Y. W. Ma

Inducing ferromagnetism in ZnO through doping of nonmagnetic elements

In this work, Zn, Al, Pt, Ag, and Au nonmagnetic metallic films were deposited on the surface of ZnO film, followed by high vacuum annealing at different temperatures. Results showed that (Zn,Al,Pt)∕ZnO films possessed room temperature ferromagnetism (RTF) after the vacuum annealing, while (Ag,Au)∕ZnO films did not. Our detailed structural investigations (transmission electron microscopy and x-ray photoelectron spectroscopy) revealed that this RTF was associated with the presence of metal clusters. The RTF disappeared in Al∕ZnO after a subsequent annealing in air, as metal clusters were oxidized. Pt∕ZnO remained ferromagnetic, as the metal cluster structure was stable subjected to the air annealing.

Enhancement of room temperature ferromagnetism in C-doped ZnO films by nitrogen codoping

The effect of nitrogen on the magnetic properties of C-doped ZnO has been investigated. It has been found that a low concentration of N doping does not lead to an apparent change of the magnetization in C-doped ZnO films. When N doping concentration exceeds 0.05 at. %, the magnetization of C–ZnO films increases significantly (more than 85%). The increased magnetization is mainly due to the enhanced moment of carbon, resulted from N doping. The successful fabrication of p-type diluted magnetic semiconductor may be of interest for spintronic applications.

Room temperature ferromagnetism in Teflon due to carbon dangling bonds

The ferromagnetism in many carbon nanostructures is attributed to carbon dangling bonds or vacancies. This provides opportunities to develop new functional materials, such as molecular and polymeric ferromagnets and organic spintronic materials, without magnetic elements (for example, 3d and 4f metals). Here we report the observation of room temperature ferromagnetism in Teflon tape (polytetrafluoroethylene) subjected to simple mechanical stretching, cutting or heating. First-principles calculations indicate that the room temperature ferromagnetism originates from carbon dangling bonds and strong ferromagnetic coupling between them. Room temperature ferromagnetism has also been successfully realized in another polymer, polyethylene, through cutting and stretching. Our findings suggest that ferromagnetism due to networks of carbon dangling bonds can arise in polymers and carbon-based molecular materials.

Room temperature ferromagnetism of ZnO nanocrystals in amorphous ZnO–Al2O3 matrix

In this work, we demonstrate room temperature ferromagnetism of ZnO nanocrystals (NCs) embedded in alumina-based amorphous matrix. Our study has shown that solid solution occurred for lower Al concentration [≤x=0.09 for (Zn1−x,Alx)O]. No ferromagnetism appears in Al-doped ZnO with x≤0.09. With higher Al concentration, alumina-based amorphous structure appeared and fully amorphous was found in the composition of (Zn0.30,Al0.70)O. Ferromagnetism was found in (Zn1−x,Alx)O with a threshold of 16 mol % of Al. The room temperature ferromagnetism was found to be attributed to ZnO NCs in an alumina-based amorphous ZnO–Al2O3 matrix. Ferromagnetism reached a maximum with a volume percentage of ZnO NCs to be ∼27 vol %. The detailed investigations (transmission electron microscopy, x-ray photoelectron spectroscopy, and superconducting quantum interference device) show that the optimized size of these NCs is in the range of 4–5 nm. No ferromagnetism appears if the sample contains a fully amorphous structure.

Mechanism of room temperature ferromagnetism in ZnO doped with Al

ZnO recently receives extensive interest owing to its potential applications in the dilute magnetic semiconductor. In this work, Al was deposited onto the surface of ZnO film followed by high vacuum annealing. The film showed the room temperature ferromagnetism (RTF). The saturation magnetization (Ms) highly depends on both the thickness of the Al top layer and the thickness of the ZnO film. The RTF disappeared when the film was further annealed in air atmosphere. The detailed structure characterizations (x-ray diffraction and x-ray photoelectron spectroscopy) revealed that the RTF was associated with a charge transfer between Al and Zn.

Structural and magnetic studies of Cu-doped ZnO films synthesized via a hydrothermal route

Highly-textured n-type ZnO:Cu films with room-temperature ferromagnetism have been hydrothermally grown in aqueous solution at 90 °C using PLD-derived ZnO seed layers on different substrates (quartz, silicon, glass and sapphire). The crystallinity of the ZnO seed layers on different substrates plays an important role in the microstructure of the subsequently grown ZnO:Cu films. Better crystallinity and texture of the seed layers is favourable to the growth of continuous films. Thick Zn0.98Cu0.02O films (ca. 1.2 μm) exhibit high value of magnetic moment (0.40 μB per Cu), comparable to the previously reported value of thin films, indicating that thick ZnO:Cu films with high magnetization can be synthesized via this hydrothermal route. The incorporation of hydrogen into the ZnO lattice plays a dominant role in ferromagnetism in this ZnO:Cu system.

Growth of highly textured manganese zinc ferrite films on glass substrates

Highly textured manganese zinc ferrite (Mn0.7Zn0.3Fe2O4) films have been successfully fabricated on glass substrates by pulse laser deposition at relatively low temperatures. Investigations indicated that the strain, which is induced by high deposition rate and the difference of thermal coefficient between the film and glass substrate, is attributed to the growth of textured structure. Growth of highly textured cobalt ferrite film was also achieved using the same method. This work provided a possible technique for fabricating high quality ferrite films on glass substrates.

Synthesis And Characterization Of Ferromagnetic Nanowires Using Aao Templates

Ferromagnetic (Ni, Co, Fe) nanowires were electrodeposited using AAO template. XRD analysis indicated that the ferromagnetic wires had preferred orientation. For longer deposition time, the nanowires with textured structure changed to single crystal. The investigation on the growth mechanism showed that in the initial stage, the nanowires were polycrystalline. Our TEM analysis revealed the transition from polycrystalline structure to single crystal from the bottom to the top. The growth mechanisms are probably due to the competition growth of the adjacent grains and the confinement of growth in the nano-sized hole of the AAO template.

Room temperature ferromagnetism and hopping conduction in Pt NCs/Al2O3 films

In this work, Pt nanocrystals (NCs) were embedded into different oxide films by codeposition of Pt and oxide at 400 °C and 10−8 Torr using a pulse laser deposition system. Films of Pt (25 mol. %) NCs embedded in (Al2O3, ZnO, or SnO2) matrix showed room temperature ferromagnetism (RTFM), while Pt (25 mol. %) NCs/(MgO or SiO2) films were not magnetic. The detailed study of Pt NCs/Al2O3 films with different concentrations of Pt revealed that RTFM depended on Pt NC size and conductivity of the film. Resistance-temperature study of this film showed that the hopping conduction may be associated with RTFM.