Magnetoresistance temperature dependence of LSMO and LBMO perovskite manganites
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La0.7Sr0.3MnO3 (LSMO) and La0.7Ba0.3MnO3 (LBMO) Polycrystalline manganite nanoparticles were prepared by combustion method using glycine fuel. The ignition process was done at 360 °C and 330 °C for LSMO and LBMO, respectively. Both of the samples have rhombohedral structure using XRD analysis. The reduction is observed in electrical resistivity when external magnetic field is applied during the decrease in temperature from 300 to 89 °K, which is due to the tunneling between particles. The magnetoresistance (MR) of samples was measured in this range of temperature in both the presence and absence of a magnetic field of about 10 kG. The colossal role of a kind of extrinsic MR which is 19% and 22% for LSMO and LBMO, respectively, has been investigated in this research. The metal–insulator transition temperature of 200 °K is recorded for LBMO.
KeywordsManganites Magnetoresistance Low temperature Combustion method
In manganites, there are two types of MRs. CMR occurs in high magnetic field (several Tesla) at room temperature due to intrinsic properties of single crystalline manganites. LFMR occurs in low magnetic field at a range of temperatures and depends on extrinsic properties of polycrystalline manganites like the method of preparation [7, 8, 9, 10].
The intrinsic MR of manganites leads to decrease in resistance due to suppression of spin fluctuations in applied magnetic field. As this type of MR needs high magnetic fields, the practical applications of this effect are restricted. The extrinsic MR which is because of the existence of intergrain spin polarized tunneling (SPT) or spin-dependent scattering (SDS) between the grain boundaries in the presence of external magnetic field can be increased as the particles size decrease , or another elements are doped in manganites . In fact, external factors such as preparation method, sintering temperature and particle morphology are of major impacts on grain boundaries which are the main reasons for extrinsic MR effect [10, 12, 13, 14].
Among RE1−xAExMnO3 compounds, La1−xAExMnO3 (AE = Sr, Ca, Ba) were analyzed both experimentally and theoretically in recent years . According to these analyses around x ~ 0.3 these materials are in metal ferromagnetic phase, and at lower and higher than x ~ 0.3 they are antiferromagnetic insulators .
There are several methods, such as milling , sol–gel , solid state reaction  and combustion , through which manganites are prepared. These routes have some drawbacks such as complex procedures, long reaction time and very high temperatures requirement. The combustion rout which we applied in this work is economic and time-saving. During this process, there is no gases generation and particle size growth. In fact, it is a popular method for nanomaterials [3, 17, 18]. The main purpose of this research is the substitution of divalent atoms with different atomic radius for La in La1−xAExMnO3, which affects the magnetic and electrical properties of it [19, 20, 21]. As a result, La0.7Sr0.3MnO3 (LSMO) and La0.7Ba0.3MnO3 (LBMO) were prepared, and their structural and magnetoresistance properties were analyzed. As well as this, applying combustion method for LBMO synthesis is the outstanding aspect of this research.
LSMO and LBMO were synthesized by the combustion method using nitrates such as La(NO3)3·6H2O, Mn(NO3)2·4H2O, Sr(NO3)2 and Ba(NO3)2 as the initial ingredients and glycine as a fuel [3, 18]. The process consists of two steps. Firstly, the stoichiometric amounts of the nitrates, including La(NO3)3·6H2O,Mn(NO3)2·4H2O and Sr(NO3)2 for LSMO, La(NO3)3.6H2O, Mn(NO3)2.4H2O and Ba(NO3)2 for LBMO and glycine, were dissolved separately in distilled water and formed the uniform solution using magnetic stirrer. Then, the uniform evaporated gel of primary materials was formed keeping the product on magnetic stirrer for 30 min at 100 °C. Following which, the temperature was enhanced gently so as to ignition takes place. The ignition occurred during the sintering process with a voluminous and high speed flame at 360 °C for LSMO and 330 °C for LBMO. Finally, the fluffy blackish or dark brown powder of LSMO and LBMO obtained as final product along with brown vapor of nitrogen dioxide which is due to the decomposition of nitrates releasing CO2 and O2 gases. The prepared LSMO and LBMO powder were sintered at 900 °C for 5 h, while the powder was pelletized by pressing at 10 ton pressure.
In the present work, the LBMO combustion reaction was also carried out like the above reaction adding barium nitrate instead of strontium nitrate with the same amount of glycine fuel. The stoichiometric amounts of nitrates and glycine, respectively, as an oxidizing (O) and a reducing agent (F), were used. These amounts are calculated based on total oxidizing and reducing valences of O and F. The equivalence ratio of O/F acts as an essential numerical coefficient in combustion process. When it becomes unity, heat is released at its maximum level [18, 22]. This kind of combustion is called stoichiometric one [3, 18, 23]. Using appropriate fuel as an agent has very important role in both the temperature of decomposition and extension of CO2 and H2O gases. Glycine fuel as a soluble amino acid in water makes it possible for metal ions to compound in the solution. It is an affordable and available material. It causes gradients to precipitate simultaneously during the combustion [3, 18].
X-ray diffraction (XRD) patterns were recorded for structural analyzing by X’Pert PRO MPD model using Cu-Kα radiation in the 2θ range of 20° to 80° with the wave length of 1.5406Å, voltage of 40 kV, current of 40 mA. The results were analyzed with Rietveld refinement by Maud software. Fourier transform-Infrared (FT-IR) analyses were carried out by a thermo Nicolet, (Model Nexus No. 870, USA) in the range of 400–4000 cm−1 in order to checking chemical bonding of the material. The compositional analyses were done by Energy-dispersive analysis of X-ray spectroscopy (EDAX) using Philips XL30. The surface morphology and elemental detection were recorded with a KYKY-EM3200 model of scanning electron microscope (SEM). The resistivity measurements were done by four-point probe method in a magnetic field of 10 kG at low temperatures to 89 °K.
Results and discussion
However, for LSMO sample, its resistivity decreases with the drop of temperature and in the range of 89–300 °K there is no transition up to temperatures higher than the room temperature . As at temperatures below 300 K, the particles are closer and grain boundaries are narrower, which is observed in SEM analyses, it is highly likely to more electrons tunneling occurs, through which the resistivity decreases and metal behaviors appear. Also applying magnetic field of 10 kG leads to decrease in the resistivity of both samples. Furthermore, Ba doping in LMO increases the resistivity comparing with Sr doping [10, 25]. At low temperatures together with the presence of the magnetic field, the decrease in resistivity is more in comparison with the absence of it. TMI in mixed valence manganites is explained by double exchange (DE) interaction which is a type of magnetic exchange interaction between ions in different oxidation state . In DE interaction, first an electron, among those which are all parallel spin-up, transfers from Mn3+ to oxygen ion and from it to Mn4+ ion simultaneously which is based on bond length of Mn–O–Mn. This interaction is the reason for metallic ferromagnetic behavior .
In conclusion, LBMO, for the first time, and LSMO polycrystalline nanoparticles were prepared by combustion method using glycine as a fuel. The TMI of LBMO is 200 °K, and there is no transition temperature for LSMO in this temperature range. MR of LSMO and LBMO, at 89 °K, is 22% and 19%, respectively. As the temperature declines in the presence of the external magnetic field, so does the resistivity. With decrease in electrons scattering, in the presence of magnetic field, electrical resistivity decreases (MR is negative) and this effect is due to the ordered alignment of spins in grain boundaries in the presence of the magnetic field. Therefore, as the intensity of magnetic field increases, so does the amount of MR.
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