 348.2 349.2 347.6 samples the FC and ZFC thermomagnetic curves split at a certain so-called irreversibility temperature, Tr (TroTC) [7] The remarkable feature of magnetic properties of our samples is the appearance of shortrange ferromagnetic interaction, which confirmed by observing a spin-glass-like behavior at low temperatures and low field range [7–9] In the inset of Fig 2c, a plot of coercivity of sample No as a function of temperature is given Clearly, this sample is soft magnetic material with low magnetic anisotropy ARTICLE IN PRESS 692 N.H Luong et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 690–693 The TC of the samples determined from the M(T) measurements are listed in Table and are in agreement with values of TC for similar compositions [4,5] The magnetic entropy change as a function of temperature for samples x ¼ 0:2 and 0.3 is presented in Fig From this figure we can see that a sharp peak in |DSm(T)| is occurred around TC and there is the existence of giant magnetocaloric effect (GMCE) in sample La0.7Cd0.3MnO3 with |DSm|max of 2.88 J/kg K With decreasing x, the total amount of Mn4+ ions becomes smaller, leading to the strong decrease of |DSm|max in sample No The largest |DSm|max in this work is larger than that of the similar composition content La0.7Sr0.3MnO3 [10], La0.7Pb0.3MnO3 [11] as well as La0.7Sr0.3CoO3 [9] but is a little less than that of small Ni doping sample in system La0.7Sr0.3Mn1ÀxNixO3 (x ¼ 0:02; |DSm|max ¼ 3.54 J/kg K) by switching magnetic field of 13.5 kOe [12] The resistance of samples versus temperature is displayed in Fig It is clear that conductivity of all samples is metallic at low temperatures and semiconducting at high temperatures There are no coincidence between the metallic–semiconducting phase transition Table Some parameters of La1ÀxCdxMnO3Àd Sample x ¼ 0:1 x ¼ 0:2 x ¼ 0:3 /rAS (A˚) TC (K) |DSm|max (J/kg K) TRm (K) Er (eV) d 1.204 115 — 83 0.19 0.007 1.193 155 1.01 69 0.27 0.0083 1.181 150 2.88 76 0.28 0.0093 /rAS: mean ionic radius of A sites in ABO3 structure; TRm: temperature where maximum of R(T) curve occurs; Er: activation energy in paramagnetic state; d: determined by dicromatic titration method Fig The magnetic entropy change as function of temperature for samples No and Fig Resistance as a function of temperature for studied samples temperatures, TRm, and the magnetic transition temperatures, TC (see Table 2) Our results are different from those reported in [4,5] for La1ÀxCdxMnO3 which showed that all samples have semiconducting-type resistance up to 77 K Furthermore, our results are qualitatively in agreement with those reported in [6] for polycrystalline samples as well as thin film of La0.67Cd0.33MnO3 in which an insulatorto-metal transition is occurred We suppose that the cation distribution does not exactly correspond to the 2ỵ 3ỵ 4ỵ dominal chemical formula La3ỵ 1x Cdx Mn1x Mnx ịO3 because resistance of the samples increases with increasing x (Fig 4) It is well known that the smaller the A site cation relative to the anion, the greater distortion to orthorhombic symmetry [13] and the distortion of the MnO6 octahedra is more when the ionic radii of the divalent ion are smaller compared to the La3+ ion Moreover, it was observed that the increase in distortion of MnO6 octahedra is also created with the increase in insulating behavior and decrease in TC [14] Our results of M(T), TC and resistance measurements are fully supported by above mentioned remarks Obviously, the properties of studied samples cannot be explained by using double-exchange (DE) and superexchange (SE) model only [13,15] In addition to the DE and SE, a Jahn–Teller lattice distortion plays an important role From Table we can see also that the higher Cd content sample, the higher oxygen deficiency is In conclusion, the manganites La1ÀxCdxMnO3 (x ¼ 0:1; 0.2, 0.3) were prepared with single-phase rhombohedral structure There is spin glass-like state occurring in the samples The paramagnetic-ferromagnetic transition temperatures exhibiting in thermomagnetic curves are not coinciding with those of metal–insulator transitions In the sample with x ¼ 0:3; |DSm|max reached 2.88 J/kg K and the sample reveals GMCE ARTICLE IN PRESS N.H Luong et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 690–693 The authors acknowledge the financial support from the Vietnam National Fundamental Research Program (Project 421004) References [1] R von Helmolt, J Wecher, B Holzapfel, L Schultz, K Samwer, Phys Rev Lett 71 (1993) 2331 [2] C.N.R Rao, B Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific Publishing Co., Singapore, 1998 [3] M.D Coey, M Viret, S von Molnar, Adv Phys 48 (1999) 67 [4] I.O Troyanchuk, D.D 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Phys Rev B 54 (1996) 8992 [15] C Zener, Phys Rev 82 (1951) 403 ... behavior at low temperatures and low field range [7–9] In the inset of Fig 2c, a plot of coercivity of sample No as a function of temperature is given Clearly, this sample is soft magnetic material... diffraction patterns Cd causes the deformation of geometric structure of samples because divalent ion Cd2+ has smaller radius than that of La3+ The structure analysis showed that the samples are of single... magnetic field of 13.5 kOe [12] The resistance of samples versus temperature is displayed in Fig It is clear that conductivity of all samples is metallic at low temperatures and semiconducting at high