Qingqing Fang

Magnetic properties and formation of Sr ferrite nanoparticle and Zn, Ti/Ir substituted phases

Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization ss and coercive field strength Hc are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization ss ¼ 74 emu/g and coercive field strength Hc ¼ 6:4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 5001C for several hours . It can prevent the formation of intermediate g-Fe2O3 and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn 2+ ,Ti 4+ and Ir 4+ are discussed. For an amount of substitution 0oxp0:6; the (Zn, Ti)x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)x substituted phase. r 2001 Elsevier Science B.V. All rights reserved.

On the origin of an additional Raman mode at 275 cm−1 in N-doped ZnO thin films

A systematic investigation on the optical properties of N-doped ZnO thin films was performed in order to understand the origin of an additional Raman mode at 275 cm−1. This Raman peak was observable only at N2 pressures lower than 30 Pa during pulsed laser deposition. Its intensity decreased with an increase of N2 pressures and eventually vanished at pressures above 30 Pa. N substitution of O (NO) was identified by photoluminescence and x-ray photoelectron spectroscopy and correlated well with the Raman intensity. The electrical measurements showed significant changes in resistivity, charge carrier concentration, and mobility due to the presence of N acceptors. Investigations on undoped ZnO films grown in Ar without N2 further confirm that N doping plays a key role in the Raman scattering. The experimental data indicate that the Raman mode originates from NO related complexes, likely in the form of Zni-NO. These investigations help to understand the doping mechanisms and underlying physics of the addition...

Temperature dependence of magnetic properties of zinc and niobium doped strontium hexaferrite nanoparticles

Zinc and niobium doped strontium hexaferrite nanoparticles, Sr(Zn0.7Nb0.3)xFe12−xO19 (x=0–1.0), were fabricated using a sol-gel method for high density magnetic recording. The structure and temperature dependence of magnetic properties are investigated. The experiments show that strontium hexaferrite with small Zn and Nb substitutions still remains a hexagonal magnetoplumbite phase. The coercive force is reduced from 6.7 to about 2.3 kOe, while the values of saturation magnetization increased to 67–74 emu/g in the substitution range of x=0–1.0. This indicates that the saturation magnetization and coercivity of strontium hexaferrite nanoparticles can be held over a very wide range by an appropriate amount of Zn and Nb doping contents. Simultaneously, it is found that the doped strontium hexaferrite nanoparticles show a small positive temperature coefficient of coercivity. The substitution of Zn2+ and Nb4+ ions for Fe3+ ions also monitors the temperature dependence of magnetization and Curie temperature, an...

Pulsed laser deposition of Li–N dual acceptor in p-ZnO:(Li, N) thin film and the p-ZnO:(Li, N)/n-ZnO homojunctions on Si(100)

This article showed the p-type ZnO thin films which were co-doped with different components of Li and N by pulsed laser deposition (PLD) on Si(111) substrates. According to Hall-effect data, the Li-N co-doped ZnO:(Li, N) exhibited stable room-temperature p-type behavior. Combining the XRD, UV-vis transmittance spectrum, and Hall-effect data, the preferable preparation condition was proposed for growing high quality p-type ZnO:(Li, N) film with comparatively low resistivity of 0.09 Ω cm and relatively high carrier concentration of 2.64 × 1017 cm−3, which were obtained at 0.1 at. % lithium composition. Furthermore, compared with the photoluminescence spectrum of ZnO:(Li) with ZnO:(Li, N) thin film, the existence of Li–N dual acceptor, which activated the acceptor in ZnO:(Li, N) thin film, was confirmed. On the basis of these, the p-ZnO:(Li, N)/n-ZnO homojunctions on Si(100) substrate was realized by PLD. All these data proved that Li-N dual acceptor would be effective when activated by PLD doping and this a...

Synthesis of 1D and heavily doped Zn1−xCoxO six-prism nanorods: improvement of blue–green emission and room temperature ferromagnetism

In this work, one-dimensional (1D) and heavily doped Zn1−xCoxO (x = 0.05, 0.1, 0.15 and 0.2) nanorods (NRs) were successfully fabricated by a new solvothermal method. Such a Zn1−xCoxO nanorod exhibits a hexagonal prism-like microcrystal with a pyramidal top. Analyses from transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) indicated that Zn1−xCoxO NRs possess perfect single crystal wurtzite structures. The influence of Co doping on the structural, optical and magnetic properties of NRs was investigated in detail. It was verified that Co was successfully doped into the ZnO wurtzite lattice structure by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The Zn1−xCoxO NRs show an obvious blue–green emission except for the weak UV emission. The corresponding luminescence mechanism was discussed, and the environment of cobalt in the ZnO wurtzite lattice was also identified through UV-vis spectroscopy. In addition, the magnetic hysteresis (M–H) curves demonstrated that the Zn1−xCoxO NRs possess obvious ferromagnetic characteristics at room temperature. These Zn1−xCoxO NRs are suggested to have potential applications in spintronic, field emission and optoelectronic devices, etc.

Temperature dependence of structure and magnetic properties in MnBi-based alloy films

MnBi alloy films, which have large magnetocrystalline anisotropy and high remanence, are synthesized with small dopants of rare-earth elements (RE=Dy, Sm, or Tb) by vacuum evaporation method. In contrast with most ferromagnetic materials, the low temperature phase (LTP) of MnBi film shows a large coercivity, Hc, over an extended temperature range. The hexagonal crystal structure of MnBi LTP thus has good thermal stability. The experiments found that doping further extends the temperature range through which the LTP is stable. Moreover, MnBi-doped films still exhibit square hysteresis loops at high temperatures. We attribute the unusual temperature dependence of the coercivity to an increased perpendicular anisotropy field, Ha, with temperature. For this reason, MnBi-based alloy films have great potential as permanent magnets at high temperatures.

The ZnO p-n homojunctions modulated by ZnMgO barriers

In this paper, we fabricated the ultrathin ZnO p-n homojunctions, which modulated by ZnMgO asymmetrical double barriers (ADB). The ADB p-n homojunctions displays step-like curve in the absorption spectrums, this is the first time that quantum confinement effect has been observed in the absorption spectrums at room temperature (RT). The Hall-effect data confirm there is 2-dimensional electron gas in the interface of the ZnMgO ADB p-n junctions. The quantum confinement effect enhances the hall-mobility μ to 103 cm2V −1s−1 based on the polarity of the films. There was no rectification property in the ZnO homojunctions with thickness of 250nm, however, when the ADB was added in the n-type layer of the homojunctions, it displays a typical Zener diode rectification property in the I-V curve.

Optical and Magnetic Properties of ZnO-based Semiconductors Regulated by Cu Ions

Cu doped ZnCoAlO (ZCAO) films deposited on single-crystal Si (100) substrates were prepared by pulsed laser deposition (PLD). The microstructures of the ZCAO films with different Cu concentrations were studied by X-ray diffraction (XRD); the findings show that the quality of the crystal structure increases with Cu ion doping. The near-band gap and deep-level-defect emissions at room temperature (RT) are observed in the Cu-doped samples via photoluminescence (PL) measurement, revealing its effects on the short-wavelength emission for the Cu-doped film compared with the ZCAO film. The magnetism and electron transport in the Cu doped ZCAO films were also investigated. The results show that interstitial Cu ions affect the carrier concentration. The magnetic measurement shows that the Cu ions occupying adjacent cation positions breaks the magnetic double exchange interaction and weakens the magnetism of the samples.

High mobility ultrathin ZnO p–n homojunction modulated by Zn0.85Mg0.15O quantum barriers

The adding of ZnMgO asymmetric double barriers (ADB) in p-ZnO: (Li, N)/n-ZnO homojunction affects the p–n junction device performance prominently. Two different homojunctions are fabricated on Si (100) substrates by pulsed laser deposition; one is the traditional p-ZnO: (Li, N)/n-ZnO homojunction with different thicknesses named as S1 (250 nm) and S2 (500 nm), the other is the one with ADB embedded in the n-layer named as Q (265 nm). From the photoluminescence spectra, defect luminescence present in the S-series devices is effectively limited in the Q device. The current–voltage curve of the Q device shows Zener-diode rectification property because the two-dimensional electron gas tunnels through the narrow ZnMgO barrier under a reverse bias, thus decreasing the working p–n homojunction thickness from 500 nm to 265 nm. The ADB-modified homojunction shows higher carrier mobility in the Q device. The electroluminescence of the ZnO homojunction is improved in Q compared to S2, because the holes in p-type ZnO (Li, N) can cross the wide ZnMgO barrier under a forward bias voltage into the ZnO quantum well. Therefore, electron–hole recombination occurs in the narrow bandgap of n-type ZnO, creating an ultraviolet light-emitting diode using the ZnO homojunction.

Magnetic Properties and Exchange Bias Effect of Cu and Cr doped Sm2Co17 Alloy Film

Cu and Cr-doped Sm2Co17 films were synthesized using pulsed laser deposition method with the substrate of Si(100), and our results show that Cu and Cr incorporation can reduce the grain size and increase the coercivity of Cu and Cr-doped Sm2Co17 films. Furthermore, incorporations of Cu and Cr will introduce a positive effect for the exchange bias of Cu and Cr-doped Sm2Co17 films. The magnetic measurement of external magnetic field and zero-field-cooled (FC-ZFC) under the temperature ranging from 10K to 300 K prove that the critical value of FM-AFM transform is ~80 K in the Cu and Cr-doped Sm2Co17 films, which showed that the exchange bias origins from the coupling competition between ferromagnetic phase and antiferromagnetic phase.