Magneto-electrique and magnetic studies of the Sr- doped Samarium manganite synthsesed by citrate gel
method.
E. Lakhal, A. Amira, R. Chihoub NDT Lab, Faculty of Science and Technology,
University MSBY-Jijel 18000, Jijel 18000, Algeria [email protected]
S.P. Altintas, A. Varilci, C. Terzioglu
Department of Physics, Faculty of Arts and Sciences, AIB University,
Bolu 14280, Turkey
Abstract— In This paper we report study the effect of sintering temperature on the magnetotransport properties of Sm1-xSrxMnO3(with x=0.3) manganite syntesized by citrate gel method. The Sm0.7Sr0.3MnO3has been sintered at 700C (SSM7), 900C (SSM9) and 1300C (SSM13). XRD confirms that phase formation starts at 700C. All the samples are single phasic having orthorhombic unit cell. The lattice parameters decrease on lowering the sintering temperature. The crystallite as well as particle size also show strong dependence on the sintering temperature. All the samples possess insulator-metal (TIM) as well as paramagnetic-ferromagnetic (TC) transitions. The TC shows a small variation [274K to 256K]. The temperature causes a great increase of resistivity and lowers the temperature of the metal-insulator transition TP. Under applied magnetic field, a significant reduction in the resistivity and a shift of TPto higher temperatures values are observed. In the insulating region (T>TP), the resistivity curves are well fitted by the small polaron hoping mechanism while in the metallic region (T<TP), the simple model of small hopping of spin-polarons is used. Some physical parameters are extracted. The highest obtained magneto- resistance MR values is about 34.32% at 0.5 Tesla for for SSM13 .
Keywords—manganite, citrate, magnetotransport, structure.
I. INTRODUCTION
Manganites doped-perovskite R1-xAxMnO3, were R is a rare-earth cations and A is a alkaline earth cation, have been extensively stadied not for their interesting fundamental science, but also for their rich properties and to their various and potential applications in different sectors [1-3]. The interest in these materials is due to their unusual magnetic and electronic properties along with possible technological applications in magnetic information storage and faster reading devices [4-5]. However this substitution leads to a mixed valence state (Mn3+/Mn4+) and can induce a metal- insulator transition (M-I), marked by a peak in the electrical resistivity at a temperature TP accompanied by a change from a paramagnetic state to a ferromagnetic one. The partial substitution in manganites compounds causes distortions in structure involving rotations of the MnO6 octahedral and
changes of the Mn-Mn bond lengths. These facts has been explained by zener in 1951 [6-7], who introduced a new mechanism called the double exchange (DE) mechanism that describes the hopping of electrons in eg orbitals between neighboring Mn3+and Mn4+sites .
In this work, the magneto-transport properties in the manganites Sm0.7Sr0.3MnO3, prepared and characterized previously. Our results reveal that modifying the temperature and investigated their impact on the structure and magnetotransport properties.
II. EXPERIMENTAL SECTION
We have adopted the sol-gel based polymeric precursor route to synthesize Sm0.7Sr0.3MnO3 (SSMO) samples having nano size particles at a significantly lower sintering temperature.
In this technique, aqueous solution of high purity Sm(NO3)3.6H2O, SrO and Mn(NO3)2.4H2O have been taken in the desired stoichiometric proportions. An equal amount of ethylene glycol has been added to this solution with continuous stirring. This solution is then heated on a hot plate at a temperature of ~ 90-120 C till a dry thick brown colour sol is formed. At this temperature ethylene glycol polymerizes into polyethylene glycol, which disperses the cations homogeneously forming cation-polymer network. This has been further decomposed in an oven at a temperature of ~ 300 C to get polymeric precursor in the form of black resin like material. The polymeric precursor thus obtained is then sintered at different temperatures ranging from 500 to 900 C.
Several time periods ranging between 5 to 6 hrs were employed. It was found that sintering for ~4 hrs gave optimum results for all temperatures. Phase pure completely crystalline samples have been obtained at the temperature as low as 700 C. The SSMO samples sintered at 500 C, 700 C and 900 C will hereafter be referred to as SSM5, SSM7 and SSM9 respectively. All the synthesized samples have been subjected to gross structural characterizations using powder X-ray diffraction (XRD) measurements are carried out on a Siemens D8-advance diffractometer in the Bragg–Brentano
10 20 30 40 50 60 70 80 90 100 0
20 40 60 80 100 120 140 160 180 200 220 240
Intensity(u.a)
2 Theta
900 °C
700 °C
500 °C Sm0.7Sr0.3MnO3
20 30 40 50 60 70
-500 0 500 1000 1500 2000 2500 3000 3500
Intensity (a.u.)
2 theta
1300°C Sm0.7Sr0.3MnO3
geometry using CuKα radiation. The microstructural study of the sample done on a JEOL JSM-6390 LV scanning electron microscope (SEM) .The resistivity of the sample in 0, 0,5T magnetic field was measured by the standard four- probe method on a cryodine CTI-Cryogenics closed cycle cryostat.The magnetic characterizations have been carried out by ac susceptibility measurements. Magnetoresistance (MR) refers to the relative change in the electrical resistivity by the application of an external magnetic field. It is given by :
100
%
0 0
H
MR
Where ρ0 and ρH represent the resistivities under zero and magnetic field H, respectively.
III. RESULTS AND DISCUSSION
XRD RESULTS
Fig.1. Show the X-ray diffraction pattern of the elaborated sample. This pattern reveal that All the samples are orthorhombic and single phasic without any detectable secondary phase when compared to previously published results. The XRD data is refined using Jana 2006 software [8] and it is displayed that the structure of the sample is orthorhombic, space group Pnma (N°62). The refined cell parameters are a=5.4510(6) Å, b=5.4359(5) Å and c=
7.6759(5) Å in agreement with previously reported values.
Fig. 1. X-ray diffraction patterns for bulk samples of Sm0.7Sr0,3MnO3sintered at 500°C , 700 °C and 900°C
SEMANDEDS
The scanning electron microscopy (SEM) photographs of the sample are shown in Fig. 3. The sample SSMO has grain size ranging from 1 to 3μm and randomly distributed. EDAX spectra of the samples were shown in the same figure. The presence of all used elements in elaboration is confirmed.
Fig 2. EDX spectra and scanning electron micrograph of SSMO S13 synthesyzed by citrate method at 1300°C.
SUSCEPTIBILITY
The paramagnetic-ferromagnetic transition temperature (TC) has been measured by measuring the ac susceptibility (χ) in the temperature range of 300-80K. The variation of χ with temperature for the samples SSM5, SSM7, SSM9 and SSM13 is shown in Fig.4, which depicts a ferromagnetic ordering transition for all the samples. We have observed only a slight variation in TC for the samples sintered at different temperatures, which have been examined by the peaks in (dχ/dT). The values of TC are given in Table 1. As is clear from this table TC shows an increase from 256K for the sample SSM5 to 274K for the sample SSM13.
(1)
Fig.2. Room temperature X-ray diffraction pattern of SSMO sintered at 1300°C
40 60 80 100 120 140 160 180 200 0
5 10 15 20 25 30 35 40
MRMax=29.32%
MR%
Temperature T(K)
Sm0.7Sr0.3MnO3 MR% (0-0.5T)
Fig.4. AC susceptibility vs. temperature cuves depiting paramagnetic-ferromegnetic transition temperature.
Sintering
Temperatere Tc(K) TIM
500 256 108
700 268 112
900 272 124
1300 274 125
Table.1. Paramagnetic-ferromagnetic transition temperature (Tc) insulator- metal transition temperature (TIM).
RESISTIVITY AND MAGNETORESISTANCE
The evolution of the resistivity with temperature under a magnetic field up to 0 and 0.5 Tesla for samples Sm
0.7Sr0.3MnO3illustrate infig.3 .
0 100 200 300
0 200 400
Sm0.7Sr0.3MnO3 0T0.5T
Temperature (K)
Fig. 5. Temperature dependence of resistivity under 0, 0.5 T magnetic fields for Sm0.7Sr0.3MnO3samples at 1300C.
The temperature dependence of magnetoresistance (MR) at magnetic field 0.5T for a sample at 1300C is shown infig6.
FIT OF RESISTIVITY CURVES To discuss in more details the magnetotransport results, we tried to fit the resistivity curves with the most accepted models.
In order to elucidate the transport mechanism at low temperature (T<TP) of the samples, we have used the empirical equation (2) based on the simple model of small spin-polaron hopping [9] :
F=
0exp
1
exp TB
A ………(2)
Where A=
0
2
a
E and B=
0B
.
The term ρ0 represents resistivity due to grain boundary and point defects. kBis Boltzmann’s constant, Ea is the activation energy for polaron hopping and ω0 is the magnon frequency.
For illustration plot of electrical resistivity vs. temperature along with best fit are shown inFig.7for the undoped sample at zero Tesla. The fit parameters obtained for both sample SSM13 at magnetic field 0 and 0,5 T are tabulated inTable 2.
SSM13
SSM9
SSM7 SSM5
Fig.6 . Magnetoresistance vs. temperature plots of Sm0.7Sr0.3MnO3sample at 1300C .
160 180 200 220 240 260 280 300 0
20 40 60 80 100 120 140
160 Sm0.7Sr0.3MnO3 (H=0T)
.Cm)
Temperature T(K)
50 60 70 80 90 100
140 150 160 170 180 190 200 210 220
.Cm)
Temperature T(K) Sm0.7Sr0.3MnO3 (H=0T)
Fig.5. Theoretical fit of low temperature resistivity data for Sm0.7Sr0.3MnO3 at H=0 Tesla (observed (symbols) and calculated (line)).
H=0T H=0.5T
Sm0.7Sr0.3MnO3
ρ0(Ωcm) 139.7274 98.1298
Ea(ev) 0,0199 0.0198
ω0(mev) 47.1360 52.0534
Table 3. Fit parameters of low temperature resistivity data
At high temperature region(T>TP), the adiabatic small polaron hopping model [10], given by the following equation, is used to fit the experimental resistivity curves :
ρ =ραT exp (Ea/KBT)…………. (3)
Where Ea represent the activation energy ,ραis residual the resistivity and KB is Boltzmann’s constant. Fig.6 shows the best fit for the undoped sample at H=0 Tesla. The activation energy and the residual resistivity values are calculated and are presented intable 4.
CONCLUSION
In summarm, we have studied the effect of sintering temperature on microstructure and mgnetotransport propreties of polycrystalline Sm0.7Sr0.3MnO3, wich have been successful synthesized by citrat method. All the samples are single phasic and have orthorhombic unit cell. The paramagnetic- ferromagnetic and isolator-metal have been observed in all the samples.
The fit of resistivity curves at low temperatures (T<TP) was realized by the simple model of small jump of spin- polarons. For the fit at high temperatures (T>TP), the Small polaron hopping model was used. The CMR effect is present in both samples. A strong magnetoresistance is obtained and is about 34.32% for sample SSM13.
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H=0T H=0.5T
Sm0.7Sr0.3MnO3
ρα (Ωcm) 4,0301.10-6 9,9836.10-6
Ea(ev) 184.985 174.604
Figure 6. Theoretical fit of high temperature resistivity data for Sm0.7Sr0.3MnO3 at H=0 Tesla (observed (symbols) and calculated (line)).
Table 4. Fitting parameters of the adiabatic small polaron hopping model for Sm0.7Sr0.3MnO3samples.
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