Positive thermoresistive coefficient and colossal magnetoresistance effects were observed in the samples consisting of 90 mol% BaTiO3 and 97 mol% La0.7Sr0.3MnO3.. The temperature depen-de
Trang 1Study of La 0.7 Sr 0.3 Mn 0.96 Co 0.04 O 3 ,La 0.7 Sr 0.3 MnO 3
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,La0.7Sr0.3MnO3and 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
r2002 Elsevier Science B.V All rights reserved
Keywords: Composites; Magnetic-ferroelectric perovskite
1 Introduction
Why are there so few multiferroic materials,i.e
materials that are both ferromagnetic and
ferro-electric 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 BaTiO3and
the ferromagnetic colossal magnetoresistance
(CMR) perovskite La0.7Sr0.3Mn0.96Co0.04O3 and
La0.7Sr0.3MnO3 BaTiO3 is an insulator and
becomes semiconducting with positive
thermore-sistive coefficient (PTC) by small doping of
rare-earth oxides [2] La0.7Sr0.3Mn1xCoxO3 (xo0.1)
presents also a PTC effect above room
tempera-ture,due to a metal to insulator transition (MIT)
[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
2 Experimental procedure Two types of samples (A and B) were produced
by the usual standard ceramic method with mole ratio compositions described in Table 1 A-type composite samples were prepared by taking ready BaTiO3and La0.7Sr0.3Mn0.96Co0.04O3 [4]as start-ing materials The final sinterstart-ing process was carried out at 12501C in air during 4 h The components of the B-type composite,BaTiO3and
La0.7Sr0.3MnO3,were prepared separately,from BaCO3 and TiO2,and from La2O3,SrCO3 and MnO,respectively The constituent phases were
*Corresponding author Fax: +84-4-8589496.
E-mail address: bcong@phys-hu.edu.vn (B.T Cong).
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 4 6 - 5
Trang 2presintered separately at 10001C for 2 h and
the composites were subjected to final heating
at 11001C during 3 h The structural
characteriza-tion of the samples was carried out with a X-ray
diffractometer D5005 The temperature
depen-dence of the dielectric function was measured
using a capacitance method with a RCL meter
PM 6303 at frequency 1 kHz,on disk samples The
DC resistance measurement was performed by a four- or two-probe method on disks with diameter
8 mm and thickness 2 mm,or on parallelepiped bars with 2 2 8 mm3 dimension Hysteresis curves were recorded using a magnetometer VSM-880
Table 1
Sample compositions
Components of composition Composite sample
A1 (%) A2 (%) B1 (%) B2 (%) B3 (%) B4 (%) B5 (%) B6 (%)
La 0.7 Sr 0.3 Mn 0.96 Co 0.04 O 3 50 25
A1 BaTiO3
B6 B5 B4 B3 B2 B1 A2
2θ (deg.) Fig 1 Room temperature X-ray patterns for the samples.
Table 2
Lattice constants of the predominant phases
Trang 33 Results and discussion The powder X-ray patterns given in Fig 1 indicate that the BaTiO3 samples,A1,A2 (B5, B6) are single (almost single) phase,and that the others are multiphase Table 2 shows the lattice constants of the predominant phases The multi-phase structure is due to the low temperature and the short time for final heating Except the rhombohedral structure in sample B6,the pre-dominant phases in the other samples can be considered as compressed or expanded tetragonal BaTiO3 structures Figs 2 and 3 show the tem-perature 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 semi-conducting behavior except sample B5 We em-phasize that the sample B5 has a positive thermoresistive effect with averaged large PTC coefficient a ¼ 27%(1C)1around 501C One may see inFig 3that the maximum of eðT Þ of BaTiO3
0
100
200
300
0 1000 2000 3000 4000
A1
T (C)
0
0.5
1
1.5
2
2.5
600 800 1000 1200 1400 1600 1800 2000
A2
T (C)
(a)
(b)
Fig 2 Temperature dependence of the resistivity and dielectric
function of samples A1 and A2.
0
5
10
15
20
25
1103
8103
-200 -180 -160 -140 -120 -100
B1
T (C)
0
0.2
4103
8103 1.210 4
B2
T(C)
0
2
4
6
8
10
2000 4000 6000
T (C)
B3
0 10 20 30 40 50 60 70
1000 1500 2000 2500 3000 3500 4000
B4
T (C)
0
50
100
150
200
250
1600 2000 2400 2800 3200 3600
B5
T (C)
1103
2103
3103
4103
510 3
6103
BaTiO3
T (C)
Fig 3 The same as in Fig 2 for samples B1–B6 Sample B5 exhibits the PTC effect clearly.
Trang 4occurs at TmaxE1271C—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 2 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 inTable 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 4
demonstrates the low-field hysteresis loops of
samples A1 and A2,measured at T ¼ 1531C
The remanent magnetization and the coercive field
are equal to 1.385 102emu/g and 33.26 Oe
(8.72 103emu/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 1531C The field dependence of the
resistivity of sample B1 and sample B6,measured
at room temperature,is plotted in Fig 5 The
magnetoresistance ratio (MR) increases with
increasing content of the magnetic component
At 7 kOe this ratio is equal to 0.3% for sample
B1 and to 3% for sample B6 The room
temperature and low-field MR of the
magnetic-component rich B6 sample is comparable with the corresponding value of pure La0.7Sr0.3MnO3 [5] From the experimental data shown inFigs 2 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.
-0.2
-0.1
0
0.1
0.2
-0.1 -0.05 0 0.05 0.1
A2 A1
H (Oe)
Fig 4 Low-field hysteresis loops for samples A1 and A2,
measured at T ¼ 1531C.
409 409.5 410 410.5
0.92 0.93 0.94 0.95 0.96 0.97 0.98
H (kOe)
B1
B6
Fig 5 Field dependence of the resistivity of the samples B1 and B6,measured at room temperature.