Magnetoresistance and Transport Properties of La
0.9Ba
0.1MnO
3Nanoparticles
B.M. Nagabhushana
1, R. P. Sreekanth Chakradhar
2, K. P. Ramesh
2,V. Prasad
2, S.V. Subramanyam
2, G.T. Chandrappa
1*1Department of Chemistry, Central college Campus, Bangalore University, Bangalore – 560 001, India
2Department of Physics, Indian Institute of Science, Bangalore - 560 012, India.
Abstract
This paper describes studies on the magnetoresistance and transport properties of nanocrystalline La0.9Ba0.1MnO3 powders prepared by low temperature solution combustion process. Powder XRD data reveals that the as formed and calcined La0.9Ba0.1MnO3 are in cubic phase with Pm3m space group. The microstructure and composition of the compound have been examined by SEM and EDS. Two active IR vibrational modes have been observed at 400 and 600 cm-1. The magnetoresistance measured on sintered La0.9Ba0.1MnO3 exhibits a broad metal- insulator transition (TM-I) at around 228 K. At 1 T applied magnetic field La0.9Ba0.1MnO3 shows MR of 10%, whereas for 4 and 7 T the %MR are in the range 51 and 60 % respectively at TM-I.
INTRODUCTION
In recent years, the phenomenon of giant magnotoresistance (GMR) has attracted wide attention in the hole doped magnetic perovskites of the type Ln1- xAxMnO3 (Ln=La, Nd, Pr and A =Ca, Ba, Sr, Pb). These materials have interesting electrical and magnetic properties and also have potential applications for magnetic recording, magnetic switches and magnetic sensors. Generally, doped manganites reveal a metal- insulator (M-I) transition with peak in the electrical resistance at a temperature TM-I, which is close to the ferromagnetic transition temperature. A giant negative magnetoresistance (MR) is generally encountered in the region near Tc. The magnitude of MR is related to the Mn4+ content, ionic radius of the A-site cation and the particle size of the manganites. The lanthanum manganites are usually prepared by the ceramic route that needs higher temperature (1000 – 1500 οC) and long annealing time to get homogeneous composition of desired structures. On the other hand, the solution combustion process has several advantages like fast heating rates, short reaction time (~ 5 minutes), besides producing nano size, porous and homogeneous product.
In the present work, low temperature solution combustion route has been used for the preparation of nanocrystalline La0.9Ba0.1MnO3 powders by low temperature solution combustion route. The transport and magnetic properties in well-characterized La0.9Ba0.1MnO3 nanoparticles are presented.
EXPERIMENTAL
La0.9Ba0.1MnO3 nanocrystalline powders were prepared by low temperature combustion process using metal nitrates as oxidizers and oxalyl dihydrazide (ODH) as a fuel. The detailed calculation of stoichiometry and
combustion preparation was reported elsewhere [1,2].
The combustion products were characterized by XRD, TG/DTA, SEM and FTIR techniques. The elemental analysis and homogeneity of samples were investigated by means of quantitative energy dispersive spectroscopy (EDS). The MR measurements were performed on sintered samples using four-probe method in a liquid helium cryostat (Janis Supervaritemp Cryostat) from 300 K down to 77 K with magnetic field from 0-7 T, using super conducting magnet. The resistance measurements were carried out by standard four-probe technique. The magnitude of MR is then defined as
[∆R/R(0)]= [R(H) - R(0)]/ R(0)
Where R(H) and R(0) are the resistances at a given temperature in presence and absence of a magnetic field, H respectively.
RESULTS AND DISCUSSION
Fig. 1 shows PXRD of as formed and calcined (900 oC, 6 h) La0.9Ba0.1MnO3. The crystal structure and lattice parameters have been evaluated by the Rietveld analysis program Fullprof method. The as formed and calcined La0.9Ba0.1MnO3 are indexed in cubic symmetry (Pm3m).
The lattice parameter and cell volume for calcined sample were obtained as a = 3.891 Å and V=58.924 (Å)3. The broad and intense peaks of XRD reveal that the materials are in nano dimension, which is also confirmed by Scherrer’s formula (45 nm). The existence of strong oxidizing atmosphere at elevated temperature during combustion reaction leads to excess of Mn4+ (>
30 %) in as made and in calcined (900 °C, 6 h) La0.9Ba0.1MnO3 samples stabilizing cubic phase.
The Mn4+ content is a crucial factor in determining the magnetic and transport properties. The Mn4+
content in La0.9Ba0.1MnO3 was estimated from the iodometric titration and it is found that the Mn4+ content is 36 and 32 % for as made and calcined La0.9Ba0.1MnO3
samples respectively.
The SEM micrographs (not shown)of as formed and calcined (900 oC, 6 h) La0.9Ba0.1MnO3 samples show dumbbell shaped agglomerated particles of size in the range 0.5 to 1.0 µm. A slight weight loss in TG curve (not shown) starts at 650 oC and a broad exothermic peak around 900 oC in DTA curve correspond to the reduction of Mn4+ to Mn3+ state. The two characteristic IR bands (not shown), one around 600 cm-1 corresponds to the stretching mode νs of the Mn-O-Mn or Mn-O bond and another band around 400 cm-1 attributed to the bending mode νb, which is sensitive to Mn-O-Mn bond angle.
Fig.2 shows the temperature dependence of the resistance of La0.9Ba0.1MnO3 pellets (sintered at 900 oC for 6 h) at different magnetic fields. The sample at zero field exhibit a broad metal insulator transition at around 228 K. In fact, application of a magnetic field will give the field-induced ferromagnetic ordering and hence reduce the resistance through whole temperature range we have investigated.
20 30 40 50 60
2θ (degree)
(a)
(211)
(210)
(200)
(111)
(110)
Intensity (arb. units) (100)
(b)
50 100 150 200 250 300
0 10 20 30 40 50 60 70 80 90
50 100 150 200 250 300
0 10 20 30 40 50 60 70 80 90
50 100 150 200 250 300
0 10 20 30 40 50 60 70 80 90
50 100 150 200 250 300
0 10 20 30 40 50 60 70 80 90
-MR(%)
7 T
4T
1 T
Temperature (K)
Fig. 1. Powder XRD patterns of (a) as formed
and (b) calcined (900 oC, 6 h) La0.9 Ba0.1MnO3. Fig. 3. Temperature variation of %MR of La0.9Ba0.1MnO3 sample at 1, 4 and 7 T
Fig. 3 shows the temperature variation of negative %MR in La0.9Ba0.1MnO3 samples as a function of magnetic field. The %MR decreases monotonically with increasing temperature for all the applied fields. At metal insulator transition temperature (228 K) La0.9Ba0.1MnO3
samples exhibit 10, 51 and 60 % of negative MR for 1, 4, and 7 T applied magnetic fields respectively. The presence of a sufficient concentration of ferromagnetic clusters seems to suffice for the observation of GMR in La0.9Ba0.1MnO3 sample.
50 100 150 200 250 300
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
50 100 150 200 250 300
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
50 100 150 200 250 300
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
50 100 150 200 250 300
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0T 1 T
4 T Resistance (ohm) 7 T
T e m p e ra ture (K )
CONCLUSIONS
Nonocrystalline, single phase La0.9Ba0.1MnO3 manganite has been prepared through low temperature combustion route. We observed a metal-insulator transition temperature around 228 K. La0.9Ba0.1MnO3
sample exhibit field-induced ferromagnetic orderings and shows negative magneto resistance.
Fig. 2 .Temperature variation of the resistance of La0.9Ba0.1MnO3 sample at 0, 1, 4 and 7 T.
REFERENCES
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2. R.P. Sreekanth Chakradhar, B.M. Nagabhushana, G.T. Chandrappa, K P.Ramesh and J. L. Rao, J. Chem. Phys. 121 (2004) 10250.
