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Physica B 327 (2003) 370–373 Study of La0.7Sr0.3Mn0.96Co0.04O3, La0.7Sr0.3MnO3 and BaTiO3 composites B.T Cong*, N.N Dinh, D.V Hien, N.L Tuyen Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Hanoi, Viet Nam Abstract Composites with varying composition of ferromagnetic La0.7Sr0.3Mn0.96Co0.04O3, La0.7Sr0.3MnO3 and ferroelectric BaTiO3 have been prepared using a solid-state ceramic method The structure, temperature dependence of DC resistivity, dielectric constant, magnetoresistance, and the hysteresis loops of some samples have been investigated Positive thermoresistive coefficient and colossal magnetoresistance effects were observed in the samples consisting of 90 mol% BaTiO3 and 97 mol% La0.7Sr0.3MnO3 The intermediate compositions (50 mol% each other) are good candidates for application as multiferroic material r 2002 Elsevier Science B.V All rights reserved Keywords: Composites; Magnetic-ferroelectric perovskite Introduction Why are there so few multiferroic materials, i.e materials that are both ferromagnetic and ferroelectric in the same phase? This is a great fundamental unsolved problem of physics [1] The aim of this contribution is the preparation and the study of some properties of composites consisting of the typical ferroelectric BaTiO3 and the ferromagnetic colossal magnetoresistance (CMR) perovskite La0.7Sr0.3Mn0.96Co0.04O3 and La0.7Sr0.3MnO3 BaTiO3 is an insulator and becomes semiconducting with positive thermoresistive coefficient (PTC) by small doping of rareearth oxides [2] La0.7Sr0.3Mn1ÀxCoxO3 (xo0.1) presents also a PTC effect above room temperature, due to a metal to insulator transition (MIT) *Corresponding author Fax: +84-4-8589496 E-mail address: bcong@phys-hu.edu.vn (B.T Cong) [3,4] CMR perovskites are good conductors Thus, by combining them with barium titanate, one expects to find also materials with PTC and, hence, the possibility to observe the mutual influence of ferroelectricity and ferromagnetism Experimental procedure Two types of samples (A and B) were produced by the usual standard ceramic method with mole ratio compositions described in Table A-type composite samples were prepared by taking ready BaTiO3 and La0.7Sr0.3Mn0.96Co0.04O3 [4] as starting materials The final sintering process was carried out at 12501C in air during h The components of the B-type composite, BaTiO3 and La0.7Sr0.3MnO3, were prepared separately, from BaCO3 and TiO2, and from La2O3, SrCO3 and MnO, respectively The constituent phases were 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) - B.T Cong et al / Physica B 327 (2003) 370–373 371 Table Sample compositions Components of composition BaTiO3 La0.7Sr0.3Mn0.96Co0.04O3 La0.7Sr0.3MnO3 A1 (%) A2 (%) B1 (%) B2 (%) B3 (%) B4 (%) B5 (%) 50 50 75 25 50 75 80 85 90 50 25 20 15 10 97 Intensity (a.u) mol% Composite sample B6 (%) B6 B5 B4 B3 B2 B1 A2 A1 BaTiO3 20 30 40 50 60 70 2θ (deg.) 80 90 Fig Room temperature X-ray patterns for the samples Table Lattice constants of the predominant phases Sample BaTiO3 A1 A2 B1 B2 B3 B4 B5 B6 Lattice parameters of dominated phase a b c 3.994 3.918 3.956 3.994 3.990 3.990 3.986 3.986 5.515 3.994 3.918 3.956 3.994 3.990 3.990 3.986 3.986 5.515 4.039 3.962 4.000 4.039 4.305 4.305 4.031 4.031 13.265 presintered separately at 10001C for h and the composites were subjected to final heating at 11001C during h The structural characterization of the samples was carried out with a X-ray diffractometer D5005 The temperature dependence of the dielectric function was measured using a capacitance method with a RCL meter Tmax (1C) emax 127 À98 163 À155 136 144 170 128 5600 3741 1915 4284 13444 6326 3518 2795 PM 6303 at frequency kHz, on disk samples The DC resistance measurement was performed by a four- or two-probe method on disks with diameter mm and thickness mm, or on parallelepiped bars with   mm3 dimension Hysteresis curves were recorded using a magnetometer VSM-880 B.T Cong et al / Physica B 327 (2003) 370–373 372 300 4000 Results and discussion 3000 The powder X-ray patterns given in Fig indicate that the BaTiO3 samples, A1, A2 (B5, B6) are single (almost single) phase, and that the others are multiphase Table shows the lattice constants of the predominant phases The multiphase structure is due to the low temperature and the short time for final heating Except the rhombohedral structure in sample B6, the predominant phases in the other samples can be considered as compressed or expanded tetragonal BaTiO3 structures Figs and show the temperature dependence of the resistivity and the dielectric function ðeðTÞÞ: The measurement was done in the most interesting temperature region, where the MIT occurs All samples show semiconducting behavior except sample B5 We emphasize that the sample B5 has a positive thermoresistive effect with averaged large PTC coefficient a ¼ 27%(1C)À1 around 501C One may see in Fig that the maximum of eðTÞ of BaTiO3 100 -200 -80 -120 T (C) -160 (a) 2.5 2000 1600 1.5 1400 1200 1000 0.5 800 600 100 T (C) 50 (b) 150 200 Fig Temperature dependence of the resistivity and dielectric function of samples A1 and A2 20 15 10 -180 -160 -140 T (C) -120 -100 6000 4000 2000 100 T (C) 50 150 250 200 ρ (MΩ cm) B3 ε ρ (MΩ cm) 10 4103 70 60 50 40 30 20 10 0 40 80 T(C) 120 160 4000 B4 3500 3000 2500 2000 1500 50 100 150 T (C) 200 200 3200 150 2800 100 2400 50 2000 50 100 T (C) 150 1600 200 1000 250 610 3600 B5 ε ρ (MΩ cm) 8103 -200 1.210 110 B2 ρ (MΩ cm) B1 ε ρ (MΩ cm) 0.2 810 25 ε BaTiO3 5103 4103 ε ρ (MΩ cm) 1800 A2 ρ (MΩ cm) 1000 ε 2000 ε 200 ε ρ (MΩ cm) A1 310 210 50 100 T (C) 150 Fig The same as in Fig for samples B1–B6 Sample B5 exhibits the PTC effect clearly 110 B.T Cong et al / Physica B 327 (2003) 370–373 0.1 410.5 0.98 A1 -0.1 410 0.96 R (Ω) M (emu/g) M (emu/g) A2 -0.2 -500 0.97 0.05 0.1 B6 409.5 0.94 B1 -0.05 -250 H (Oe) 250 -0.1 500 Fig Low-field hysteresis loops for samples A1 and A2, measured at T ¼ À1531C occurs at Tmax E1271C—the Curie temperature of the ferroelectric–paraelectric (FE–PE) transition The maximum is shifted and its shape is changed in the other samples, but remains the evidence for this transition The values of Tmax ; emax are given also in Table It should be noted that eðTÞ for samples B1 and B5 seems to have two maxima and that the given values Tmax and emax belong to the distinguished maximum From the data in Table 2, one sees that sample B2 has an extremely high value of emax as compared to that of the BaTiO3 sample The Tmax of this sample is 1361C higher than what obtained for BaTiO3 This phenomenon may be treated as enhancement of the dielectric constant by the magnetic component Fig demonstrates the low-field hysteresis loops of samples A1 and A2, measured at T ¼ À1531C The remanent magnetization and the coercive field are equal to 1.385  10À2 emu/g and 33.26 Oe (8.72  10À3 emu/g and 45.55 Oe), respectively, for sample A1 (A2) The hysteresis loops show the presence of ferromagnetic order in the samples A1 and A2 at À1531C The field dependence of the resistivity of sample B1 and sample B6, measured at room temperature, is plotted in Fig The magnetoresistance ratio (MR) increases with increasing content of the magnetic component At kOe this ratio is equal to À0.3% for sample B1 and to À3% for sample B6 The room temperature and low-field MR of the magnetic- 0.95 R (Ω) 0.2 373 0.93 409 -15 -10 -5 H (kOe) 10 15 0.92 Fig Field dependence of the resistivity of the samples B1 and B6, measured at room temperature component rich B6 sample is comparable with the corresponding value of pure La0.7Sr0.3MnO3 [5] From the experimental data shown in Figs and 4, we can say that the composite compound A1 (probably also B1) is a good application as a multiferroic material The PTC and CMR effects in semiconducting BaTiO3 and CMR manganese perovskites originate from grain boundary effects These phenomena are related to the change of the potential barrier for carriers at grain boundaries, for temperatures near the FE–PE (ferromagnetic– paramagnetic) transition temperature Probably, the same explanation holds for composite samples Acknowledgements The authors thank the projects 410301 and KC.02.12 for support References [1] N.A Hill, J Phys Chem B 104 (2000) 6694 [2] Y Xu, Ferroelectric Materials and their Application, North-Holland, Amsterdam, 1991 [3] X.J Fan, J.H Zhang, X.G Li, et al., J Phys.: Condens Matter 11 (1999) 3141 [4] B.T Cong, D.L Minh, N Chau, et al., Bull Amer Phys Soc 2000, March Meeting, p 58 [5] R Mahendiran, R Mahesh, A.K Raychaudhuri, C.N.R Rao, Sol State Commun 99 (1996) 14 ... the data in Table 2, one sees that sample B2 has an extremely high value of emax as compared to that of the BaTiO3 sample The Tmax of this sample is 1361C higher than what obtained for BaTiO3. .. A2 A1 BaTiO3 20 30 40 50 60 70 2θ (deg.) 80 90 Fig Room temperature X-ray patterns for the samples Table Lattice constants of the predominant phases Sample BaTiO3 A1 A2 B1 B2 B3 B4 B5 B6 Lattice... experimental data shown in Figs and 4, we can say that the composite compound A1 (probably also B1) is a good application as a multiferroic material The PTC and CMR effects in semiconducting BaTiO3 and
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