Lixia Ren

Photocarrier transport and dynamics in mixed-phase BiFeO 3 films.

We report a remarkable photoinduced relaxation process and its dependence of thickness and temperature in mixed-phase BiFeO3 films grown on (001) LaAlO3 substrates. When the films are illuminated by the light above the bandgap, their resistances are reduced with the increase of temperature. The photoinduced change of resistance reaches to the maximum of about 2.17 × 105% at 300 K. It is noted that the relaxation processes of the resistance are significantly different between T-like phase and T-R mixed phase due to structural strain, symmetry breaking and built-in electric field at the phase boundaries. These results provide more insights into intrinsic mechanisms of mixed-phase multiferroic materials and potential applications in all-oxide photoelectric devices.

The Frustration-induced Ferroelectricity of a Manganite Tricolor Superlattice with Artificially Broken Symmetry

In this paper, [(La0.9Sr0.1MnO3)n/(Pa0.9Ca0.1MnO3)n/(La0.9Sb0.1MnO3)n]m superlattices films have been deposited on (001) Nb:SrTiO3 substrates by a laser molecular-beam epitaxy technology. Expected ferroelectricity arise at well-defined tricolor superlattice at low temperature, composed of transition metal manganite, which is absent in the single-phase compounds. Furthermore, the ferroelectric properties of the superlattices are enhanced by increasing the periodicity m, which may be attributed to the accumulation of the polarization induced by the frustration. As for the magnetic hysteresis loop characteristics of the multilayer structures, the saturation magnetization and magnetic coercivity of films present definitely a strong periodic dependence. It also indicates that the frustration may exist in the tricolor superlattice. Our results further verify the previous theoretical research of generating multiferroics experimentally paving a way for designing or developing the novel magnetoelectric devices based on manganite ferromagnets.

Tunability of Band Gap and Photoluminescence in CH 3 NH 3 PbI 3 Films by Anodized Aluminum Oxide Templates

Hybrid organic-inorganic halide CH3NH3PbI3 perovskite films are deposited on anodized aluminum oxide templates with the different pore diameters via one-step spin coating method. The obvious 0.082 eV blue shift of optical band gap is observed in films with decreasing the diameters of pores from 400 to 30 nm. And numerical simulations based on finite element modeling are carried out to represent the absorption edge and consistent with the experiment results. It is interesting that the films show the intense photoluminescence with the excitation intensity of less than 1 μW. Moreover, the photoluminescence intensity is increased with increasing pore diameters, which is attributed to the radiative recombination rate of photogenerated electrons and holes. These results pave a way for the further understanding of tunable photophysical properties of perovskite films.

Modulation of persistent magnetoresistance by piezo-strain effect in manganite-based heterostructures

Persistent magnetoresistance effects in the phase-separated Pr0.65(Ca0.25Sr0.75)0.35MnO3/SrTiO3 and Pr0.65(Ca0.25Sr0.75)0.35MnO3/0.7PbMg1/3Nb2/3O3–0.3PbTiO3 heterostructures under a low magnetic field are investigated. It is observed that the persistent magnetoresistance effects decrease with increasing temperatures and the values for the heterostructures on 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 and SrTiO3 substrates are about 86.6% and 33.2% at 40 K, respectively. More interestingly, the applied electric field on the 0.7PbMg1/3Nb2/3O3–0.3PbTiO3 substrate can suppress the persistent magnetoresistance effect, indicating that different energy landscapes can be dramatically modulated by the piezo-strain. These results are discussed in terms of the strain-induced competition in the ferromagnetic state and the charge-ordering phase by the energy scenario, which provide a promising approach for designing devices of electric-magnetic memories in all-oxide heterostructures.

Thickness dependence of photoresponsive properties at SrTiO3-based oxide heterointerfaces under different strains

The photoresponsive characteristics of SrTiO3-based oxide heterointerfaces at different thicknesses and strains are investigated. In addition to the typical persistent photoconductivity, the transient photoconductivity is observed at the heterointerfaces below the critical thickness. More intriguingly, it shows a transition from the transient photoconductivity to the persistent photoconductivity for the interfaces at the critical thickness with enhancing temperatures, which is related to the dominant role of thermal energy. Moreover, larger strain at the LaAlO3/SrTiO3 interfaces produces a roughly greater photoinduced change in the resistance than the strain-relaxed (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 interfaces. Our results provide deeper insight into the photoresponsive properties and intrinsic mechanisms of two-dimensional electron gas at complex oxide interfaces.

Quasi-two-dimensional electron gas at γ-Al2O3/SrTiO3 heterointerfaces fabricated by spin coating method

Complex oxide heterointerfaces with the two-dimensional electron gas have drawn a lot of attention due to their emerging properties. However, most of them are prepared using the method with relatively high energy particles, which would inevitably lead to some defects. Here, a key challenge in the quasi-two-dimensional electron gas at spinel-type Al2O3/SrTiO3 heterointerfaces using a spin coating method is addressed. The grown Al2O3 films have the atomic-scale smooth surface and the thickness is about 70 nm. The γ-Al2O3 layer with a thickness of about 3 nm in proximity to SrTiO3 and the amorphous Al2O3 layer on the top of γ-Al2O3 are observed for the heterointerface annealed at 800 °C. The heterointerfaces at annealed temperatures above 750 °C exhibit a metallic behavior, which is attributed to the dominant layer of γ-Al2O3. The sheet carrier density is about 3.1 × 1015 cm−2 and the Hall mobility is 4924.4 cm2 V−1 s−1 at 15 K at the heterointerface annealed at 800 °C. Our work provides a low-cost way for the large-scale and large-area production of two-dimensional electron gas at high-quality oxide interfaces.Complex oxide heterointerfaces with the two-dimensional electron gas have drawn a lot of attention due to their emerging properties. However, most of them are prepared using the method with relatively high energy particles, which would inevitably lead to some defects. Here, a key challenge in the quasi-two-dimensional electron gas at spinel-type Al2O3/SrTiO3 heterointerfaces using a spin coating method is addressed. The grown Al2O3 films have the atomic-scale smooth surface and the thickness is about 70 nm. The γ-Al2O3 layer with a thickness of about 3 nm in proximity to SrTiO3 and the amorphous Al2O3 layer on the top of γ-Al2O3 are observed for the heterointerface annealed at 800 °C. The heterointerfaces at annealed temperatures above 750 °C exhibit a metallic behavior, which is attributed to the dominant layer of γ-Al2O3. The sheet carrier density is about 3.1 × 1015 cm−2 and the Hall mobility is 4924.4 cm2 V−1 s−1 at 15 K at the heterointerface annealed at 800 °C. Our work provides a low-cost way for t...

The tunable optical magneto-electric effect in patterned manganese oxide superlattices

The optical magneto-electric (OME) effect has been widely investigated in magnetic materials, but obtaining the large and tunable OME effect is an ongoing challenge. We here design a tri-color superlattice composed of manganese oxides, Pr0.9Ca0.1MnO3, La0.9Sr0.1MnO3, and La0.9Sb0.1MnO3, where the space-inversion and time-reversal symmetries are broken. With the aid of the grating structure, the OME effect for near-infrared light in tri-color superlattices is investigated systematically through the Bragg diffraction method. The relative change of diffracted light intensity of the order n = ±1 has a strong dependence on the magnetization and polarization of the tri-color superlattice, whether the superlattice is irradiated in reflection or transmission geometries. Otherwise, the relative change of diffracted light intensity increases with the increase in the superlattice period and with the decrease in the grating period. The maximum relative change of diffracted light intensity in tri-color superlattices with the grating structure patterned is as large as 8.27%. These results pave the way for designing next-generation OME devices based on manganese oxides.The optical magneto-electric (OME) effect has been widely investigated in magnetic materials, but obtaining the large and tunable OME effect is an ongoing challenge. We here design a tri-color superlattice composed of manganese oxides, Pr0.9Ca0.1MnO3, La0.9Sr0.1MnO3, and La0.9Sb0.1MnO3, where the space-inversion and time-reversal symmetries are broken. With the aid of the grating structure, the OME effect for near-infrared light in tri-color superlattices is investigated systematically through the Bragg diffraction method. The relative change of diffracted light intensity of the order n = ±1 has a strong dependence on the magnetization and polarization of the tri-color superlattice, whether the superlattice is irradiated in reflection or transmission geometries. Otherwise, the relative change of diffracted light intensity increases with the increase in the superlattice period and with the decrease in the grating period. The maximum relative change of diffracted light intensity in tri-color superlattices w...

Orientation-Dependent Optical Magnetoelectric Effect in Patterned BaTiO3/La0.67Sr0.33MnO3 Heterostructures

The optical magnetoelectric effect has been widely investigated, but obtaining the large and tunable optical magnetoelectric effect at room temperature is still a big challenge. We here design ferroelectric/ferromagnetic heterostructures with various orientations, which are composed of titanate BaTiO3 and manganese oxide La0.67Sr0.33MnO3. This artificial bilayer structure presents room-temperature ferroelectric and ferromagnetic properties. After patterning a 4 μm grating structure on the bilayer thin film, the optical magnetoelectric effect for near-infrared light is investigated systematically through the Bragg diffraction method. The relative change of diffracted light intensity of the order n = 1 has a strong dependence on the magnetization and polarization of the thin films, whether the superlattice is irradiated in reflection or transmission geometries. For (100)- and (111)-oriented samples, both show the room-temperature optical magnetoelectric effect, while the (111)-oriented thin film has a stronger optical magnetoelectric effect. These results pave the way for designing next-generation optical magnetoelectric devices based on the ferroelectric/ferromagnetic structure.