Physica B 327 (2003) 270–278 Structure, magnetic, magnetocaloric and magnetoresistance properties of La1ÀxPbxMnO3 perovskite Nguyen Chaua,*, Hoang Nam Nhata, Nguyen Hoang Luonga, Dang Le Minhb, Nguyen Duc Thoa, Nguyen Ngoc Chaua b a Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Department of Solid State Physics, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Abstract La1ÀxPbxMnO3 (x ¼ 0:1; 0.2, 0.3, 0.4, and 0.5) perovskites were prepared by a solid-state reaction Except for x ¼ 0:5 (cubic) and x ¼ 0:4 (rhombohedral), the structure of the other compositions was pseudo-rhombohedral with P1 symmetry The particle size of the grains is depending on the Pb content of the samples The Curie temperature Tc increases from 235 K for x ¼ 0:12310 K for x ¼ 0:2 and is almost constant (about 360 K) for xX0:3: The field-cooled and zero-field-cooled thermomagnetic curves measured at low field show a split below a so-called irreversibility temperature Tr ; which is somewhat smaller than Tc except for x ¼ 0:1; where it is 270 K From a series of magnetic isotherms the magnetic entropy changes DSðTÞ were determined for a field step of 500 Oe The maximum value of DSmax increases with increasing x till x ¼ 0:3 and then decreases with further increasing x: The conductivity of perovskites is metallic at low temperatures and semiconducting at high temperatures Magnetoresistance measurements have been performed r 2002 Elsevier Science B.V All rights reserved Keywords: Perovskite structure; Magnetic oxides; Magnetocaloric effect Introduction The Ln1ÀxAxBO3 perovskites (Ln=rare earth, A=alkaline earth element, B=Mn or Co) are attracting considerable interest because they exhibit interesting physical effects and have potential for applications due to the complex relationship between crystal structure, electrical, magnetic and thermal properties Colossal magnetoresistance in manganese perovskites [1] and large magneto*Corresponding author Tel.: +84-4-858-9496; fax: +84-4858-9496 E-mail address: chau@cms.edu.vn (N Chau) caloric effects around the Curie temperature in La1ÀxSrxCoO3 (LSCO) [2] have been found The metal–insulator transition [3], an anomaly in the thermal expansion and hysteresis of the resistance around the Curie temperature [4] are interpreted as first-order transitions [4–6] The magnetocaloric effect has not only been studied in cobaltites but also in manganites The manganites also have potentials as solid electrolytes, catalysts, sensors and novel electronic materials Their rich electronic phase diagrams reflect the fine balance of interactions, which determine the electronic ground state These compounds represent in microcosm, the interplay 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) - N Chau et al / Physica B 327 (2003) 270–278 of experiment, theory and application, which is at the heart of solid-state physics [7] Most research on manganites and cobaltites is concerned with alkaline-earth elements like Sr, Ba or Ca or combinations of these Young et al [8] have studied the crystal structure and magnetic properties of La0.7Pb0.3Mn1ÀxCoxO3 (0pxp1) perovskites For x ¼ 0; these authors found a rhombohedral structure and also a rather large magnetoresistance effect, about 50% below 200 K and about 25% around 300 K Troyanchuk et al [9] have observed that the La1ÀxPbxMnO3 perovskites (x ¼ 0:420:6) have a rhombohedral (slightly distorted) cubic structure Huang et al [10] have studied the crystal structure and the magnetic scaling behaviour of La1ÀxPbxMnO3 perovskites (x ¼ 0:020:5) and have shown that all the samples crystallize in the rhombohedral structure Moreover, the substitution of Pb+2 ions on La+3 sites induces a mixed-valence state of Mn3+/Mn4+ and enhances the magnetic transition temperature in this system In this work, we report on our study of structure, magnetic, electric and magnetocaloric properties of La1ÀxPbxMnO3 manganite Experimental La1ÀxPbxMnO3 samples (x ¼ 0:1; 0.2, 0.3, 0.4, and 0.5) were prepared using a conventional powder solid-state reaction method Stoichiometric amounts of 3N purity La2O3, PbO and MnCO3 powders were homogeneously mixed and completely ground Then, the mixed samples were presintered at 9001C for 15 h The heated samples were cooled to room temperature, reground to fine particles, and pressed into pellets and sintered at 9201C for 15 h All high-temperature treatments were performed at ambient atmosphere with a programmed heating and cooling rate of 501C– 1501C/h The structure of the samples was examined in a Brucker D 5005 X-ray diffractometer The microstructure and chemical composition were studied in a 5410 LV Jeol scanning electron microscope (SEM), which includes an energy dispersion spectrometer (EDS) Thermal phase transitions 271 were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) using TA Instruments Apparatus SDT 2960, with a heating rate of 201C/min Magnetic measurements were performed in a vibrating sample magnetometer (VSM) DMS 880 in magnetic fields up to 13.5 kOe Results and discussion Since Pb is an element having a low melting temperature and easily evaporates, presintering and sintering were performed at not too high temperatures Fig shows SEM photographs of some representative samples In the samples with small Pb content (x ¼ 0:1—Fig 1a, x ¼ 0:3—Fig 1b) the crystallites are homogeneous and small (about 0.3–0.4 mm) For a larger amount of Pb (x ¼ 0:5), the size of the crystallites increases up to 0.5– 0.7 mm and there are several particles with sizes up to l mm (Fig 1c) This allows us to suggest that in the sample with x ¼ 0:5 the crystalline particles develop easily due to existence of local liquid phases Fig presents the EDS spectrum of La0.7Pb0.3MnO3 One can see that there are no strange elements and the sample composition is similar to the nominal one, i.e there has been no evaporation of Pb So, the sintering temperature was not too high Fig presents the X-ray diffraction (XRD) patterns of the investigated samples We can see that all five samples are of single phase For La1ÀxPbxMnO3 (x ¼ 0:120:5) the refinement was successful for pseudo-cubic lattices with symmetries decreasing from cubic (x ¼ 0:5) to rhombohedral (x ¼ 0:4) and triclinic (x ¼ 0:3; 0.2, 0.1) For x ¼ 0:3 (where the symmetry changes from rhombohedral to triclinic), the metal-atom positions must be fixed while the oxygen positions were refined; for x ¼ 0:2; 0.1 (P1) all atomic positions including the metal sites could be refined The existence of a cubic cell for x ¼ 0:5 is very similar to the results obtained for La1ÀxSrxMnO3 [11] as well as for La1ÀxSrxCoO3 [12,13] The variation of unit-cell volume is DV =V ¼ 1:0% i.e 272 N Chau et al / Physica B 327 (2003) 270–278 Fig SEM photographs of the surface of La1ÀxPbxMnO3: (a) x ¼ 0:1; (b) x ¼ 0:3; (c) x ¼ 0:5: Fig Energy dispersion spectrum of La0.7Pb0.3MnO3 Fig XRD patterns of La1ÀxPbxMnO3 perovskites N Chau et al / Physica B 327 (2003) 270–278 ( The largest volume a max–min variation=0.6 A occurs for x ¼ 0:2; 0.3 and the smallest volume for x ¼ 0:5: The lattice constants vary from 3.877 Table ( and bond angles (1) for La1ÀxPbxMnO3 Bond lengths (A) x to Mn–O Mn–O–Mn x Mn–O Mn–O–Mn 0.1 O1 1.962, 1.965 164.3 O2 1.972, 1.979 158.7 O3 1.961, 1.966 162.3 1.964, 1.971 162.6 0.3 1.960, 1.966 164.5 1.937, 1.974 169.7 0.2 O1 1.979, 1.984 158.5 O2 1.967, 1.979 161.0 O3 1.956, 1.956 166.9 0.4 1.943 180.0 0.5 1.939 180.0 273 ( (x ¼ 0:3), i.e 0.5% Table (x ¼ 0:5) to 3.895 A shows the bond lengths and bond angles for Mn– O and Mn–O–Mn Note that in triclinic symmetry (x ¼ 0:1; 0.2, and 0.3) there are three independent oxygen positions O1, O2, O3, so the values are given for each case individually The average Mn– O–Mn bond angle increases clearly with increasing Pb2+ content and reaches a maximum for x ¼ 0:4 and 0:5 (1801), whereas the bond Mn–O lengths decrease continuously to a minimum for x ¼ 0:5 ( resulting in the most tight bonding (1.939 A), between Mn and O atoms The La1ÀxPbxMnO3 compounds have been investigated in Refs [10,14] with hexagonal cells of space group R3c (for all x ¼ 0:020:5) To Fig DSC curves for La1ÀxPbxMnO3 (a), DSC and TGA curves for the raw material La2O3 (b) 274 N Chau et al / Physica B 327 (2003) 270–278 Fig Thermomagnetic curves of the La1ÀxPbxMnO3 perovskites in a magnetic field of 20 Oe (a) x ¼ 0:1; (b) x ¼ 0:3; (c) x ¼ 0:5: compare these results with the present ones, the pseudo-cubic lattice should be transformed by the transformation matrix [(1,1,À1)(0,1,À1)(2,2,2)] Except for x ¼ 0:1 and 0.2, where small angular deformations are seen in our samples (o0.21), all other cases differ only in the cell constants The cells reported in the literature are all of the same symmetry and appear to be smaller, except for x ¼ 0:4: On the average, these cells are 0.5% smaller ( 3/unit cell Because of the lower sensitivity i.e 0.3 A to angular deformation of the hexagonal cell, the development of Mn–O–Mn angles was not seen in Ref [14] as clearly as in our samples Fig shows the results of thermal analysis for La1ÀxPbxMnO3 samples (Fig 4a) as well as DSC and TGA curves for the starting material La2O3 (Fig 4b) All DSC curves show one broad endothermal peak around 801C and one sharp endothermal peak around 3951C It is suggested that the first peak is due to evaporation of water out of the samples and that the second one corresponds to the decomposition of La2O3 Á nH2O into intermediate phases The fuzzy endothermal peak around 8601C can be considered to correspond to the start of the reaction in which the perovskite structure is formed For all samples, zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were performed in a magnetic field of 20 Oe Fig shows that the FC and ZFC curves for x ¼ 0:1; 0.3, and 0.5 are separated from each other at low temperatures The ferromagnetic transition temperature, TC was determined from these thermomagnetic curves and is presented in Table The Curie temperature increases with increasing Pb content from x ¼ 0:1 (TC ¼ 235 K) N Chau et al / Physica B 327 (2003) 270–278 to x ¼ 0:4 (TC ¼ 360 K) and after that TC becomes somewhat lower in the sample with x ¼ 0:5: Clearly, substitution of Pb2+ for La3+ induces a mixed-valent state of Mn3+/Mn4+ and enhances the ferromagnetic transition temperature The dependence of TC on x in the system Table Magnetic-transition temperatures of La1ÀxPbxMnO3 TC : ferromagnetic transition temperature; Tr : irreversibility temperature; Tg : spin–glass transition temperature x 0.1 0.2 0.3 0.4 0.5 TC (K) Tr (K) Tg (K) M (emu/g) HC (Oe) MR (%) 235 270 — — — 0.40 310 300 260 3.7 11 3.40 358 350 295 24 31 5.26 360 357 300 20 27 5.60 355 345 275 14 24 4.20 275 La1ÀxPbxMnO3 is in good qualitative agreement with results for La1ÀxSrxCoO3 [12] In low applied field, the ZFC and FC magnetization curves are splitted at temperatures below a so-called irreversibility temperature, Tr (oTC ) (Fig 5) Note that in sample with x ¼ 0:1 (Fig 5a) due to the low sintering temperature, obviously the sample is in large homogeneity The magnitude of the splitting and the temperature Tr decrease with increasing external field In addition, the low field ZFC of the MðTÞ curves clearly show a cusp at a so-called spin freezing (or spin–glass transition) temperature Tg : As the strength of external magnetic field increase, Tg also shifts to a lower temperature and the cusp in the ZFC MðTÞ is smeared out to broad maximum These phenomena are identifying features of a spin-glass or cluster-glass state [15,16] The maximum will be the result of the competition between (local) Fig Isothermal magnetization curves for La0.7Pb0.3MnO3 (a); DSðTÞ for La0.7Pb0.3MnO3 (b); and DSðTÞ for La0.5Pb0.5MnO3 (c) N Chau et al / Physica B 327 (2003) 270–278 276 Fig Temperature La0.7Pb0.3MnO3 Fig Hysteresis loops of La1ÀxPbxMnO3 at room temperature (a) x ¼ 0:2; (b) x ¼ 0:5: anisotropy (decreasing with increasing temperature, so allowing an increasing magnetization) and eventually the decrease of the magnetic order, when Curie temperature is approached In Table 2, also the values of Tr and Tg are presented The composition dependence of Tr and Tg has a similar tendency as that of TC : MðHÞ isotherms have been measured for all La1ÀxPbxMnO3 samples, at various temperatures in a narrow temperature interval around the Curie temperature, in magnetic fields up to 13.5 kOe The entropy change resulting from the spin ordering, induced by the applied magnetic field, can be obtained according to the thermodynamic relation [17]: DSðT; HÞ ¼ SðT; 0Þ À SðT; HÞ Z H max ¼ fqMðT; HÞ=qTgH dH: ð1Þ dependence of the coercivity of Here, SðT; 0Þ and SðT; HÞ represent the entropy without and with applied magnetic field, respectively From the set of isothermal MðHÞ curves we have evaluated the entropy change, DSðTÞ; for the field change from to 13.5 kOe, as a function of temperature, and then the maximum value DSðTÞmax was evaluated Examples are given in Figs 6a–c The maximum magnetic-entropy change, DSðTÞmax is found to be 0.65, 1.30, 1.53, 0.87, 0.81 J/kg K for x ¼ 0:1; 0.2, 0.3, 0.4, and 0.5, respectively, so is maximal for x ¼ 0:3: These materials can be considered as good magnetic refrigerant materials operating at temperatures ranging from below to above room temperature For checking the ferromagnetic state of the samples, their hysteresis loops were measured at room temperature in a maximum field of 700 Oe Fig presents hysteresis loops for x ¼ 0:2 and 0.5 The sample with x ¼ 0:1 is paramagnetic at room temperature The magnetic parameters derived from these loops are shown in Table The magnetization jumps from a low value at x ¼ 0:2 to a maximum value at x ¼ 0:3; and then decreases at further increase of x: The coercivity has a similar concentration dependence This result is different from that in Ref [18], where the authors showed that HC is almost independent of the substitution of Ag for La As usual for softmagnetic materials, the coercivity of perovskites depends on temperature As an example, Fig shows this dependence for x ¼ 0:3: We suggest N Chau et al / Physica B 327 (2003) 270–278 that the temperature dependence of the magnetocrystalline anisotropy has a similar behaviour Examination of the electrical properties of the perovskites shows that the conductivity is Fig Temperature La0.6Pb0.4MnO3 dependence of the resistance of 277 metallic at low temperatures and they are semiconducting at high temperatures Fig presents the temperature dependence of the resistance of La0.6Pb0.4MnO3 as an example This dependence is in agreement with results in other systems of perovskites [1,7] We have determined the magnetoresistance (MR) of all samples Fig 10 presents the results at room temperature for x ¼ 0:2; 0.3, and 0.5 The MR is determined as the ratio ẵRHị R0ị=R0ị; where RðHÞ and Rð0Þ correspond to the resistance of the sample with and without applied magnetic field, respectively The MRðHÞ curve for x ¼ 0:2 (Fig 10a) is of second order in H: It should be noted that for this composition TC is close to the measuring temperature The rupture point in the MRðHÞ curves for the samples with x ¼ 0:3 and 0.5 (Fig 10b and c) is expected to be related to irreversible displacement of domain walls in the magnetizing process Table shows the MR values of all samples studied at the maximum applied magnetic field of 10 kOe We Fig 10 Magnetoresistance of perovskites La1ÀxPbxMnO3 at room temperature: (a) x ¼ 0:2; (b) x ¼ 0:3; (c) x ¼ 0:5: 278 N Chau et al / Physica B 327 (2003) 270–278 see that MR reaches a maximum value of 5.6% at room temperature for La0.6Pb0.4MnO3 It is well known that MRðTÞ normally reaches a maximum at a temperature close to the Curie temperature The study of MR at different temperatures is in progress Conclusions Single phase La1ÀxPbxMnO3 (0:1pxp0:5) perovskites were prepared The symmetry decreases from cubic (x ¼ 0:5) to rhombohedral (x ¼ 0:4) and triclinic (x ¼ 0:3; 0.2, 0.1) The Curie temperature increases from 235 K for x ¼ 0:12310 K for x ¼ 0:2 and then remains almost constant with further increasing x: The coercivity of perovskites at room temperature depends on the Pb content in the samples The studied compounds may be considered as magnetic refrigerant materials operating at temperatures ranging from below to above room temperature The MR value of La0.6Pb0.4MnO3 reaches 5.60% at room temperature in a field of 10 kOe This is the highest room-temperature value of the samples investigated Acknowledgements This work was supported by the Natural Science Council of Vietnam and by the National Basic Research Program No KT 420101 References [1] R von Helmolt, J Wecker, D Holzapfel, L Schultz, K Samwer, Phys Rev Lett 71 (1993) 2331; S Jin, T.H Tiefel, M McCormark, R.R Fastnacht, R Ramesh, L.H Chen, Science 264 (1994) 413 [2] S Chaudhary, V.S Kumar, S.B Ruy, P Chaddah, S.R Krishnakumar, V.G Sathe, A Kumar, D.D Sarma, J Magn Magn Mater 202 (1999) 47 [3] P.E Schiffer, P.A Ramirez, W Bao, S.W Cheong, Phys Rev Lett 75 (1995) 3336 [4] M.R Ibarra, A.P Algarabel, C Marquina, J Blasco, J Garcia, Phys Rev Lett 75 (1995) 354 [5] H.Y Hwang, S.W Cheong, P.G Radaelli, M Magegio, B Batlogg, Phys Rev Lett 75 (1995) 914 [6] P.G Radaelli, D.E Cox, M Marezio, S.W Cheong, P.E Schiffer, A.P Ramirez, Phys Rev Lett 75 (1995) 4488 [7] J.M.D Coey, M Viret, S.V Molnar, Adv Phys 48 (1999) 167 [8] S.L Young, Y.C Chen, Lauce Horng, T.C.Wu, H.Z Chen, J.B Shi, J Magn Magn Mater 289 (2000) 5576 [9] I.O Troyanchuk, D.D Khalyavin, H Szymczak, Mater Sci Bull 32 (1997) 1637 [10] T.S Huang, C.H Chen, M.F Tai, Mater Res Soc Symp Proc 674 (2001) U 3.4.1 [11] H.D Chinh, N Hanh, N Chau, Proceedings of the Fifth Vietnam National Conference on Physics 3/2001, p 783 [12] N.H Luong, N Chau, P.M Huong, D.L Minh, N.N Chau, B.T Cong, M Kurisu, J Magn Magn Mater 242–245 (2002) 760 [13] L.T.C Tuong, P.V Phuc, N.N Toan, Proceedings of IWOMS’99, p 399 [14] C.H Chen, T.S Huang, M.F Tai, Mater Res Soc Symp Proc 674 (2001) U 3.8.1 [15] M Itoh, I Natori, S Kubota, K Motoya, J Phys Soc Japan 63 (1994) 1486 [16] D.N.H Nam, K Jonason, P Nordblad, N.V Khiem, N.X Phuc, Phys Rev B (1999) 4189 [17] A.M Tishin, J Magn Magn Mater 184 (1998) 62 [18] T Tang, K.M Gu, Q.Q Cao, D.H Wang, S.Y Zhang, Y.W Du, J Magn Magn Mater 222 (2000) 110 ... mixed-valence state of Mn3+/Mn4+ and enhances the magnetic transition temperature in this system In this work, we report on our study of structure, magnetic, electric and magnetocaloric properties of La1ÀxPbxMnO3... La0.6Pb0.4MnO3 dependence of the resistance of 277 metallic at low temperatures and they are semiconducting at high temperatures Fig presents the temperature dependence of the resistance of La0.6Pb0.4MnO3... temperature dependence of the magnetocrystalline anisotropy has a similar behaviour Examination of the electrical properties of the perovskites shows that the conductivity is Fig Temperature