Shih-Jye Sun

Origin of ferromagnetism in nitrogen embedded ZnO?:?N thin films

Nitrogen embedded ZnO : N films prepared by pulsed laser deposition exhibit significant ferromagnetism. The presence of nitrogen ions contained in ZnO is confirmed by the secondary ion microscopic spectrum and by Raman experiments, and the embedded nitrogen ions can be regarded as defects. According to the experimental results, a mechanism is proposed based on one of the electrons in the completely filled d-orbits of Zn that compensates the dangling bonds of nitrogen ions and leads to a net spin of one-half in the Zn orbits. These one-half spins strongly correlate with localized electrons that are captured by defects to form ferromagnetism. Eventually, the magnetism of nitrogen embedded ZnO : N films could be described by a bound magnetic polaron model.

Influence of the substrate temperature on the electrical and magnetic properties of ZnO : N thin films grown by pulse laser deposition

We investigated the effect of the substrate temperature on the magnetic, electrical and optical properties of nitrogen-doped ZnO thin films. Experiments showed that a high substrate temperature depresses the deposition rate and produces much thinner films displaying more robust ferromagnetism. In addition, the resistivity decreased as the substrate temperature increased and all of the nitrogen-doped films, at different substrate temperatures, had larger gaps than the pure ZnO film.

Observation of bias-dependent low field positive magneto-resistance in Co-doped amorphous carbon films

We report a considerable intrinsic positive magnetoresistance (PMR) effect in Co-doped amorphous carbon films by radio frequency magnetron sputtering. The kind of PMR effect is bias dependence and its ratio reaches a peak at a particular voltage. At room temperature, the maximum PMR ratio is about 10% among these samples. The x-ray absorption spectroscopy and Raman spectra results support the appearance of the bias-dependent PMR effect strongly depends on the sp2 states and Co dopants. A phenomenological model related to orbital Zeeman splitting has been proposed to describe the resistance, which is controlled by voltage and magnetic field.

The role of carriers in spin current and magnetic coupling for ZnO:Co diluted magnetic oxides

The role of carriers in the electric conduction and magnetic coupling of diluted magnetic oxides is essential to the spin current formation. This study elucidates the conduction of electrons originating from oxygen vacancies and the magnetic coupling between major doped transition ions. The findings indicate that electrons may conduct in the conduction band or by hopping within discrete localized states. Furthermore, because d-orbital of doped transition ions overlap with these localized states, only hopping electrons contribute to magnetic coupling and spin current formation. Those electrons in the conduction band have no observable effect on magnetic coupling.

Electronic structure of Al-doped ZnO transparent conductive thin films studied by x-ray absorption and emission spectroscopies

This study used O K-, Zn L3-, Zn K-, and Al K-edges x-ray absorption near-edge structure (XANES) and O K-edge x-ray emission spectroscopy (XES) measurements to investigate the electronic structure of transparent Al-doped ZnO (AZO) thin film conductors. The samples were prepared on glass substrates at a low temperature near 77 K by using a standard RF sputtering method. High-purity Ne (5N) was used as the sputtering gas. The crystallography of AZO thin films gradually transformed from the ZnO wurtize structure to an amorphous structure during sample deposition, which suggests the suitability to grow on flexible substrates, eliminating the severe degradation due to fragmentation by repeated bending. The O K- and Zn L3-edges XANES spectra of AZO thin films revealed a decrease in the number of both O 2p and Zn 3d unoccupied states when the pressure of Ne was increased from 5 to 100 mTorr. In contrast, Al K-edges XANES spectra showed that the number of unoccupied states of Al 3p increased in conjunction with t...

The role of anomalous Hall effect in diluted magnetic semiconductors and oxides

A serious debate has arisen in the development of spintronics regarding contradictory findings on whether or not anomalous Hall effect (AHE) represents the spin polarization nature of carriers in diluted magnetic semiconductors(DMS) and oxides (DMO). Based on our results and on the common AHE characteristics of others reports, here we suggest that only those AHEs for DMSs or DMO which match quantitatively with the magnetic hysteresis loop and which follow the 1.6 scaling relation represent the spin polarization nature of carriers. However, these criteria cannot be used to determine the percentage of magnetic precipitation or of the spin polarized current.

Dynamical behaviour of 7-1 gold nanowire under different axial tensile strains

The phonon spectrum has in recent years attracted much interest in investigating electrical transfer in nanowires. In this study, molecular dynamics is employed to calculate the phonon property of thinness for 7-1 gold nanowire at room temperature. This study finds that the phonon density distribution of 7-1 nanowire is different from both that of bulk face-centred cubic (fcc) structure and that of nanoparticles. We also observed that the high frequency of the phonon spectrum in the bulk fcc structure has a higher intensity than the low frequency of the phonon spectrum. This phenomenon, however, does not appear in the 7-1 nanowire. The strain effect of 7-1 nanowire on the phonon property has also been studied; moreover, the phonon spectra of core–core and shell–shell structures of nanowire exhibit two different modes. The relationship between the phonon profile of these core and shell structures is analysed in this study.

UV-assisted deposition of ZnO nanorods

An enhanced preferred crystalline orientation in ZnO nanorods is achieved by applying ultraviolet (UV) illumination during sample deposition. The crystal field, generated from internal charge transfer between different energy bands by carrier excitations, drives crystalline growth in a preferred orientation. This work applies a novel crystalline engineering model, such that the preferred crystalline orientation in sample growth can be controlled by exciting carriers between assigned bands, which match the direction of crystal momentum between assigned bands via UV illumination.

Observation of the amorphous zinc oxide recrystalline process by molecular dynamics simulation

The detailed structural variations of amorphous zinc oxide (ZnO) as well as wurtzite (B4) and zinc blende (B3) crystal structures during the temperature elevation process were observed by molecular dynamics simulation. The amorphous ZnO structure was first predicted through the simulated-annealing basin-hopping algorithm with the criterion to search for the least stable structure. The density and X-ray diffraction profiles of amorphous ZnO of the structure were in agreement with previous reports. The local structural transformation among different local structures and the recrystalline process of amorphous ZnO at higher temperatures are observed and can explain the structural transformation and recrystalline mechanism in a corresponding experiment [Bruncko et al., Thin Solid Films 520, 866-870 (2011)].

Tunable magnetic order of Co nanoparticles and magnetotransport in Co/ZnO nanocomposites

We demonstrate tunable magnetic order of cobalt nanoparticles in Co∕ZnO nanocomposites. High-density electronic states in ZnO formed during high vacuum annealing help generate bound and free charge carriers, which in turn enable the stable magnetic ordering of Co nanoparticles in the Co∕ZnO nanocomposites in a tunable manner. This is demonstrated by the following experimental observations: (i) enhanced spontaneous magnetization and coercivity, (ii) transition from semiconducting to metallic electrical-transport, and (iii) transverse magnetotransport transition from negative magnetoresistance to the anomalous Hall effect. The work explores a route to manipulate the magnetic order of magnetic nanoparticles by means of intentionally generated defects in oxides.