Siau Wei Ng

Effect of Sr Substitution in La-Ca-Mn-O Compound on Structural and Electrical Properties

Polycrystalline manganites compound of La0.67A0.33MnO3 (A= Sr, Ca) has prepared by conventional solid state reaction method. In this work, an effort has been made to study the influences of substitution with different alkaline earth in La-site. The structural of La0.67A0.33MnO3 was studied by X-ray diffraction patterns (XRD), XRD spectrums of La0.67Ca0.33MnO3 (LCMO) showed in orthorhombic structure (space group Pbnm) while La0.67Sr0.33MnO3 (LSMO) was in hexagonal structure (R-2c). The LCMO and LSMO bulks were investigated magnetically in view to understanding the ferromagnetic-paramagnetic (FM-PM) behaviors using vibrating sample magnetometer (VSM) at room temperature. LSMO bulk sample exhibited ferromagnetic with high in magnetization while LCMO bulk was in paramagnetic behaviors. The Curie temperature and metal-insulator transition temperature (Tp) were measured using AC Susceptibility (ACS) at temperature range 78 K- 300 K and four-point probe technique. The Tc for LCMO was 260 K, the curves coincide at temperatures above 240 K and become zero around the Curie critical temperature, where the FM-PM transition occurs and Tp for LCMO was reported around 250 K. Meanwhile, the Tc and Tp for LSMO were above 300 K. The difference in grain sizes of the microstructure images of LCMO and LSMO are might be due to the substitution of variance alkaline earth ions that differs in grain growth were observed using scanning electron microscopy (SEM). By replacing Sr2+ in La-Ca-MnO, the crystal structure transform from orthorhombic to hexagonal with highly symmetrical, thus the MnO6 octahedral are less distortion and the local spins are relatively more aligned. Therefore, electron hopping interaction at sub-orbital (3d:eg) increase, resulting the Tc and Tp are shifted to higher values

Grain Size Effect on Electrical and Magnetotransport Properties of La0.67Ca0.33MnO3 synthesized via Sol-Gel Method

The effect of grain size on electrical resistivity and magnetoresistance (MR) effect on La0.67Ca0.33MnO3 prepared via sol-gel method was reported with different sintering temperature starting from 600°C to 1200°C. X-ray diffraction (XRD) patterns show that La0.67Ca0.33MnO3 was in single phase and had orthorhombic structure with space group Pbnm (62). The crystallite size as well as particle size show strong dependence on sintering temperature (Ts). The metal-insulator transition (TMI) and the magnitude of magnetoresistance are significantly influenced by the effective grain boundaries in LCMO polycrystalline. The TMI and its resistivity increases as the grain size and crystallite size increases (Ts increase). The highest low-field magnetoresistance (LFMR), which extrinsic percentage of MR is more dominant, observed in LC-SG8 (-12.89%) with nano-sized distribution that showing double crystallite structure. However, the highest CMR is found in LC-SG10 (-23.48%) at 80 K with 1 Tesla. These percentage variations of MR for all samples were influenced by the grain size and crystallite size variations which consequently affect the spin polarized-tunneling scattering at the grain boundary layer.

Electrical Transport Properties and Magnetoresistance of Pr0.67Sr0.33MnO3-NiO Composites

A series of polycrystalline bulk samples (Pr0.67Sr0.33MnO3)1-x/ (NiO)x were prepared by solid state reaction, and its structure, electrical transport and magnetoresistance properties were investigated. X-ray analysis showed that parent compound of Pr0.67Sr0.33MnO3 (PSMO) formed in single phase with crystal structure of orthorhombic while secondary phase of NiO can be detected with the addition of composite. The electrical properties showed that the resistivity increased with the addition of NiO due to enhancement of spin dependent-tunneling scattering across the artificial grain boundaries of NiO layer. Magnetic field dependence MR curve at various temperatures clearly indicates that extrinsic magnetoresistance had been enhanced due to addition of NiO as the artificial grain boundary.

Lattice Strain Effect in Structural, Magnetoresistance and Electrical Properties of La0.67Sr0.33MnO3 Bulk and Thin Film System

Polycrystalline La0.67Sr0.33MnO3 (LSMO) powder prepared via conventional solid state reaction was pressed into pellet form. The pellets became target to growth thin films on corning glass (LSMO-C), fused silica (LSMO-FS) and MgO (100) (LSMO-M) substrate via pulsed laser deposition (PLD) method. XRD results showed that all samples were hexagonal structure with R-3C space group. Thin films showed relatively smaller crystallite size compared to bulk samples. From Rietveld Refinement analysis, all thin films experienced lattice strain when deposited on different substrate. LSMO compound deposited in different substrate induced structure distortion and lattice strain. Compression along c-axis occurred when the lattice strain increased thus shifted the metal-insulator transition temperature to lower temperature and increased its resistivity.

Sintering Temperature Study on Structure, Magnetic Properties and Magnetoresistance of Pr0.67Ba0.33MnO3

Polycrystalline perovskite manganite of Pr0.67Ba0.33MnO3 (PBMO) bulk ceramic samples were prepared by conventional solid‐state reaction method. The structures, typical magnetic properties and magnetoresistance were studied. At lower sintering temperature (900 °C to 1100 °C), formation of PBMO phase accompany by secondary phases of BaMnO3 and Pr6O11 were observed. However, at 1200 °C, pure single phase of PBMO was obtained. PBMO compounds become denser upon the increase in sintering temperature. A reduction of secondary phases as sintering temperature increased lead to the enhancement of magnetization value. The highest room temperature %MR of 17.1% was found in sample sintered at 1200 °C in 10 kG external magnetic field. In summary, higher sintering temperature reduced multiphase formation and enhanced the magnetic and magnetoresistance properties.

Influence of Sintering Temperature on Microstructure and Electrical Properties of La0.67Ba0.33MnO3 Ceramic

In this study we report the effect of the sintering temperature of polycrystalline La0.67Ba0.33MnO3 (LBMO) manganites synthesized via solid state reaction method. LBMO have been sintered at 900 to 1200 °C in intervals of 100 °C. X‐ray diffraction spectrum confirms that LBMO phase formation starts at 900 °C accompany by a minor second phase of BaMnO3 and fully forms into single phase of LBMO at 1200 °C. SEM images showed that, the grain size increased with sintering temperature and leading to densifications or lower porosity. The Tp was not affected by the sintering temperature. However, the electrical resistances of samples were decreased with the sintering temperature. At higher sintering temperature, due to grain growth and reduction of insulating BaMnO3 phase, the intergrain connectivity has been improved and the resistance drops.