M. M. Awang Kechik

Growth and characterization of La5/8Sr3/8MnO3 thin films prepared by pulsed laser deposition on different substrates

AbstractnColossal magnetoresistance La5/8Sr3/8MnO3 (LSMO) thin films were directly grown on MgO(100), Si(100) wafer and glass substrates by pulsed laser deposition technique. The films were characterized using X-ray diffraction (XRD), field emission-scanning electron microscope and atomic force microscopy (AFM). The electrical and magnetic properties of the films are studied. From the XRD patterns, the films are found to be polycrystalline single-phases. The surface appears porous and cauliflower-like morphology for all LSMO films. From AFM images, the LSMO films deposited on glass substrate were presented smooth morphologies of the top surfaces as comparing with the films were deposited on Si(100) and MgO(100). The highest magnetoresistance (MR) value obtained was −17.21xa0% for LSMO/MgO film followed by −15.65xa0% for LSMO/Si and −14.60xa0% for LSMO/Cg films at 80xa0K in a 1T magnetic field. Phase transition temperature (TP) is 224xa0K for LSMO/MgO, 200xa0K for LSMO/Si and above room temperature for films deposited on glass substrates. The films exhibit ferromagnetic transition at a temperature (TC) around 363xa0K for LSMO/MgO, 307xa0K for LSMO/Si and 352xa0K for LSMO/Cg thin film. TC such as 363 and 352xa0K are the high TC that has ever been reported for LSMO films deposited on MgO substrate with high lattice mismatch parameter and glass substrates with amorphous nature.

Simple preparation and characterization of bismuth ferrites nanoparticles by thermal treatment method

In the present work, nanoparticles of multiferroic bismuth ferrites (BiFeO3) were synthesized via a simple thermal treatment method. BiFeO3 was prepared from an aqueous solution containing bismuth nitrate and iron nitrate as starting materials, polyvinyl pyrrolidone (PVP) as a capping agent and nitric acid to dissolve the bismuth nitrate, respectively. It is followed by thermal treatment at various calcination temperatures at 350, 450 and 550xa0°C. The samples were characterized by thermogravimetric analysis, X-ray diffractometer (XRD), transmission electron microscope (TEM), vibrating sample magnetometer and electron spin resonance (ESR) spectroscopy. XRD results indicate that the samples, calcined at 350, 450 and 550xa0°C, crystalized in rhombohedral crystal structure (space group R3c). The crystallinity of samples increased with increasing calcination temperature. Morphology study using TEM confirmed the growth of BiFeO3 nanoparticles with the average particle’s size increases from ~30xa0nm up to ~80xa0nm with the increasing of calcination temperature from 350 to 550xa0°C. Magnetic saturation Ms, of samples decreased from 2.15 to 0.25xa0emu/g while the coercivity Hc, increased from 54.41 to 272 G when the calcination temperature increased from 350 to 550xa0°C. ESR revealed increment of g-factor value from 2.14 to 2.64 and peak-to-peak linewidth from 129.33 to 201.61 Oe with the increasing of calcination temperature from 350 to 550xa0°C. The results demonstrate that by using thermal treatment method, the BiFeO3 nanoparticles can be obtained at low temperature, i.e. 350xa0°C.

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

Transport critical current density of Bi-Sr-Ca-Cu-O/Ag superconductor tapes with addition of Fe3O4 as flux pinning center

This paper reports on the flux pinning capability of micron size Fe3O4 in Bi-Sr-Ca-Cu-O superconductor tapes. Ag sheathed high temperature superconductor tapes with starting compositions (Bi,Pb)2Sr2Ca2Cu3O10 (2223) and (Bi,Pb)2Sr2Ca2Cu3O10-(Fe3O4)0.01 were fabricated using the powder in tube method. The Bi-Sr-Ca-Cu-O powders were prepared by using the co-precipitation technique. The effects of Fe3O4 addition on the microstructure, phase formation, critical temperature and transport critical current density, Jc were studied. The Jc value of the Fe3O4added tapes is higher (6,090 A/cm2 at 77 K and 24,500 A/cm2 at 30 K, in zero field) than the non-added tapes (3,730 A/cm2 at 77 K and 13,3180 A/cm2 at 30 K, in zero field). A sudden decrease of Jc in low magnetic fields (B < 0.12 T) when applied parallel (B∥) and perpendicular (B⊥) to the tapes surface was observed. The destruction of weak links played an important role in the early Jc suppression. The rate of decrease of Jc was observed to decrease when the magnetic field was increased further. Improvement in the flux pinning was observed in (Bi, Pb)2Sr2Ca2Cu3O10-(Fe3O4)0.01 tapes. This study shows that magnetic particles such as Fe3O4 can act as effective pinning centers leading to the enhancement of Jc in the system.

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.

Effects of (La0.67Ca0.33MnO3)x (x= 0.0, 0.8, 1.0) Additions on the Structural and Superconducting Transition Temperature of YBa2Cu3O7-δ System

The effect addition of manganites compound on the superconducting properties of YBCO was investigated through X-Ray diffraction (XRD), resistance-temperature (R-T) measurement, AC Susceptometer, and Scanning Electron Microscope (SEM). Samples with addition of (La0.67Ca0.33MnO3)x (LCMO) (x = 0.0, 0.8, 1.0) in YBa2Cu3O7-δ (YBCO) superconductor were prepared via solid state method. Addition of LCMO in YBCO superconductor changed the properties of samples. XRD analysis showed the addition of LCMO (0.8 wt% - 1.0 wt%) formed three new peaks of LCMO. The crystal structure is orthorhombic with space group Pmmm for pure Y123 but after addition 0.8 wt% and addition 1.0 wt% of LCMO the crystal structure was changed to tetragonal and space group P4/mmm. The temperature dependent of electrical resistance showed the samples were metallic above the transition temperature even though after addition. The Tc-on superconducting transition temperature decreased from 86 K to 84 K while the Tc-offdecreased from 76 K to 70 K. Diamagnetic onset transition temperature, Tc-on(dia)decreased from 90.2 K to 89.8 K. This suppression was due to the magnetic behaviour of the LCMO addition. Morphology observation showed that the grain size and grains boundary are decreased while the porosity was increased as the addition of LCMO increases.

Effect of Thermo‐Mechanical Treatment on the Current Transport Properties of Bi(Pb)‐Sr‐Ca‐Cu‐O Superconductor Tapes with Nanopowder CoFe2O4 Addition

We study the effect of various thermo‐mechanical treatment on the current transport properties electrical Bi(Pb)‐Sr‐Ca‐Cu‐O tapes with nanopowder CoFe2O4 addition. We found that pinning strength of the tapes without intermediate pressing increases as the sintering temperature increases from 835°C to 855°C, whereas tapes applied with intermediate pressing have lower pinning strength compared with tapes without the pressing. On the contrary, self‐field Jc of tapes without intermediate pressing decreases with increasing sintering temperature. Intermediate pressing are found effective to increase self‐field Jc. The results obtained will be explained with the help of the XRD spectrums and SEM micrographs on the tapes.