Mingjing Chen

Epitaxial growth and transport properties of Bi2Sr2Co2Oy thin films by metal organic deposition

Epitaxial Bi2Sr2Co2Oy thin films have been grown on LaAlO3 (001) by metal organic deposition. Detailed x-ray diffraction texture measurements reveal the excellent c-axis and ab-plane alignments. At 300 K, the ab-plane power factor is estimated to be more than two times larger than that of the single crystals due to the very low resistivity and the reasonably large Seebeck coefficient of the films. In addition, a large ab-plane negative magnetoresistance of 38% related to the suppression of the spin scattering is also observed in the films at low temperature.

Properties of expitaxial Ca3Co4O9 thin films fabricated by chemical solution method

Epitaxial Ca3Co4O9 thin films were deposited on c-cut sapphire substrates by the chemical solution method. The room temperature resistivity and the Seebeck coefficient of the films are about 11 mΩ cm and 130 µV K−1, which are comparable to those of single crystals. A large nonlinear resistivity related to the suppression of spin-density-waves by an external electric field is also observed at low temperature.

Enhanced polarization and dielectricity in BaTiO3:NiO nanocomposite films modulated by the microstructure

Parallel and vertical interfaces in vertically and parallelly aligned nanocomposite thin films have been shown to be an effective method to manipulate functionalities. However, tuning the physical properties by modulating the microstructure of the self-assembled nanocomposite films and understanding the physical properties underlying the manipulation is still a challenge. In this work, BaTiO3:NiO (BTO:NiO) nanocomposite films with nanomultilayer, nanocolumnar and nanogranular structures have been prepared on Nb:SrTiO3 (Nb:STO) substrates by a pulsed laser deposition (PLD) method. These films have been used as a model system to investigate the relationship between the microstructure and the ferroelectric properties. The polarization, dielectricity and leakage can be separately modulated by tuning the microstructures. The experimental results show that the remanent polarization of the nanocomposite films is much higher than that of the pure BTO films. By precisely modulating the microstructure, a significantly enhanced polarization (>70% higher than pure BTO for the nanomultilayer structure) and dielectricity (>60% and >240% higher than pure BTO for the nanomultilayer and nanocolumnar structure, respectively) are realized in these films. These results demonstrate that tunable ferroelectric properties can be realized by controlling the microstructures in the epitaxial BTO:NiO nanocomposite thin films, which will be expected to be applied in the devices such as supercapacitors, solar cells and non-volatile memory applications.

Polarization and charge-transfer effect on the transport properties in two-dimensional electron gases/LaNiO3 heterostructure

The film thickness dependent transport properties of the LaNiO3 (LNO) layer epitaxially grown on LaAlO3/SrTiO3 (LAO) 2-dimensional electronic gas (2DEG) have been investigated. The ultrathin LNO films grown on the 2DEG have a sheet resistance below the values of h/e2 in all temperature ranges. The electron density is enhanced by more than one order of magnitude by capping LNO films. X-ray photoelectron spectroscopy shows that the interface undergoes unambiguous charge transfer and electronic reconstruction, leading to modulation doping of such atomically engineered complex oxide heterointerfaces. The polar-catastrophe of the 2DEG is directly linked to the electronic structure and transport properties of the LNO. The transport properties can be well modulated by the thickness of the LAO in the 2DEG, and the data can be well fitted with the polar-catastrophe scenario. These results suggest a general approach to tunable functional films in oxide heterostructures with the 2DEG.

Influence of Annealing on Photoluminescence of ZnO:Mn Thin Flims Grown by Sol-Gel Technique

ZnO:Mn thin flims were fabricated by Sol-gel technique. the effect of annealing time on the photoluminescence (PL) properties of the prepared flims has been investigated. The room temperature PL of ZnO:Mn films is shown to consist of an UV emission band due to radiative recombination of free excitons and a weak green band signated as the radiative transition from Zn vacancies(VZn) to Oxygen antisites. It is shown that whereas increasing annealing time make both of emission bands increase, an enhacement for the relative intensity of green emission compared to that of UV emission is obtend. Meanwhile, An increse of saturation magnetizations was also observed. All results provided clear evidences that although the annealing have the effect of ameliorate the microstructure of ZnO:Mn flim, the accompanied increased VZn defects make the magnetic moments become more alignd. Keywords-sol-gel; ZnMnO; PL; ferromagnetism; VZn

The role of pd hybridization in the metal-insulator transition in NdNiO3 heterostructure

ABSTRACT Understanding and controlling phase transition in the transition-metal oxides is interesting from the fundamental physics point of view. The pd hybridization are of considerable issue to tune the phase transition temperature. Here, the transport properties of the NdNiO3 with artificial ultra-thin NiO insert layer have been investigated. The relationship between the essential parameters of the pd hybridization (T) and the metal-insulator transition (MIT) temperature has been clearly elucidated a monotone decreasing relationship. This work realized the combination of the engineering hybridization and the phase transition temperature, which might be significant for the development of multifunctional materials in the transition-metal oxides. GRAPHICAL ABSTRACT IMPACT STATEMENT The pd hybridization are of considerable issue to tune the phase transition temperature. Here, the pd hybridization have been tuned and elucidated a monotone decreasing relationship with the metal–insulator transition temperature.

Charge transfer effect at the La 0.7 Ca 0.3 MnO 3 /NiO heterojunction and the novel interface ferromagnetism

We report the formation of a new ferromagnetic (FM) states in the antiferromagnet (AFM) NiO at the interface with ferromagnet (FM) La_0.7Ca_0.3MnO₃ (LCMO). The new magnetization temperature at 80 K is observed and can be ascribed to Ni³⁺-O-Mn³⁺ superexchange interactions. Mn 3s and Ni 3p core-level spectra have been measured by x-ray photoelectron spectroscopy (XPS) which show a direct evidence of charge transfer effects of the type Mn⁴⁺-Ni²⁺ → Mn³⁺-Ni³⁺ at the interface region. The valence band offset (VBO) at the LCMO/NiO interface can be determined to be ΔE_VBO ~ 0.77 eV. The valence band of NiO is shifted to higher binding energy compared to LCMO. Thus, charge transfer occurred owing to the valence band edge shifting at the heterostructure interfaces.

Control of the ferromagnetic to paramagnetic transition in SrRuO 3 epitaxial films by electric field

The external electric field controlled the phase transition behavior for ferromagnetic thin films is important to realize its practical applications as spintronic devices. In this work, we report the electric field controlled ferromagnetic (FM) to paramagnetic (PM) transition in a SrRuO3/Pb(Mg1/3Nb2/3)O3-PbTiO3 (SRO/PMN-PT) heterostructures by adding a bias voltage. The FM to PM transition and the magnetoresistance (MR) have been dynamic controlled by the electric field. The Curie temperature (Tc) is changed from 154 K to 162 K as a result of external electric field applied to the PMN-PT substrates. It is demonstrated that the SRO film is closely associated with the strain states, which can be directly controlled by the bias voltage. Our finding offer novel opportunities for modulating phase transitions in a reversible non-volatile way.

Effects of substrate temperature on structural and optical properties of S-doped ZnO films

S-doped ZnO films were fabricated on sapphire (0001) substrates by pulsed-laser deposition at different substrate temperatures. Effects of the substrate temperature on the structural and optical characteristic of S-ZnO films were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-Vis spectrophotometer and fluorescence spectrometer. The results show that the S-doped ZnO thin films are wurtzite crystal structure. And the crystalline quality of S-doped ZnO films improves with increasing the substrate temperature from 400°C to 500°C. And the S/O ration is slowly decreased with substrate temperature elevation attributed to instability of S under the higher deposition temperature conditions. The absorption edges of S-doped ZnO thin films slightly shift blue as the substrate temperature increase from 400°C to 600°C. The photoluminescence (PL) spectra indicates that the visible emissions of S-doped ZnO films are enhanced, as well as the UV emission is weakened comparing to the pure ZnO thin film. It is illuminated that S-doping of ZnO can modify the optical properties of ZnO, which is promising for optoelectronic applications that work in the visible emission region.