Structural Electrical and Magnetic Properties of La2/3ca1/3mno3 Perovskites With In, Ga And Al Substitution at Either La Or Ca Site
The structure, electrical and magnetic properties of colossal magnetoresistance material La2/3Cal13Mn0(3L CMO) substituted with In, Ga and A1 at both La and Ca site have been studied. Samples of (La1.,In,)2/~Cal/~MnO~(L ICMO), (Lal. ,GaX)2nCalnMnO3 (LGCMO), (Lal-,A1,)213Cal/3Mn03 (LACMO), La2n(Ca...
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| Format: | Thesis |
| Language: | English English |
| Published: |
2004
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/6367/1/FSAS_2004_32.pdf |
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| Summary: | The structure, electrical and magnetic properties of colossal magnetoresistance
material La2/3Cal13Mn0(3L CMO) substituted with In, Ga and A1 at both La and Ca
site have been studied. Samples of (La1.,In,)2/~Cal/~MnO~(L ICMO), (Lal.
,GaX)2nCalnMnO3 (LGCMO), (Lal-,A1,)213Cal/3Mn03 (LACMO), La2n(Cal-
,InX)lmMn03 (LCIMO), Lazn(Cai-xGax)1/3Mn03 (LCGMO), Lam (Cal-,AlX)lnMnO3
(LCAMO) with x=O.O to 1.0 were prepared using solid state reaction method. X-ray
diffraction (XRD) patterns shows single phase pattern at low concentration with
increasing intensity of secondary phases at high concentration of dopant. All samples
except sample LICMO x=0.6, exhibit orthorhombic structure. Sample LICMO x=0.6
exhibits tetragonal structure. The AC susceptibility studies indicates LICMO,
LGCMO, LACMO exhibit wide variety of magnetic phases. For LICMO, LACMO
and LGCMO system, ferromagnetic to paramagnetic transition are observed from the
undoped sample x=O.O to 0.5,0.4 and 0.3 respectively. With further doping at La site,
spin glass transition is observed followed by antiferromagnetic to paramagnetic
transition with increasing dopant concentration. The Curie temperature, Tc decreases
as indium, gallium and aluminum doping increases indicates weakening of ferromagnetic interactions, but the antiferromagnetic interactions is getting stronger
with increasing dopant, resulting spin glass system and antiferromagnetism with
further doping concentration. With In, Ga and Al substitution at the Ca site, all
samples with the exception of LCIMO x=1 .O, exhibit ferromagnetic to paramagnetic
transition. For LCIMO sample x=1.0, AC susceptibility study indicates
antiferromagnetic to paramagnetic transition. The electrical properties show the metal
to insulator transition and this property is limited to certain doping level for both La
and Ca site substitution, i.e. until x=0.9 for LICMO, x=0.8 for LGCMO, LACMO,
LCIMO and LCGMO, and x=0.5 for LCAMO system. Beyond the specific doping
level, the samples become insulator for La site substitution, and semiconducting
behaviour for Ca site substitution. This phenomenon is due to the ionic size of dopant
for La site substitution, and both ionic size of dopant and decreasing Ivln4+/Mn3+ra tio
due to decreasing ca2+ ions. Fitting of adiabatic small polaron hopping model to high
temperature ln(R/T), indicates the activation energies of all samples within range of
0.03eV to 0.17eV which is consistent with reported values in the literature,
confirming small polaron hopping activities beyond Tp. Magnetoresistance
measurements show that magnetoresistance (MR) ratio is maximum at temperature
close to Tp for all samples, and increases with increasing dopant concentration for La
site substitution. However, for Ca site substitution, the magnetoresistance's
maximum is not as high as La site substitution, and decreases with increasing dopant
concentration for x > 0.3, because of the low ~ n ~ + / I v frnat~io+ t hat weakened the
Zener double exchange interactions and thus the metallic conductivity and
ferromagnetism. High MR values are 80% for LICMO sample x=0.4, 95% for
LGCMO sample x=0.6 and 87% for LACMO sample x=0.2, compares to 40% of
LCMO sample. The Scanning Electron Microscopy (SEM) micrographs indicate fused and denser grains for all samples. Large abnormal growth is seen only in
LICMO for x=O.l and 0.2 samples and increasing level of porosity with increasing
dopant is seen for LACMO, LCGMO and LCIMO samples. LICMO and LGCMO
samples exhibit decreasing level of porosity with increasing substitution while
LCAMO system has low level of porosity in all samples. |
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