Zhongqiang Hu

Enhanced multiferroic properties of the high-valence Pr doped BiFeO3 thin film

High-valence Pr-doped BiFeO3 (BPF) multiferroic thin film was prepared by pulsed laser deposition on a Pt∕TiO2∕SiO2∕Si substrate. X-ray diffraction analysis indicates that the BPF film is of pure phase with a polycrystalline perovskite structure. The BPF film exhibited enhanced multiferroic properties: (i) a rectangular-shaped electric hysteresis loop with a large Pr of 75μC∕cm2 and a low Ec of 250kV∕cm at Emax ∼620kV∕cm, (ii) a saturated magnetic hysteresis loop with a large Ms of 58emu∕cm3, and (iii) a fatigue-free behavior after being subjected to 1.2×109 switching cycles, which were superior to, in some aspects, those of La- or Nd-doped BiFeO3 films reported before.

Temperature-dependent energy storage properties of antiferroelectric Pb0.96La0.04Zr0.98Ti0.02O3 thin films

The energy storage properties of antiferroelectric (AFE) Pb0.96La0.04Zr0.98Ti0.02O3 (PLZT 4/98/2) thin films were investigated as a function of temperature and applied electric field. The results indicated that recoverable energy density (Ure) and charge-discharge efficiency (η) of PLZT (4/98/2) depend weakly on temperature (from room temperature to 225 °C), while Ure increases linearly and η decreases exponentially with increasing electric field at room temperature. These findings are explained qualitatively on the basis of the kinetics of the temperature-induced transition of AFE-to-paraelectric phase and the field-induced transition of AFE-to-ferroelectric phase, respectively. The high Ure (≈61 J/cm3) and low leakage current density (≈3.5 × 10−8 and 3.5 × 10−5 A/cm2 at 25 and 225 °C, respectively) indicate that antiferroelectric PLZT (4/98/2) is a promising material for high-power energy storage.

Enhanced multiferroic properties of BiFeO3 thin films by Nd and high-valence Mo co-doping

Pure BiFeO3 (BFO), Nd-doped BFO (BNF) and Nd and Mo co-doped BFO (BNFM) multiferroic thin films were prepared by pulsed laser deposition on Pt/Ti/SiO2/Si substrates. Among these films, the BNFM film exhibited the lowest dielectric loss and leakage current density, a well-saturated P–E loop, excellent charge-retaining ability and fatigue-free behaviour. Furthermore, improved ferromagnetism was observed in this film with a saturation magnetization of ~15 emu cm−3. These significant improvements in ferroelectric and ferromagnetic properties could be preliminarily attributed to the elimination of oxygen vacancies and the increase in the Fe2+ ions in the film by Nd and high-valence Mo co-doping, respectively.

Ferroelectric memristor based on Pt/BiFeO3/Nb-doped SrTiO3 heterostructure

We report a continuously tunable resistive switching behavior in Pt/BiFeO3/Nb-doped SrTiO3 heterostructure for ferroelectric memristor application. The resistance of this memristor can be tuned up to 5 × 105% by applying voltage pulses at room temperature, which exhibits excellent retention and anti-fatigue characteristics. The observed memristive behavior is attributed to the modulation effect of the ferroelectric polarization reversal on the width of depletion region and the height of potential barrier of the p-n junction formed at the BiFeO3/Nb-doped SrTiO3 interface.

Structural Transition and Multiferroic Properties of Eu-Doped BiFeO3 Thin Films: Structural Transition and Multiferroic Properties of Eu-Doped BiFeO3

The Bi1−xEuxFeO3 (x=0, 0.05, 0.10, 0.15, 0.20, and 0.30) thin films were grown by pulsed laser deposition on Pt/Ti/SiO2/Si substrates, and the effects of Eu content on the structure and multiferroic properties were investigated. A structural transition from rhombohedral R3c to orthorhombic Pbnm occurred in the film with x=0.15 due to the increase of the Eu content. Superior multiferroic properties were obtained in the film with x=0.10, giving a remanent polarization Pr=74 μC/cm2 and a coercive field Ec=250 kV/cm at an applied electric field E=560 kV/cm, a good antifatigue property, and a saturated magnetization Ms=3.5 emu/cm3. These significantly improved ferroelectric properties could be attributed to the appropriate lattice distortion and the lower leakage current due to the doping, while the enhanced ferromagnetism might be ascribed to the substitution-induced suppression of the spiral spin modulation.

Pressure-induced cubic-to-orthorhombic phase transformation in the negative thermal expansion material HfW2O8

The effect of pressure on the crystal structure of HfW2O8 has been investigated by neutron powder diffraction. At a hydrostatic pressure of 0.62 GPa at room temperature the cubic material transforms, with a 5% reduction in volume, to the same orthorhombic phase that is seen in the isostructural compound ZrW2O8 above 0.21 GPa. The transformation is sluggish, requiring about 24 h to complete at constant pressure. Once formed, the orthorhombic phase is retained upon release of pressure. Upon heating to 360 K, the metastable orthorhombic phase transforms back to the cubic phase. The substantially higher pressure for the cubic-to-orthorhombic transition in HfW2O8, compared to ZrW2O8, may be important for the application of this material in composites with controlled thermal expansion because rather large local pressures can occur in such composites.

Voltage control of magnetism in FeGaB/PIN-PMN-PT multiferroic heterostructures for high-power and high-temperature applications

We report strong voltage tuning of magnetism in FeGaB deposited on [011]-poled Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) ternary single crystals to achieve more than 2 times broader operational range and increased thermal stability as compared to heterostructures based on binary relaxors. Voltage-induced effective ferromagnetic resonance field shift of 180 Oe for electric field from −6.7 kV/cm to 11 kV/cm was observed in FeGaB/PIN-PMN-PT heterostructures. This strong magnetoelectric coupling combined with excellent electric and temperature stability makes FeGaB/PIN-PMN-PT heterostructures potential candidates for high-power tunable radio frequency/microwave magnetic device applications.

Effect of Ce and Zr codoping on the multiferroic properties of BiFeO3 thin films

Pure BiFeO3(BFO), Ce-doped BiFeO3(BCFO), Zr-doped BiFeO3(BFZO), as well as Ce- and Zr-codoped BiFeO3(BCFZO) thin films were successfully prepared on Pt/Ti/SiO2/Si substrates by chemical solution deposition. The effects of Ce and Zr doping on the structure, surface morphology, electrical and magnetic properties of BFO films were studied. X-ray diffraction and atomic force microscopy analysis revealed structure transition and decreased grain sizes in the doped BFO films. In comparison with the other doped BFO films studied here, the Ce- and Zr-codoped BCFZO film showed the lowest dielectric loss and leakage current density and exhibited a well-squared hysteresis loop with a remanent polarization Pr of 64??C/cm2 and a coercive electric field Ec of 339?kV/cm, as well as the fatigue-free characteristics. Meanwhile, the largest magnetization was also observed in this codoped film. The possible reasons for the enhancement of the ferroelectricity and the ferromagnetism of these films were discussed.

Enhancement of properties of dye-sensitized solar cells by surface plasmon resonance of Ag nanowire core–shell structure in TiO2 films

A series of TiO2 nanocomposite photoanodes with different amounts of Ag nanowire (AgNW) coated with SiO2 in a AgNW@SiO2 core–shell structure are prepared by a ball milling method and applied to assemble a series of dye-sensitized solar cells (DSSCs). The influence of AgNW@SiO2 on the performance of the TiO2–AgNW photoanodes and DSSCs is investigated. Studies indicates that the range and strength of light absorption of the TiO2 photoanodes, the photon capture ability of the dye molecules, and the short-circuit current density (Jsc) are significantly increased while the dark current is decreased due to the incorporation of AgNW@SiO2. The optimal properties are obtained in a DSSC with a Jsc of 11.83 mA cm−2, and an overall photoelectric conversion efficiency (η) of 6.26%, significantly superior to those of the DSSC with a pure TiO2 photoanode. The enhancements of Jsc and η are attributed to the increase of light coupling and thus the light absorption of the dye due to the localized surface plasmon resonance and the possible enhanced light scattering of AgNW@SiO2 in the photoanode.

Nonvolatile bipolar resistive switching in an Ag/TiO2/Nb : SrTiO3/In device

A TiO2 thin film was deposited on a Nb : SrTiO3 substrate by pulsed laser deposition to form an Ag/TiO2/Nb : SrTiO3/In device. The bipolar resistive switching (RS) effect of this device was investigated. The current–voltage characteristics exhibited pronounced and stable bipolar RS features. The device could be switched to a low resistance state (LRS) at forward voltage and returned to a high resistance state (HRS) at reverse voltage, and the RS ratio RHRS/RLRS reached up to 2 × 103 at a read voltage of −0.5 V. Moreover, the RS ratio could be adjusted by changing the maximum value of the forward or reverse voltage, which shows promise for multilevel memories. These results are discussed by considering carrier injection-trapped/detrapped process of the heterostructure and show high potential for nonvolatile memory applications.