Xuebin Zhu

Electrical, optical and structural properties of CuCrO2 films prepared by pulsed laser deposition

Transparent conducting CuCrO2 thin films were prepared by pulsed laser deposition (PLD) from a nanocrystalline CuCrO2 target. The derived CuCrO2 films were highly c-axis oriented deposited at 800 K. The microstructural, electrical as well as optical properties were studied. It was found that the films were relatively smooth and behaved as semiconductors. The energy band of the CuCrO2 films is constructed based on the Mott–Davis model in order to investigate the conduction mechanism. The transmittances of the films in the visible region are about 60–80% with direct band gaps of about 3.2 eV. The results suggested that CuCrO2 films could be successfully prepared by the PLD method, which can broaden the applications of the transparent conducting oxide films.

Magnetic and electrical properties of p-type Mn-doped CuCrO2 semiconductors

Mn-doped CuCrO2 polycrystalline semiconductors were synthesized by the sol–gel method. The valence state of the Mn ions in the CuCr1−xMnxO2 samples was identified as 3+ by analysing their lattice parameters, their electrical properties and their x-ray photoelectron spectra. The temperature dependence of the susceptibility of all samples exhibits paramagnetic behaviour at high temperature. For x = 0.20 the samples exhibit a clear ferromagnetic (FM) transition at 120 K. Clear hysteresis loops indicate that FM order exists in the Mn-doped samples at 50 K. All samples behave like semiconductors. The electrical and magnetic properties indicate that the ferromagnetism can be attributed to the double exchange interaction between the Mn3+ and Cr3+ in CuCr1−xMnxO2 semiconductors.

Enhanced low-field magnetization and magnetoresistance in nano-MgO added La2 3Ca1 3MnO3 composites

We report the significant enhancement of low-field magnetization and magnetoresistance (MR) in La2/3Ca1/3MnO3 (LCMO) compounds with 5 at.% nano-MgO addition in the form of both nanoparticle and nanowire. It is found that the magnetization of the nano-MgO added compounds is highly sensitive to a low applied field below 7 kOe compared with the MgO-free LCMO. In addition, the enhancement of low-field MR is found in nano-MgO added compounds and the enhancement for the nanowire MgO-added composite is more remarkable than that for the nanoparticle MgO-added one. Moreover, two ferromagnetic phases with low-TC and high-TC in nano-MgO added compounds are observed. These results demonstrate that the low-field transport and magnetic properties of LCMO compounds can be effectively manipulated by controlling the morphology and dispersion of the insulating barrier using nano-MgO addition.

Growth of Ca3Co4O9 films: Simple chemical solution deposition and stress induced spontaneous dewetting

Chemical solution deposition as a simple method for preparation of oxide films is used to fabricate misfit structured Ca3Co4O9 thermoelectric films using cheap precursors, Ca and Co acetates, which opens up a simple route to fabricate large-area Co-based oxide thermoelectric films. The results show that when the annealing temperature is higher than 800°C crystallized Ca3Co4O9 films can be obtained on single-crystal LaAlO3 substrates, whereas the Ca3Co4O9 films are amorphous for lower annealing temperatures. Experimental evidence shows that the amorphous Ca3Co4O9 films behave like semiconductors with enhanced resistivity and Seebeck coefficient within the measured temperature range. The magnetoresistance is positive indicating the absence of long-range order of the Co-ion spins. For the crystallized Ca3Co4O9 films, it can be seen that the thinner films show the characteristics of spontaneous dewetting induced by the stress, which suggests that it is possible to prepare nanostructured Ca3Co4O9 films through...

Structural, magnetic and dielectric properties of the Aurivillius phase Bi6Fe2−xMnxTi3O18 (0 ≤ x ≤ 0.8)

The n = 5 Aurivillius phase ceramics Bi6Fe2−xMnxTi3O18 (BFMTO) (0 ≤ x ≤ 0.8) were synthesized with a conventional solid-state reaction method. All samples can be indexed with an orthorhombic structure with the space group B2cb. The magnetic measurements indicate that these ceramics are predominantly paramagnetic with the presence of short-range antiferromagnetic interactions and a weak ferromagnetic ordering state, implying that the predicted Fe3+–O–Mn3+ 180° ferromagnetic superexchange interaction based on the Goodenough–Kanamori rule might not be achieved in BFMTO ceramics through Mn substitution for Fe in the n = 5 Aurivillius phase. The dielectric loss of the x = 0.3 and 0.4 samples demonstrates the relaxation process and the rather large activation energy (2.63 eV for the x = 0.3 sample and 2.10 eV for the x = 0.4 sample) implies that this relaxation process is not due to the thermal motion of oxygen vacancies. The 0.5 ≤ x ≤ 0.8 samples exhibit a paraelectric–ferroelectric phase transition and the ferroelectric Curie temperature decreases upon increasing the doping level of Mn.

BiFeO3 thin films prepared on metallic Ni tapes by chemical solution deposition: effects of annealing temperature and a La0.5Sr0.5TiO3 buffer layer on the dielectric, ferroelectric and leakage properties

In this work, BiFeO3 (BFO) thin films were prepared on metallic Ni (200) tapes with and without a La0.5Sr0.5TiO3 (LSTO) buffer layer at different temperatures by chemical solution deposition. The effects of the annealing temperature as well as the LSTO buffer layer on the dielectric, leakage and ferroelectric properties have been studied in detail. The crystallite size, dielectric constant and leakage current density increase, while the coercive field decreases with increasing annealing temperature. The BFO thin films deposited directly on the Ni tapes are prone to wrinkling, while the wrinkles are smoothed by introducing a thin LSTO buffer layer. Decreased compressive microstrain as well as improved ferroelectric and leakage properties are observed in the BFO thin films deposited on the LSTO buffered Ni tapes. The results will provide an instructive route to optimize BiFeO3-based thin films on metallic tapes by chemical solution deposition methods.

Evolution of the resistive switching in chemical solution deposited-derived BiFeO3 thin films with dwell time and annealing temperature

Different resistive switching has been observed in the Au/BiFeO3/Pt structure composed with the BiFeO3 (BFO) thin films annealed at different temperature for different dwell time. Resistive switching decreases and disappearances with increasing annealing temperature, while reappear with prolonging dwell time. The evolution in the resistive switching is understand by the polarization modulated interfacial barriers for the low temperature annealing BFO films, oxygen vacancies rectifying interfacial depletion layer barriers for the high temperature and long time, and p-n junction for the films annealed for the longest time at high temperature.

Synthesis and characterization of ordered and disordered polycrystalline La2NiMnO6 thin films by sol–gel

Polycrystalline La(2)NiMnO(6) thin films are prepared on Pt/Ti/SiO(2)/Si substrates by the sol-gel method. Through controlling the processing parameters, the cation ordering can be tuned. The disordered and ordered thin films exhibit distinct differences for crystal structures as well as properties. The crystal structure at room temperature characterized by X-ray diffraction and Raman spectra is suggested to be monoclinic (P2(1)/n) and orthorhombic (Pbnm) for the ordered and disordered thin films, respectively. The ferromagnetic-paramagnetic transition is 263 K and 60 K for the ordered and disordered samples respectively, whereas the saturation magnetic moment at 5 K is 4.9 μ(B) fu(-1) (fu = formula unit) and 0.9 μ(B) fu(-1). The dielectric constant as well as magnetodielectric effect is higher for the ordered La(2)NiMnO(6) thin films. The magnetodielectric effect for the ordered thin film is dominantly contributed to the intrinsic coupling of electric dipole ordering and fluctuations and magnetic ordering and fluctuations, while it is mainly contributed to Maxwell-Wagner (M-W) effects for the disordered thin film. The successful achievements of ordered and disordered polycrystalline La(2)NiMnO(6) thin films will provide an effective route to fabricate double-perovskite polycrystalline thin films by the sol-gel method.

Enhanced remnant polarization in ferroelectric Bi6Fe2Ti3O18 thin films

For practical applications of ferroelectric memory, a large remnant polarization in nontoxic Pb-free ferroelectric materials is required. We prepared Bi6Fe2Ti3O18 thin films on Pt/Ti/SiO2/Si (100) substrates by chemical solution deposition using different annealing processes and found that a well-defined ferroelectric hysteresis loop with a rather large remnant polarization (Pr ≈ 21.5 μC cm−2) occurs in Bi6Fe2Ti3O18 thin films prepared by rapid thermal annealing in air. This value of remnant polarization is superior to those of other ferroelectric Aurivillius compounds such as Bi2WO6, Bi3TiNbO9, Bi4Ti3O12, Bi5FeTi3O15, and Bi6Fe2Ti3O18 thin films prepared by chemical solution deposition and pulsed laser deposition. The results of X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and ferroelectric P–E hysteresis loops revealed that the grain size, surface morphologies, oxygen vacancies and lattice distortion play very important roles in the determination of the remnant polarization. The present study provides an effective route to prepare layered Aurivillius thin films with a large remnant polarization by chemical solution deposition.

Synthesis and Characterization of Self-assembled c-axis Oriented Bi2Sr3Co2Oy Thin Films by the Sol gel Method

Bi(2)Sr(3)Co(2)O(y) thin films are prepared on SrTiO(3) (100), (110) and (111) single crystal substrates using the sol-gel method. All the thin films are c-axis oriented regardless of the orientation of the substrate suggesting self-assembled c-axis orientation, and X-ray photoelectron spectroscopy results give evidence of coexistence of Co(3+) and Co(2+) ions in the derived films. Transmission electronic microscopy observations reveal that all samples are c-axis oriented with no obvious differences for different samples, and the c-axis lattice constant is determined as ~15 Å suggesting the misfit structure. A phenomenological thermodynamic phase diagram for self-assembled c-axis orientation is established for misfit cobaltate-based films using chemical solution deposition. All samples behave like semiconductors due to the coexistence of Co(3+)/Co(2+) ions, and the resistivity at 350 K is ~47, 39 and 17 mΩ cm for the thin films on SrTiO(3) (100), (110) and (111), respectively, whereas the Seebeck coefficient at 300 K is 97, 89 and 77 μV K(-1). The successful attainment of Bi(2)Sr(3)Co(2)O(y) thin films with self-assembled c-axis orientation will provide an effective prototype for investigation of growth mechanisms in complex oxide thin films with a misfit structure.