Tezer Fırat

Electrical and optical properties of point defects in ZnO thin films

We show that the deposition of ZnO films under varying oxygen partial pressure and annealing conditions allows for the controllable formation of specific defects. Using x-ray diffraction and photoluminescence, we characterize the defects formed and show that these defects are responsible for changes in film carrier density, carrier type, sheet resistivity and mobility.

Synthesis of SiO2 coated NiFe2O4 nanoparticles and the effect of SiO2 shell thickness on the magnetic properties

In this work, the results of synthesis of core-shell NiFe2O4 nanoparticles and influence of silica coating on the magnetic properties of nanoparticles are presented. Spherical NiFe2O4 nanoparticles were prepared via a normal micelles process. NiFe2O4 nanoparticles homogeneously coated with SiO2 of various shell thickness were synthesized by reverse microemulsion. The interparticle spacing was varied by changing the amount of added tetraethylorthosilicate. The microstructures and morphologies of these nanoparticles were studied by x-ray diffraction and transmission electron microscopy techniques. The magnetic parameters such as saturation magnetizations, blocking temperatures, and magnetic anisotropies have been calculated from dc magnetization and ac susceptibility measurements.

Structural, Optic, and Magnetic Investigation of the Synthesized ZnO and Zn

Both ZnO and Zn0.99Co0.01O semiconductors were synthesized through solid state reaction via mechanical milling and thermal treatment. Initially the wurtzite ZnO structures of the synthesized particles were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Since these techniques were unable to identify both contamination atoms and Co distribution, energy dispersive X-ray spectrometry (EDS) was used. EDS showed a successful doping of Co atoms with the atomic ratio of 0.9 ± 0.1%, and also showed a contamination of tungsten (W) atoms, in the atomic ratio of 1.6 ± 0.2% for Zn0.99Co0.01O, and 1.3 ± 0.2% for ZnO. Substitutions of Co+2 ions with Zn+2 host atoms in the ZnO lattice were exposed through X-ray photo spectroscopy (XPS) data of Co 2p electronic energy levels. UV-vis absorption spectroscopy (UV-vis) was also used to prove Co substitutions in the ZnO lattice. This was revealed by a decrease in band gap from 3.25 ± 0.01 eV to 3.03 ± 0.01 eV, and the existence of newly permitted transitions between intra ionic d-d* levels. The ferromagnetic effect of Co doping in ZnO lattice was revealed by the coercivity of ~154±50 Oe and positive Curie-Weiss temperature, 79 ± 1 K. Beside ferromagnetic interactions, the calculated effective Bohr Magnetron (¿eff), 0.32±0.01 ¿B, suggested anti-ferromagnetic interactions due to be less than the theoretical spin based magnetic moment of Co2+ ions, 3.0 ¿B.

_{0.99}

Abstract DC magnetization measurements have been performed for Ni 79 Mn 21 alloys in the temperature range of 4.2 to 50 K. Taking the demagnetizing field into account, the influence of an external magnetic field, H ext , on the re-entrant spin-glass transition temperature T f of this alloy has been investigated. It has been observed that T f is independent of H ext if H ext is sma ller than the demagnetizing field H D ( = NM s ) where N is the demagnetizing factor; M s is the value of the saturation magnetization. However, for higher fields, T f is displaced towards lower temperatures. These results are interpreted in terms of anisotropy rotation based on the “domain-anisotropy” model.

Co

Influence of the hole concentration on transition temperature and on intergrain coupling were investigated in 110 K Bi superconductor. A.c. susceptibility of the sample has a two stage behavior due to phase coherence in the grains and between the grains. The measurements as a function of a.c. field amplitude show that the grain boundaries act like Josephson junctions with a depressed order parameter. The hole concentration was varied between 0.17 and 0.31 per CuO unit by heat treatment in the single phase samples at appropriate oxygen partial pressures. The dependences of intergrain (Tcj) and intragrain (Tc) diamagnetic transition temperatures on the hole concentration are similar to other p-type high-Tc compounds. Although Tc changes in a range of 2.5 K, Tcj is very sensitive to the hole concentration and varies in a range of 40 K. However, with increasing of the hole concentration Tcj approaches Tc until the maximum value of Tc is achieved, and then suddenly decreases at high levels of oxidation. These results indicate that the degree of the order parameter depression at the grain boundaries can be minimized in a sample which is prepared with a hole concentration corresponding to the maximum value of the intragrain transition temperature.

_{0.01}

The photocurrent (PC) spectrum of AgIn5S8 crystal consists of a single peak, which provides to determine the bandgap energy by applying the Moss rule. The temperature dependence of the bandgap energy was also calculated. The PC dramatically increased by pre-illumination with light having wavelength corresponding to the bandgap of AgIn5S8. The temperature-dependent PC of the sample was measured at different temperatures from 80 to 300 K and the PC spectrum consisted a single broad peak. Thermal quenching of the PC was observed to start at ∼105 K and the PC completely quenched at ∼180 K. The quenching mechanism was discussed in terms of the two-centre model. The height of the PC peak rised linearly with applied voltage up to 5.0 V under constant intensity of light. Similarly, the dark current–voltage characteristics consisted of a single region dominating an ohmic behaviour, and no space charge limited region was apparent at various temperatures up to 20 V.