Chengyue Xiong

Rectifying I-V characteristic of LiNbO3∕Nb-doped SrTiO3 heterojunction

LiNbO3∕Nb-doped SrTiO3 heterojunction was fabricated by pulsed laser deposition. The current-voltage curve of this heterojunction shows good rectifying property and changes with temperature dramatically. For the forward bias, the conduction mechanism changes from Ohmic-like for the low bias voltages to space charge limited current for the high bias voltages. While for the reverse bias, it changes from Schottky emission to avalanche breakdown with increasing bias voltage. The results were explained by considering the band structures of the junction. This work demonstrates that ferroelectric materials, combined with other oxides, can lead to some interesting property which may have potential applications.

Rectifying property and giant positive magnetoresistance of Fe3O4/SiO2/Si heterojunction

Fe3O4∕SiO2∕Si heterojunction was fabricated by growing Fe3O4 film on an n-typed Si wafer with the native SiO2 buffer layer using the pulsed laser deposition. Transmission electron microcopic study shows the high quality of the heterojunction interfaces and the SiO2 layer is 2.5nm thick. This junction shows a backward diodelike rectifying behavior and an anomalously giant positive magnetoresistance (MR) for the large reverse bias voltages. The temperature dependence of MR shows a peak around the Verwey transition temperature with a maximum MR of 87% under a −2V bias voltage. The results were discussed by considering the band structure of the heterojunction and the effect of the reverse bias voltage.

Negative magnetoresistance in undoped semiconducting amorphous carbon films

Amorphous carbon (a-C) films were fabricated by chemical vapor deposition on SiO2 substrate. The a-C films have nano-crystalline sp2 structure with the grain size of ∼5 nm and an optical band gap of ∼1.8 eV. The a-C films show negative magnetoresistance (MR) from 300 to 2 K and an anomalous shape change of MR-magnetic field curves at 10 K. Grain boundary scattering theory and weak localization theory were used to explain the MR mechanism and shape change of MR-magnetic field curves.

The rectifying property and magnetoresistance of La0.67Ca0.33MnO3∕SiO2∕Si heterojunction

We have fabricated a heterojunction by depositing La0.67Ca0.33MnO3 film on electron doped silicon wafer with a buffer layer of natural SiO2. The current-voltage measurement shows that it is a diode with a good rectifying property in a wide temperature range. At high positive voltages, the current-voltage curve shows space charge limited (SCL) current behavior, manifesting itself as a SCL diode. At low positive and negative voltages, the resistance of the junction shows a peak at a certain temperature, which decreases with increasing positive voltage and keeps constant at negative voltages. Magnetoresistance of the junction shows a similar temperature dependence as the resistance of the junction. Calculations show that these phenomena can be attributed to the depletion layer of the La0.67Ca0.33MnO3 film. This work also demonstrates that SCL diode can be realized in La0.67Ca0.33MnO3∕SiO2∕Si with the presence of the SiO2 layer.

Growth of graphene on copper and nickel foils via chemical vapour deposition using ethylene

Abstract Graphene was grown on copper and nickel foils via chemical vapour deposition (CVD), using ethylene (C2H4) as a precursor instead of methane (CH4). Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) results confirmed that ethylene can be used in the growth of graphene. The effect of ethylene pressure on graphene growth was also observed. This study’s results show that graphene can be grown on copper and nickel foils under the same growth conditions of CVD if ethylene is used as a precursor.

Magnetic field controllable nonvolatile resistive switching effect in silicon device

A Si–SiO2–MgO device showing nonvolatile resistive switching effect is fabricated. It is observed the resistance of the device changed from high value to low value at a certain transition voltage after being stimulated by a large current. In addition, the transition voltage shifts reproducibly under a reversed large current. By applying a reading voltage in the range of the transition voltages, nonvolatile resistive switching phenomena with on/off ratio of about 10, endurance of more than 200 cycles and retention of more than 104 s is obtained. More importantly, it is also observed that the magnetic field could shift the transition voltage to higher values, affecting or even suppressing the resistive switching. The magnetic field controllable nonvolatile resistive switching effect in the silicon device suggested by this work may be helpful to the silicon based industry.

Magnetoresistance sign change in iron-doped amorphous carbon films at low temperatures

Positive and negative magnetoresistance (MR) is observed in iron-doped amorphous carbon films at low temperatures. The MR of these films changes between positive and negative values by adjusting metallic doping, the temperature and the magnetic field. Wave-function shrinkage and spin-dependent scattering are found to be responsible for these changes. The former contributes to the positive MR, while the latter contributes to the negative one. These two factors compete with each other, resulting in different MR values with opposite signs. The results may be beneficial to carbon-based spintronics.

Current-voltage characteristics of phase separated La0.5Ca0.5MnO3∕Nb–SrTiO3 p-n junction and magnetic tunability

The authors report the current-voltage characteristics of La0.5Ca0.5MnO3∕Nb-SrTiO3 p-n junction. This junction shows remarkable thermal hysteresis, giant negative magnetoresistance (MR), remarkable differences of MR for the zero field cooling and field cooling processes, and memory effect of magnetic field. Magnetic force microscopy studies provide direct evidence of magnetic inhomogeneity in La0.5Ca0.5MnO3 film. These intriguing behaviors of our p-n junction can be explained by the phase separation in La0.5Ca0.5MnO3. This work demonstrates the principle of harnessing phase separation for highly tunable device applications.

Reconfigurable Magnetic Logic Combined with Nonvolatile Memory Writing

In magnetic logic, four basic Boolean logic operations can be programmed by a magnetic bit at room temperature with a high output ratio (>103 %). In the same clock cycle, benefiting from the built-in spin Hall effect, logic results can be directly written into magnetic bits using an all-electric method.

Extremely Large Magnetoresistance at Low Magnetic Field by Coupling the Nonlinear Transport Effect and the Anomalous Hall Effect

The anomalous Hall effect of a magnetic material is coupled to the nonlinear transport effect of a semiconductor material in a simple structure to achieve a large geometric magnetoresistance (MR) based on a diode-assisted mechanism. An extremely large MR (>10(4) %) at low magnetic fields (1 mT) is observed at room temperature. This MR device shows potential for use as a logic gate for the four basic Boolean logic operations.