F.M. Zhang

Effect of annealing ambient on the ferromagnetism of Mn-doped anatase TiO2 films

The magnetic properties of anatase Ti1?xMnxO2 (0 < x < 0.06) films prepared by sol?gel spin coating annealed in air and vacuum have been investigated. Room temperature ferromagnetism was observed in all the films. Enhancement of ferromagnetism is revealed in the films annealed in vacuum. The magnetic moment of the films annealed in vacuum, at 300?K, is 0.285 ? 0.004 ?B/Mn for Ti0.9618Mn0.0382 O2 and 0.366 ? 0.005 ?B/Mn for Ti0.9409 Mn0.0591O2. It is believed that the enhanced ferromagnetism could be due to the formation of oxygen vacancies and/or defects.

Enhanced magnetoresistance in self-assembled monolayer of oleic acid molecules on Fe3O4 nanoparticles

Spin transport through molecules is investigated using self-assembled monolayers of oleic acid molecules on half metallic Fe3O4 nanoparticles. Fourier transform infrared spectroscopy measurements indicate that 1 ML (monolayer) molecules chemically bond to the Fe3O4 nanoparticles and the physically absorbed molecules do not exist in the samples. The magnetoresistance (MR) of cold-pressed, molecule fully covered nanoparticles is up to 7.3% at room temperature and 17.5% at 115 K under a field of 5.8 kOe. The MR ratio is more than two times larger than that of pure Fe3O4 nanoparticles. This enhanced MR is likely arising from weak spin scattering while carriers hop through the oleic acid molecules. These results show that small molecules are promising for development of future spin-based molecular electronics.

Ferromagnetism of Co-doped TiO2 films prepared by plasma enhanced chemical vapour deposition (PECVD) method

Ti1−xCoxO2 polycrystalline films have been prepared on Si(0 0 1) substrates by the plasma enhanced chemical vapour deposition technique at 280 °C without using any carrier gas. All the films show room-temperature ferromagnetic behaviours and no ferromagnetic clusters are detected by x-ray diffraction, x-ray photoelectron spectroscopy, atomic force microscopy, Raman and superconducting quantum interference device measurements as the doping concentration is lower than 4%. In addition, the formation of non-ferromagnetic CoTiO3 under heavy doping is considered to be responsible for the degradation of magnetization in Ti1−xCoxO2 polycrystalline films. Furthermore, saturated magnetization of Ti1−xCoxO2 films is found to decrease with the increasing duration of oxygen-plasma processing, indicating that the oxygen vacancies in the films play an important role in the generation of ferromagnetic Ti1−xCoxO2 films.

Large low field magnetoresistance in ultrathin nanocrystalline magnetite Fe3O4 films at room temperature

Ultrathin (15nm) Fe3O4 nanocrystalline films with (111) spinel texture have been prepared by rapid annealing of amorphous ion oxide films. Large low field magnetoresistance (LFMR), with the values of about −6.3% at 300K and −10% at 200K under a field of 0.5T, has been observed in the films. The LFMR is mainly attributed to the boundary tunneling of high spin-polarized electrons in Fe3O4 grains of the films and nearly follows a simple relationship between MR and polarization for intergranular tunneling. The fabricating method here seems to be a good approach to prepare high quality Fe3O4 nanocrystalline films with a large LFMR at room temperature.

Large low-field magnetoresistance in Fe3O4/molecule nanoparticles at room temperature

Acetic acid molecule-coated Fe3O4 nanoparticles, 450–650 nm in size, have been synthesized using a chemical solvothermal reduction method. Fourier transform infrared spectroscopy measurements confirm one monolayer acetic acid molecules chemically bond to the Fe3O4 nanoparticles. The low-field magnetoresistance (LFMR) of more than −10% at room temperature and −23% at 140 K is achieved with saturation field of less than 2 kOe. In comparison, the resistivity of cold-pressed bare Fe3O4 nanoparticles is six orders of magnitudes smaller than that of Fe3O4/molecule nanoparticles, and the LFMR ratio is one order of magnitude smaller. Our results indicate that the large LFMR in Fe3O4/molecule nanoparticles is associated with spin-polarized electrons tunnelling through molecules instead of direct nanoparticle contacts. These results suggest that magnetic oxide-molecule hybrid materials are an alternative type of materials to develop spin-based devices by a simple low-cost approach.

Influence of the oxidative annealing temperature on the magnetism of (Mn, N)-codoped ZnO thin films

(Mn, N)-codoped ZnO films have been achieved by oxidative annealing of sputtered Zn2N3:Mn films at different temperatures in flowing O2 ambient. X-ray diffraction measurements and x-ray photoelectron spectroscopy studies indicate that both divalent Mn2+ and trivalent N3− ions are incorporated into ZnO lattice. Ferromagnetism with Curie temperature above 300 K was observed in the (Mn, N)-codoped ZnO films and found to be sensitive to the oxidative annealing temperature due to its strong effect on the carrier type and concentration. The strongest ferromagnetism has been found in the (Mn, N)-codoped ZnO films with the highest hole concentration in this study. The results indicate that holes are favorable for ferromagnetic ordering of doped Mn2+ ions in ZnO in agreement with the recent theoretical studies. The mechanism of ferromagnetic coupling in (Mn, N)-codoped ZnO is discussed.

Characterization and luminescence properties of AlON:Eu2+ phosphor for white-emitting-diode illumination

AlON phosphors with different Eu2+ doping concentrations were synthesized by solid-state reaction method. Two Eu2+ doping sites were confirmed by temperature-dependent emission intensity, band shift and biexponential decay studies. High internal quantum efficiency and tunable chromaticity coordinates make AlON:Eu2+ phosphor a suitable candidate for white light-emitting diodes application.

Effects of annealing and hydrogenation on the properties of ZnMnO polycrystalline films synthesized by plasma enhanced chemical vapour deposition

For the Mn-doped ZnO polycrystalline films fabricated with plasma enhanced chemical vapour deposition technique followed by thermal treatment at 600 °C for 5 h, the effects on both rapid thermal annealing up to 900 °C in Nitrogen ambient for 2 min and hydrogenation processing have been investigated. It has been indicated that after the rapid thermal annealing the magnetization of the films decreases. The magnetization of the films has been found to increase with the increase in hydrogenation processing duration at the early stage of the processing and gets a saturated value after about 60 min while there is a decrease with increasing electron concentration after a rapid thermal process in nitrogen ambient.

Effect of hydrogenation on the ferromagnetism in polycrystalline Si1−xMnx:B thin films

Polycrystalline Si1−xMnx films codoped with boron have been prepared by radio-frequency magnetron sputtering deposition followed by post-thermal processing for crystallization. The polycrystalline thin films were treated by hydrogen plasma excited with approach of radio-frequency plasma-enhanced chemical vapor deposition. It has been found that the saturation magnetization decreases after hydrogenation while the structural properties of the films do not show any change. At the same time, it is observed that after hydrogenation the hole concentration in the films is lower than that before the treatment, believed to be due to a combination between hydrogen and boron. The obvious correlation between the magnetic properties and the transport properties of the polycrystalline Si1−xMnx films suggests that there be a mechanism of hole-mediated ferromagnetism for Si-based diluted magnetic semiconductors.

Room-temperature ferromagnetism in p-type (Mn, N)-codoped ZnO thin films achieved by thermal oxidation of sputtered Zn3N2:Mn films

P-type (Mn, N)-codoped ZnO films have been fabricated by oxidative annealing of sputtered Zn2N3 : Mn films on silicon substrates at 550 °C in flowing O2 ambient. X-ray diffraction and x-ray photoelectron spectroscopy studies reveal that the Mn and N ions incorporated into the ZnO lattice are divalent Mn2+ and trivalent N3− ions, respectively. Ferromagnetism with Curie temperature above 300 K was observed in p-type (Mn, N)-codoped ZnO films. The results indicate that holes are favourable for ferromagnetic ordering in doped Mn2+ ions in ZnO, in agreement with recent theoretical studies.