Papers by Keyword: Metal-Insulator Transition

Abstract: Colossal magnetoresistive (CMR) materials have huge potential in modern application and it has been widely used in magnetic sensing industry. From the literature, an incorporation of secondary insulating phase into mixed-valence manganites could improve its extrinsic effect especially low-field magnetoresistance (LFMR). However, nanoparticle addition could lead to substitution and diffusion with its parent compound. In this work, the structural and electrical properties of La_0.7Ca_0.3MnO₃ (LCMO) were investigated by adding the α-Fe₂O₃ nanoparticle with ratio of 0.00, 0.05, 0.10, 0.15 and 0.20 as the artificial grain boundaries. The LCMO compound has been synthesised using sol-gel route. The samples were chosen to sinter at 800°C to obtain the pure LCMO phase by referring to the thermogravimetric analysis (TGA). The structural properties were investigated by an X-ray diffractometer (XRD) while electrical properties were measured by a four-point probe (4PP) system. XRD patterns showed the coexistence of two phases (LCMO & α-Fe₂O₃). LCMO crystallised in orthorhombic structure with space group Pnma while α-Fe₂O₃ exhibited in hexagonal form with space group R-3c. As the content of α-Fe₂O₃ increases, the resistivity of the samples increases drastically. Nevertheless, the addition of iron oxide has no significant effect on the metal-insulator transition temperature (T­_MI). From the XRD and 4PP analysis, it can be deduced that the α-Fe₂O₃ nanoparticles do not react with LCMO compound and successfully formed the La_0.7Ca_0.3MnO₃ /α-Fe₂O₃ composites. The resistivity increases when the nano-sized α-Fe₂O₃ is added into LCMO nanocomposites due to the insulator nature of α-Fe₂O₃.

Abstract: In this work, (1-x) (Nd_0.67Sr_0.33MnO₃): x (TiO₂) composites with x = 0, 0.1, 0.2, 0.3 and 0.4 have been prepared to investigate the structural and electrical properties. Nd_0.67Sr_0.33MnO₃ (NSMO) was synthesised via the solid-state reaction method before incorporated with TiO₂. The addition of TiO₂ nanoparticle as the secondary phase in manganite composite would favour the spin-polarized tunnelling near to the grain boundary and thus enhance the extrinsic magnetoresistance. Nevertheless, nanoparticle addition might contribute to substitution and diffusion with manganite compound as reported in literature. The effect of the TiO₂ nanoparticle addition into NSMO composites has been examined by an X-ray diffractometer (XRD) and a four-point probe (4PP) system. From the thermogravimetric analysis (TGA), NSMO phase formation occurred in between 756.45 - 977.59 °C. XRD patterns showed that there is no peak shift when the TiO₂ concentration increases. It can be deduced that TiO₂ was segregated at the NSMO grain boundary region and its grain surface. However, a small amount of Ti atoms are expected to replace the Mn atoms in NSMO crystal system and has caused the increase in crystallite size. The electrical study showed that the presence of TiO₂ nanoparticle and substitution of Ti in Mn sites have weaken the double exchange (DE) mechanism and suppressed the metal-insulator transition temperature (T_MI). In addition, the insulating behaviour of TiO₂ has also caused the resistivity of composites to increase drastically.

Abstract: Vanadium oxide films have been fabricated by the acetylacetonate and triethoxy vanadyl sol-gel methods on silicon substrates, as well as by magnetron sputtering on glass-ceramic substrates. Additional annealing in reducing atmosphere results in formation of vanadium dioxide or mixed phases with a VO₂ predominance. The obtained films demonstrate the metal-insulator transition and electrical switching. In the films produced from triethoxy vanadyl, the peculiarities of electrical properties are related to the size effect, heterophase character of vanadium oxide films, and different types of charge carriers in the bulk of nanocrystallites and on their surfaces. Also, the effect of doping with hydrogen by means of plasma-immersion ion implantation on the properties of vanadium dioxide is explored. It is shown that the transition parameters in VO₂ thin films depend on the hydrogen implantation dose. At doses exceeding a certain threshold value, the films are metallized, and the phase transition no longer occurs.

Abstract: We report the results of studies on the electronic state of the hole-doped Y-based pyrochlore iridate, (Y_1-x-yCu_xCa_y)₂Ir₂O₇. We carried out the resistivity, Muon Spin Relaxation (μSR), X-ray Photoemission Spectroscopy (XPS) measurements and Density Functional Theory (DFT) calculations on the non-doped (x=y=0) and doped (x=0.05, y=0.15) systems. We found in the non-doped system that the magnetic ordering of Ir spins which was accompanied by the metal-insulator transition (MIT) occurred at around 157 K and disappeared in the doped system in which MIT seems to disappear or smeared out. We suggest from the current study that a quantum critical point which shows a change in the electronic ground state from insulating to metallic to exist between those two systems.

Abstract: The effect of nickel substitution on the electrical transport properties of La_0.7Sr_0.2Ba_0.1Mn_1-xNi_xO₃ (x = 0 and 0.1) manganite have been studied. The temperature dependence of resistivity of the samples shows that nickel substitution increases the overall resistivity. Exchange probability calculation shows that the ferromagnetic (FM) coupling inside the sample decreases upon nickel substitution, which induces antiferromagnetic (AFM) coupling inside the sample. These results are suitable with the reduction in the electronic bandwidth value. Comparison of experimental data with theoretical models shows that the electrical behavior of the samples is well explained by a theory based on percolation models. Present result suggests that the electrical behavior of the sample was influenced by scattering and interaction between electron, phonon, and magnon. Nickel substitution also decrease the metal-insulator transition temperature and theoretical Curie temperature from around 328 K to 249 K and around 349 K to 275 K respectively.

Abstract: A systematic investigation of polycrystalline La_0.7Ba_0.1Sr_0.2Mn_1-xCu_xO₃ (x = 0 and 0.5) manganite has been conducted with a view to understand the effect of copper substitution on the electrical transport properties and its correlation with structural and morphological properties. The structural and morphological properties of the sample without copper content (x = 0) have been reported previously, while in this research, the structural and morphological properties of sample with 5% copper content (x = 0.5) will be reported. Structural comparison with previous research shows that 5% copper content does not change the rhombohedral structure of the sample without copper content. Comparison of resistivity (ρ) data with the theoretical model shows that the electrical properties of both samples are well described using the electron-electron, electron-phonon, electron-magnon, and kondo-like spin dependent scattering theory. Furthermore, percolation theory which have been used in the model shows that ferromagnetic phase and paramagnetic phase coexisted in both samples. Through copper substitution, the overall resistivity of the sample decrease compared to the sample without copper content. Furthermore, percolation theory demonstrates that ferromagnetic phase of the sample is decreasing as copper ions substituted manganese ion.

Abstract: The La_0.8Sr_xCa_0.2-xMnO₃ (x=0, 0.05 and 0.15) compounds were synthesized by combining sol-gel method and high temperature sintering. The effects of Sr doping content on the microstructure, metal-insulator transition and magnetoresistance (MR) of the La_0.8Sr_xCa_0.2-xMnO₃ were investigated. The results show that the La_0.8Sr_xCa_0.2-xMnO₃ exhibit single perovskite phase and the change of Sr doping content can result in phase structure transition. The La_0.8Sr_0.15Ca_0.05MnO₃ is rhombohedra structure and the La_0.8Sr_xCa_0.2-xMnO₃ shows distorted cubic structure when x is no more than 0.05. Increasing Sr doping content causes the degeneration of microstructure homogeneity and density and the decrease of average grain size of the La_0.8Sr_xCa_0.2-xMnO₃, which can be attributed to the weakening of atomic diffusion during the sintering process. The metal-insulator transition temperature (T_MI) of the compounds increases with the decrease of Sr doping content due to the improvement of microstructure. The La_0.8Sr_0.15Ca_0.05MnO₃ has the highest MR peak value, which is about 95%. The La_0.8Ca_0.2MnO₃ has the highest room temperature MR, which is about 28%. Moreover, the magnetoresistance of the La_0.8Ca_0.2MnO₃ is very stable between 125~300K.

Abstract: Vanadium oxide nanoparticles were synthesized with controlled size and dispersity by gas phase cluster beam deposition. The composition of the nanoparticle film is dominated with VO₂ nanoparticles. The VO₂ nanoparticles undergo a phase transition between the room temperature monoclinic insulator phase and the higher temperature rutile metal phase. In the metallic phase, the VO₂ nanoparticles exhibit a strong surface plasmon resonance in the near-IR region from 900nm to the 2000nm, which generates a large enhancement on the extinction coefficient. This plasmon resonance is thermally controlled by the VO₂ MIT and can be used to improve the optical switching characteristics of VO₂ based devices in the near-IR region.

Abstract: A systematic investigation of neodymium-based manganite, Nd_0.7Sr_0.3MnO₃, was undertaken with a view to understand the influence of sintering temperature on various physical properties. The materials were prepared by the a soft chemical approach of co-precipitation method by sintering at four different temperatures starting from 700 to 1000 °C, with an interval of 100 °C. X-ray diffraction (XRD), transmission electron microscopy (TEM) and D. C. four-probe resistivity were employed to study the crystal structure, average particle size and electrical property respectively. Analysis of XRD patterns shows that all the samples exhibit single phase orthorhombic crystal structure. We followed William-son Hall approach to calculate the lattice stain (ε).These materials were found to exhibit different metal-insulator transition temperature (T_MI) for the different sintering temperature. The value of T_MI increases, as the sintering temperature increases, whereas ε decreases. TEM results show that with the increment of sintering temperature, the particle size of the NSMO samples also increases, which plays a key role on electrical transport. To understand the conduction mechanism in metallic and insulating regions of resistivity, various theoretical models are discussed in this communication.

Abstract: We report the magnetic properties of Eu_xCa_1-xB₆ single crystals (0.756x1) studied in the wide range of temperatures (1.8-300 K) and magnetic fields (up to 50 kOe). It was found that low field magnetic susceptibility χ (T) follows the Curie-Weiss law χ~(T-Θ_p)^-1 at high temperatures for all the concentrations studied. The effective magnetic moment of the Eu²⁺ ion estimated from the data diminishes from the free ion value μ_eff7.93μ_B (μ_B - Bohr magneton) for x=1 to μ_eff7.3μ_B for x=0.756. A universal behavior of magnetic susceptibility χ~(T-Θ)^-α (α=1.5) is detected close to the Curie temperature T_C in the paramagnetic state at both metallic (x>x_C~0.8) and dielectric (xC.