Yuan Song-Liu

Improved multiferroic properties of La-doped 0.6BiFeO3?0.4SrTiO3 solid solution ceramics

The 0.6(Bi1?xLax)FeO3?0.4SrTiO3 (x = 0, 0.1) multiferroic ceramics are prepared by a modified Pechini method to study the effect of substitution of SrTiO3 and La in BiFeO3. The X-ray diffraction patterns confirm the single phase characteristics of all the compositions each with a rhombohedral structure. The magnetic properties of the ceramics are significantly improved by a solid solution with SrTiO3 and substitution of La. The values of the dielectric constant ?r and loss tangent tan ? of all the samples decrease with increasing frequency and become constant at room temperature. The La-doped 0.6BiFeO3?0.4SrTiO3 ceramics exhibit improved dielectric and ferroelectric properties, with higher dielectric constant enhanced remnant polarization (Pr) and lower leakage current at room temperature. Compared with a anti-ferromagnetic BiFeO3 compound, the 0.6(Bi0.9La0.1)FeO3?0.4SrTiO3 sample shows the optimal ferromagnetism with remnant magnetization Mr ~ 0.135 em?/g and ferroelectricity with Pr ~ 5.94 ?C/cm2 at room temperature.

Monte Carlo Simulation of the Temperature-Dependent Resistance for (La1-xYx)2/3Ca1/3MnO3 (0≤x≤0.2)

Resistance as a function of temperature for optimally doped manganese perovskites is simulated based on a random resistor network model which is generated through the Monte Carlo method by identifying neighbours in each cubic lattice randomly occupied by ferromagnetic metallic particles. Only one parameter, i.e. the number density of ferromagnetic particles, is used as an adjustable parameter, and it is shown that the model yields excellent agreement with measured resistance data in (La1-xYx)2/3Ca1/3MnO3 (0≤x≤0.2) for temperatures covering the whole range from the high-T insulating behaviour to the low-T metallic state.

Transport propeties of polycrystalline La2/3Ca1/3Mn1−x Cu x O3 prepared by Sol-Gel

The influence of Cu dopant (x) and sintering temperature (Ts) on the transport properties of La2/3Ca1/3Mn1−xCuxO3 series samples prepared by Sol-Gel technique was investigated. X-ray diffraction patterns show that all the samples with different Cu dopant and sintering temperatures (Ts) are of single phase without obvious lattice distortion. Experimental results indicate that the insulator-metal transition temperature is directly related to the sintering temperature and Cu dopant x. It is interesting to observe that a proper amount of Cu dopant can substantially improve magnetoresistance effects.

Effects of Thickness and Polarization Field on the Photovoltaic Properties of BiFeO3 Thin Films

We study the effects of film thickness on lattice parameters, direct band gap and photovoltaic outputs in the sol-gel derived BiFeO3 thin films. With the change of the film thickness, the great transitions will take place in the preferred orientation and lattice parameters. Furthermore, the photovoltaic outputs are significantly dependent on the film thickness. The results show that the open circuit voltage gradually increases and the short circuit current reciprocally decreases with the increase of film thickness. In particular, we demonstrate for the first time that there are tunable photovoltaic outputs with external electric field polarization switching in the polycrystalline BiFeO3 film, which is critical for the future device applications based on the photovoltaic properties of BiFeO3 films.

The synthesis and exchange bias effect of monodisperse NiO nanocrystals

Monodisperse NiO nanocrystals with an average particle size of 3 ± 0.4 nm are successfully synthesized by the thermal decomposition of Ni-oleylamine complex in an organic solvent under a continuous O2 flux. The crystalline structure and the morphology of the product are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Magnetization and alternating-current (ac) susceptibility measurements indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core and a spin-glass-like surface shell. In addition, both the exchange bias field and the vertical magnetization shift can be observed in this system at 10 K after field cooling. This observed exchange bias effect is explained in terms of the exchange interaction between the antiferromagnetic core and the spin-glass-like shell.

Effects of Pr on the structure and magnetic properties of FePt alloys

Abstract The effects of Pr on the structure and magnetic properties of PrxFe60.5-xPt39.5 alloys (x = 0, 0.5, 1.0. and 1.5) were investigated. X-ray diffraction data indicated that the phase transition temperature of FePt based alloys from disordered face-centered-cubic to ordered face-centered-tetragonal cubic decreases with the increase in Pr concentration. Pr plays the role of a grain refiner and it can enhance the exchange coupling between soft magnetic phase and hard magnetic phase. The results indicate that the replacement of Fe by Pr can significantly improve the remanence and coercivity of the Fe60.5Pt39.5 alloy. These results can be explained on the basis of phase transformation and microstructure. Both the remanence ratio and coercivity of the FePt based alloy as a function of the Pr content are increased by the optimum addition of 0.5 at.% Pr.

Electrical transport and low-field magnetoresistance in La2/3Ca1/3MnO3/YSZ composites

Electrical transport and low-field magnetoresistance (MR) are reported for (1-x)LCMO/xYSZ composites (0 2%. In particular, the composite with x≤1% shows a wide temperature window with a large constant MR below the transition. Based on microscopy analysis, a possible interpretation is presented for different observations for x 2% with emphasis on the boundary effect arising from insulating YSZ layers.

First-Principles Based Model of Spin-state Phase Transition

The nature of spin-state phase transition is investigated with [Fe(C4H4N2){Pt(CN)4}] that is a novel 3D Hofmann?like compound. The bistability of this system is obtained by the first-principles calculation. It is demonstrated that thermal expansion is the intrinsic force involved in spin-state transition. Based on these results, we suggest a thermal exciting bistable model of spin-state transition with a temperature dependent crystal-field splitting (CFS). Experimental evidence of spin-state phase transition coincides with our theoretical model. This model approaches something fundamental in the mechanism leading to the transition, and it is important in developing new and practical controllable quantum devices.

Phase Transformation and Magnetic Properties of NdxFe60.5-xPt39.5 (x = 0, 0.5, 1.0, 1.5) Alloys

Abstract The effect of Nd addition on the structure, phase transformation and magnetic properties of FePt based alloys was investigated. The results indicated that the transition temperature from ordered FCT to disordered FCC phase decreased with increasing Nd concentration, but for alloys quenched rapidly from the γ phase region into ice-water, it increased with increasing Nd. The Nd element not only effectively reduced the grain size of the ordered phase but also decreased the degree of the ordered phase and refined the grains of the FCC matrix phase. The remanence ratio and coercivity of the FePt based alloy as a function of the Nd content had maximum values, respectively.

Evidence of the Griffiths Phase in Doped Manganites Studied by Electron Paramagnetic Resonance Measurements

A systematic investigation of electron paramagnetic resonance above the Curie temperature TC is performed on polycrystalline (La1−xYx)2/3Ca1/3MnO3 (x = 0, 0.15 and 0.2) samples. The results indicate anomalous paramagnetic behaviour below a temperature TG and, the anomalous extent increases with cooling from TG. Especially, the resonance linewidth ΔHpp anomalously increases with cooling for the temperature range of TC<T<TG. The experimental observation is discussed within the framework of the Griffiths theory that predicts the existence of ferromagnetic clusters above TC. We explain the observed anomalous paramagnetic behaviour to be due to the magnetic heterogeneity caused by the ferromagnetic clusters which appear in the Griffiths phase.