Abstract
Problem statement: Microstructure of the grain will influence the properties of a polycrystalline manganites when different dopant is introduced. In this work, an effort had been made to investigate the influence of Ca, Ba and Sr substitution in La site. Approach: Polycrystalline manganites compound of La0.67A0.33MnO3 where A = Ba, Sr and Ca had been prepared via conventional solid-state reaction method. The structure, microstructure, magnetic and electrical properties had been investigated using XRD, SEM, VSM and four-point probe techniques. Results: XRD spectrums showed that LBMO and LCMO were in single-phase orthorhombic structure whereas LSMO was rhombohedral structure. Scanning electron micrographs showed that LSMO had smaller average grain size as compared to LBMO. However, for LCMO, there was no clear grain boundary that can be observed and all the grains were well connected. The difference in the microstructure image might be due to the variance A-site cation that differs in grain growth. The Curie temperature, Tc of LBMO and LSMO was 343.0 and 363.5 K, respectively. But the Tc for LCMO was lower then 300 K. The Tp for LCMO was 215 K while it was near or above 300K for LBMO and LSMO. All samples showed extrinsic Magnetoresistance (MR) effect. Higher MR was observed well below Tc or Tp where MR value increased monotonically with the decrease of temperature. This was associated with the different grain and grain boundary formation when different cation was substituted. LCMO displayed the highest low-field MR value (-15.82% at 0.2 T, 90 K) and high-field MR (-25.60% at 1 T, 300 K). Conclusion: Hence, electrical and magnetic transition temperature (Tp and Tc) were affected by the average A-site cation radius.
Highlights
In recent year there has been a lot of research study in Colossal Magnetoresistance (CMR) effects in rareearth manganite perovskite with general formula La1xAxMnO3 (A = Ba, Sr, Pb and Ca) due to their potential technology application
These compounds are Mn3+ rich and doping of divalent atoms introduces mixture valency of Mn3+and Mn4+ ions plays a major role in Double Exchange (DE) ferromagnetic interaction coupled with metallic resistivity
CMR effects of polycrystalline ceramic bulk exhibit two classes of Magnetoresistance (MR), the intrinsic and extrinsic MR. The former is referred to intragrain effect where its MR shows a maximum near paramagnetic-ferromagnetic transition temperature (Tc) and maximum electrical resistivity at metal-insulator transition temperature (Tp). The latter is due to the intergrain effect where higher MR could be observed over a wider temperature range below Tc and is characteristic of a Low-Field MR (LFMR), which is believed to be due to Spin-Polarized Tunneling (SPT) or Spin-Dependent Scattering (SDS) at grain boundary[3,4,5]
Summary
In recent year there has been a lot of research study in Colossal Magnetoresistance (CMR) effects in rareearth manganite perovskite with general formula La1xAxMnO3 (A = Ba, Sr, Pb and Ca) due to their potential technology application These compounds are Mn3+ rich and doping of divalent atoms introduces mixture valency of Mn3+and Mn4+ ions plays a major role in Double Exchange (DE) ferromagnetic interaction coupled with metallic resistivity. CMR effects of polycrystalline ceramic bulk exhibit two classes of Magnetoresistance (MR), the intrinsic and extrinsic MR The former is referred to intragrain effect where its MR shows a maximum near paramagnetic-ferromagnetic transition temperature (Tc) and maximum electrical resistivity at metal-insulator transition temperature (Tp). The electrical resistivity of all samples was measured by conventional four-point-probe technique in the temperature range 20-300 K and the data are collected in the heating mode
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