Kean Pah Lim

STRUCTURE, ELECTRICAL TRANSPORT AND MAGNETO-RESISTANCE PROPERTIES OF La5/8Ca3/8MnO3 MANGANITE SYNTHESIZED WITH DIFFERENT MANGANESE PRECURSORS

We synthesized the polycrystalline manganite of La5/8Ca3/8MnO3 with three different manganese routes prepared through a solid state reaction method. The effects of the manganese route selection on the structure, electrical transport and magneto-transport properties were examined in this study. The samples were characterized using X-ray diffraction (XRD) and SEM to identify their structure and morphology. XRD analysis confirmed that all samples were in single phase with orthorhombic structure and belonged to the Pnma space group. The average grain sized samples with manganese route of Mn2O3 and MnCO3 had a grain size of 1.2–8.7 μm and 2–7.5 μm, respectively. For the MnO2 route, the sample had a small melt-like shape with higher porosity. The metal–insulator transition temperature, TMI, for LCMO (Mn2O3), LCMO (MnO2) and LCMO (MnCO3) samples were 270 K, 266 K and 258 K, respectively. All the samples showed negative magneto-resistance with significant increase in value near the TMI temperature. The highest CMR (colossal magneto-resistance) ratio was found in LCMO (Mn2O3), -22.06% at 270 K, followed by -16.69% for LCMO (MnO2) at 80 K, and 15.2% for LCMO (MnCO3) at 100 K in a 1 T magnetic field.

Growth and characterization of ZnO multipods on functional surfaces with different sizes and shapes of Ag particles

Three-dimensional ZnO multipods are successfully synthesized on functional substrates using the vapor transport method in a quartz tube. The functional surfaces, which include two different distributions of Ag nanoparticles and a layer of commercial Ag nanowires, are coated onto silicon substrates before the growth of ZnO nanostructures. The structures and morphologies of the ZnO/Ag heterostructures are investigated using X-ray diffraction and field emission scanning electron microscopy. The sizes and shapes of the Ag particles affect the growth rates and initial nucleations of the ZnO structures, resulting in different numbers and shapes of multipods. They also influence the orientation and growth quality of the rods. The optical properties are studied by photoluminescence, UV-vis, and Raman spectroscopy. The results indicate that the surface plasmon resonance strongly depends on the sizes and shapes of the Ag particles.

The Synthesis and Characterization of Peach-Like ZnO

Peach-like ZnO microstructures are synthesized using vapor phase transport on MgO (001) substrates with a copper oxide (60 nm) buffer layer. The structure and morphology of the product are investigated using an x-ray diffractometer (XRD) and a field-emission scanning electron microscope. The peaches have an average diameter of 3 ?m and a wurtzite structure. To study the optical properties, photoluminescence (PL) and Raman spectroscopy are employed. A strong UV emission at 380 nm in the PL spectra is observed, and a sharp and dominant peak at 437 cm?1 in the Raman spectrum can be assigned to the good crystallization of obtained product. In addition, the growth mechanism of the peach-like ZnO structure is tentatively investigated based on the EDX analysis and growth time.

Structural, electrical and magnetic properties of polycrystalline La0.67(Ca1−x Sr x )0.33MnO3 manganites

Polycrystalline La0.67(Ca1−xSrx)0.33MnO3 with different substitution level of strontium element, were synthesized via solid state reaction. Structure of samples was characterized by X-ray diffraction (XRD). XRD patterns reveal that La0.67Ca0.33MnO3 exhibits orthorhombic structure with space group Pnma. Phase transitions from orthorhombic to rhombohedral take place as Ca ions were gradually substituted by Sr ions. The XRD data were further analyzed by Rietveld refinement technique. The data show that Mn–O–Mn bond angle increases as x increases. Microstructures obtained from SEM show that substitution of Sr ions has demoted the grain growth and densification process during sintering. The substitution of Sr ions has greatly influenced the hopping integral of electron via double exchange interaction, thus affecting the electrical properties and magnetic properties as well. The resistivity decreases and the metal–insulator transition temperature (Tp) shifts to higher temperature as x increases. The magnetoresistance (MR) effect gradually decreases and MR peak shifts to higher temperature as x increases. The magnetization measured at room temperature is found to be increasing as x increases.

Structural, Microstructural and Electrical Properties of La0.67(Ba,Sr)0.33MnO3 System

La0.67(Ba,Sr)0.33MnO3 were prepared via solid‐state reaction method. Rietveld refinement showed that when Ba2+(1.49A) is replaced with smaller ion, Sr2+(1.32A), its lattice parameter, volume, bond distance and angle decreases, and also different microstructure was obtained. Transition temperature, Tp shifted to higher temperature while the resistance and magnetoresistance (MR) behaviour of samples were quite similar. This phenomenon believed to be due to the variation of spin‐dependent scattering and/or spin‐polarization tunneling across the grain surface/boundary. Typical polycrystalline (intrinsic and extrinsic MR) type of MR is shown for both samples. The highest % of low‐field magnetoresistance (LFMR) (at 0.1T) of ‐11.5% and ‐10.2% at 80 K was given by La0.67Ba0.33MnO3 (LBMO) and La0.67Sr0.33MnO3 (LSMO) samples respectively.

Structure, Magnetoresistance and Magnetism of La0.67(Sr1−xBax)0.33MnO3 Perovskite

The effect of Ba‐substitution at Sr site on the structure, magnetoresistance and magnetic properties of La0.67(Sr1−xBax)0.33MnO3 samples with x = 0.00, 0.25, 0.50, 0.75 and 1.00 has been investigated. Single phase samples were obtained by a conventional solid state reaction method with rhombohedral (x = 0.00) and orthorombic (x = 1.00) structure. The grain size increases with Ba content. Low field magnetoresistance phenomenon at low temperature can be observed for all samples. However it vanishes at 300 K for x = 0.75 and 1.00. The variation of MR% value is small ranging between 16 to 18% at 100 K in an applied field of 1 T. All samples show a single magnetic transition from the ferromagnetic (FM) state to the paramagnetic (PM) state as the temperature increases. Curie temperature, Tc decrease with increasing Ba concentration.

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

Study of the thermal property of copper oxide nanowires

As a high potential candidate for field-emission emitters, gas sensors, high-critical temperature superconductors, copper oxide NWs have been intensively studied in the synthesized methods, electrical properties and chemical properties. However, there are not many literatures report on the thermal property of them. It is important for one to understand the thermal behavior in order to justify the failure limit of the material especially in nanoscale. In this paper, copper oxide NWs synthesized through direct heating in atmospheric ambient were rapidly annealed in nitrogen ambient at different temperature to check the critical thermal failure point where the free standing copper oxide NWs start to collapse. It was found that copper oxide NWs with diameters around 100 nm started to collapse after 30 minutes of annealing in the nitrogen ambient at 300°C due to the thermal shock incurred by rapid annealing. Increase in temperature will cause the NWs with bigger diameters start to fail. NWs in same diameter range will be able to withstand the temperature up to several hours if no thermal shock is induced. This was happening even when the wires were heated in a higher temperature of 600°C. This result is important for copper oxide NWs when they are incorporated into the other heat sensitive device. The results are important for justifying the failure behavior for devices based on copper oxide NWs.

Magneto-transport studies on La2/3Ba1/3(Mn1–x Alx)O3 for low field sensing applications

Abstract The magnetic and transport properties of La2/3Ba1/3(Mn1−x Al x )O3 (x=0·0, 0·1, 0·2, 0·3 and 0·4) compounds, prepared by the solid state reaction, have been investigated. Samples show a metal–insulator transition excluding the sample x=0·0. With increased Al doping, the metal–insulator transition temperature T p is shifted to lower temperatures. Grain size reduction leads to a larger resistivity and a decrease in T p. Upon analysing the data using several theoretical models, it was found that the metallic (ferromagnetic) part of the resistivity ρ (below T P) fits well with the equation ρ=ρ 0+ρ 2 T2, where ρ 0 is due to the importance of grain/domain boundary effects, and a second term ρ 2 T2 might be attributed to the electron–electron scattering. The microstructure results indicate that the porosity of the samples increased when the concentration increased. The magnetoresistance (MR) is defined as %MR=100×[ρ(H,T)–ρ(0,T)]/[ρ(0,T)], where ρ(H,T) and ρ(0,T) are the resistivities at temperature T, with an applied magnetic field H and zero applied magnetic field respectively. All samples show low-field magnetoresistance and high-field magnetoresistance regions. The highest percentage of LFMR at a temperature of 100 K is ∼210% MR/Tesla, measured for the sample x=0·2. For x=0·3, the sample reveals the highest colossal magnetoresistance value among other doped compounds with 27·27% at 100 K.

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.