Cunxu Gao

Preparation and characterization of haematite nanowire arrays

Arrays of ?-Fe2O3 nanowires embedded in anodic alumina membranes were obtained after heat-treating ?-FeOOH nanowire arrays fabricated by electrochemical deposition. Haematite polycrystalline nanowires with maximum length of about 7?m and average diameter of about 120 nm were characterized by means of x-ray diffraction and transmission electron microscopy. The Morin temperature below 80 K and N?el temperature of about 350 K for the ?-Fe2O3 nanowire arrays, far lower than those of bulk material, were measured by M?ssbauer spectroscopy and using a Magnetic Property Measurement System.

Hydrothermal epitaxial growth and nonvolatile bipolar resistive switching behavior of LaFeO3-PbTiO3 films on Nb:SrTiO3(001) substrate

Epitaxial LaFeO3-PbTiO3 (LFPTO) thin films were hydrothermally grown on the Nb-SrTiO3 (100) (NSTO) substrates with a thickness about 250 nm. As fabricated Pt/LFPTO/NSTO/Pt devices exhibit reversible bipolar resistive switching behavior. The resistance ratios between high resistance state and low resistance state exceed three orders of magnitude, which can be maintained over 6 h without observable degradation. It indicates that the Pt/LFPTO/NSTO/Pt devices reveal excellent data retention and endurance characteristics. The resistive switching mechanism of the device could be attributed to the trap-controlled space-charge-limited current conduction which is controlled by the localized oxygen vacancies in the films. Furthermore, variation of Pt/LFPTO Schottky junction depletion thickness and barriers height modulated by oxygen vacancies at Pt/LFPTO interface was suggested to be responsible for the resistance switching behaviors of the devices.

Rapid characterizing of ferromagnetic materials using spin rectification

Spin rectification is a powerful tool for dc electric detections of spin dynamics and electromagnetic waves. Technically, elaborately designed on-chip microwave devices are needed in order to realize that effect. In this letter, we propose a rapid characterizing approach based on spin rectification. By directly sending dynamic current into ferromagnetic films with stripe shape, resonant dc voltages can be detected along the longitudinal or transversal directions. As an example, Fe (010) films with precise crystalline structure and magnetic parameters were used to testify the reliability of such method. We investigated not only the dynamic parameters and the precise anisotropy constants of the Fe crystals but also the principle of spin rectification in this method.

Nonvolatile Bipolar Resistive Switching Behavior in the Perovskite-like (CH3NH3)2FeCl4

The bipolar resistive switching behavior in a device based on an crystalline iron-based organic-inorganic, perovskite-like material of (CH3NH3)2FeCl4 (MAFC), was examined and studied. Both high and low resistance states appeared to have no obvious degradation during a measurement period of 600 s with 400 cycles in a Ag/MAFC/Cu device, which also exhibited good thermal stability over a wide temperature range of 290 to 340 K. The conductivity-state switching behavior was derived from the competition between the ionic current within the MAFC and the Faradaic current that originated from oxidative reactions at the Ag/MAFC/Cu interface. A model explaining the oxidative reaction process was established to describe the symmetric resistive switching behavior in the Ag/MAFC/Cu cell. With an applied bias voltage sweeping, the oxidative layers passivated and dissipated at the Ag/MAFC/Cu interface that resulted in the competition between the induced current and the ionic current, and thus caused a symmetric resistance change. On the basis of this interfacial effect, the MAFC crystals can be used as memristor elements in devices for write-read-erase-rewrite process.

Flexible nonvolatile resistive memory devices based on SrTiO3 nanosheets and polyvinylpyrrolidone composites

Strontium titanate (SrTiO3) ultra-thin-nanosheets with an average thickness of 3.1 ± 0.2 nm have been synthesized via a solvothermal reaction route with ethylene glycol as the reaction medium solvent. The as-prepared SrTiO3 nanosheet and polyvinylpyrrolidone (PVP) (SrTiO3–PVP) composites were formed into large-scale high-integrity thin films on polyethylene terephthalate (PET) by using a spin-coating method. A flexible memory device with the configuration of Ag/SrTiO3–PVP/Pt/PET was fabricated and it showed a promising write-once read-many times memory effect with a high ON/OFF current ratio of more than 1.0 × 103, an ultralow switching voltage (0.17 V), good stability, good repeatability and flexibility. SrTiO3 nanosheets acting as trapping centers in the device play a significant role in the resistive switching mechanisms. We hope that the benefits of ultralow switching voltage and flexible characteristics will make this material promising for next generation nonvolatile functional memory applications.

Multidirectional available high-frequency response with zero-field resonance above 8 GHz in epitaxial α-Fe(001) films

We investigate the high-frequency characteristics of epitaxial single crystal α-Fe(001) thin films by permeability spectroscopy with microwave transmission along different in-plane crystal directions. The results show an in-plane isotropic static permeability above 50 with a multidirectional zero-field resonance frequency (fr) higher than 8 GHz in the films. We demonstrate that these multidirectional available high-frequency characteristics result from the strong magnetocrystalline anisotropy in the films and not from other kinds of magnetic anisotropies. The study on thickness-dependent films show fr being stable at 8.4 GHz in the films thicker than about 20 nm.

Nonvolatile bipolar resistive switching behavior of epitaxial NdFeO3–PbTiO3 thin films grown on Nb:SrTiO3(001) substrate

Epitaxial NdFeO3–PbTiO3 (NFPTO) thin films were fabricated on Nb–SrTiO3(100) (NSTO) substrates of about 300 nm thickness by a sol–gel process. Nonvolatile bipolar resistive switching has been observed in the Pt/NFPTO/NSTO structure. The resistance ratio between the high-resistance state and the low-resistance state is about one order of magnitude. After degenerating for several minutes, each memory state is stably maintained and no further degradation occurs over 15000 s. X-ray photoelectron spectroscopy results suggest that the oxygen vacancies acting as trapping centers in the films play a significant role in the resistive switching mechanisms. Analysis of the current–voltage relationship demonstrates that the trap-controlled space-charge-limited current mechanism is of considerable importance to the resistance switching.

Enhancement of magneto-photogalvanic effect in periodic GaAs dot arrays by p-n junctions coupling

To control the semiconductor device under low magnetic field is still a great challenge for semiconductor magnetoelectronics. In this work, we report the observation of the magneto-photogalvanic effect in periodic GaAs dot arrays. With an increase in magnetic field from 0 to 1500 Oe, the photovoltage increases linearly for a wide temperature range from 80 to 430 K. Compared with GaAs without the dot arrays, periodic GaAs dot arrays have a hundredfold increase of the magnetic-field-modulated photovoltage at room temperature. By changing the magnetic field orientation, the angular dependence of photovoltage reveals that the magneto-photogalvanic effect stems from the Hall electric field caused by optical current, and the enhancement of magneto-photogalvanic effect is attributed to the p-n junction coupling between GaAs dots. When the coupling between the GaAs dots is broken at the high temperatures, i.e., T = 430 K, we demonstrate that the enhancement effect disappears as expected. Our results not only illu...