Gallium nitride is a potentially interesting material system for Spintronics since it is expected to become ferromagnetic with a Curie-temperature above room temperature if doped with manganese and long spin relaxation times are detected in this material.

Ferromagnetism (FM) in GaMnN was found to be related to the position of the Fermi level relative to the Mn deep-level band. The location of the Fermi level can be altered by doping GaMnN with Si (n-type) and Mg (p-type) dopants, resulting in a ferromagnetic to paramagnetic transformation. We have found that this manipulation of the Fermi level also can affect the optical properties of GaMnN, GaMnN: Mg and GaMnN: Si. The major optical absorption bands were found either to be enhanced or diminished depending on the level and nature of the doping elements. Therefore, a correlation between optical properties and saturation magnetization of GaMnN can be established.

Systematic studies on the epitaxial growth and the effect of n-type (Al,Ga) doping on the magnetic and electrical properties of 2.0 % Cu doped ZnO dilute magnetic semiconducting thin films deposited on c-plane sapphire single crystals by pulsed laser deposition are reported here. An decrease in more than 3 orders of magnitude in resistivity from 2×101 Ohm cm for the 2.0 % Cu doped ZnO to ~5×10-3 Ohm cm for Al and Ga codoped films is observed. This increase in conductivity does not show any effect on ferromagnetic ordering thus contradicting the claim of free carrier mediated exchange as being responsible for ferromagnetic ordering in these DMS systems. A bound magnetic polaron or F-center mediated exchange is a possible explanation for the origin of ferromagnetism in these ZnO based DMS thin films.

We studied electrical transport in dilute magnetic semiconductors, which is determined by scattering of free carriers by localized magnetic moments. In our calculations of the scattering time and the mobility of the majority and minority-spin carriers we took into account both the effects of thermal spin fluctuations and of built-in spatial disorder of the magnetic atoms. These effects are responsible for the magnetic-field dependence of the mobility of the charge carriers. The application of the external magnetic field suppresses the thermodynamic spin fluctuations thus increasing the mobility. Simultaneously, depending on the type of the carriers and on parameters of the impurity potential, scattering by built-in spatial fluctuations of the atomic spins increases or decreases with the magnetic field. The latter effect is due to the change in the magnitude of the random local Zeeman splitting with the magnetic field. We discuss the role of the above effects on mobility and magnetoresistance of semiconductors where magnetic impurities are electrically active or neutral.

In this work, we carry out structural analysis of ferromagnetic Mn-doped ZnO thin films deposited by radio frequency magnetron sputtering, using transmission electron microscopy (TEM) and high resolution x-ray diffraction. On top of sapphire (0001) substrates, Mn rich precipitates and an interface reaction layer are observed following the deposition of Zn(Mn)O layers above 500°C. The crystalline quality of ZnO layers deposited by magnetron sputtering is highly improved at 500°C as well as the measured ferromagnetic response.

Gallium nitride powders and zinc oxide powders were each calcined with a few weight percent of copper oxide and/or magnesium oxide either in air or N2. Powder X-ray diffractometry, transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy were performed in order to observe calcination induced structural effects on these wurtzite type semiconductors. We note that our earlier magnetic results on Cu doped GaN are qualitatively consistent with recent first principle calculations [Wu et al., Appl. Phys. Lett. 89 (2006) 62505].

ZnO is a very promising material for spintronics applications, with many groups reporting room temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during PLD, we find an inverse correlation between magnetization and electron density as controlled by Sn doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for the ferromagnetism include the bound magnetic polaron model or exchange is mediated by carriers in a spin-spilt impurity band derived from extended donor orbitals. Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors.

Since the observation of room-temperature ferromagnetism (RTFM) in Co-doped anatase TiO2 [1], there have been many reports on the study of the magnetic properties of Co-doped TiO2 prepared by various methods with diversified results. The origin of the RTFM in these systems is still a topic of controversy today. In this work, TiO2 thin films were prepared by RF sputtering onto thermally grown oxide layers on Si substrates. Cobalt implantation was performed using a metal vapor vacuum arc (MEVVA) ion source to various doses ranging from 3×1015 cm-2 to 4×1016 cm-2. Post-implantation annealing was performed in a vacuum chamber at various temperatures ranging from 400°C to 700°C for 2 hours and 4 hours. Characterization of these films as-implanted and after thermal annealing under various conditions was performed using Rutherford backscattering spectrometry, energy filtered and high-resolution transmission electron microscopy, x-ray diffractometry, x-ray photoelectron spectroscopy, and vibrating sample magnetometry. The dependence of the magnetic properties on the implantation and annealing conditions were studied in detail. Clear RTFM properties were observed. The saturation magnetic moment per implanted Co atom (MS) seems to increase with increasing dose within the implantation dose range in this study. At a fixed dose, the MS value also shows a generally increasing trend with increasing annealing temperature and annealing time. Quite a number of samples showed MS values exceeding the bulk Co value of 1.69 ìB/Co significantly and the maximum MS value observed is about 3.16 µB/Co. Such high MS values indicate that the RTFM must not come from Co clusters alone. Possible origins of the RTFM properties will be discussed in conjunction with the structural properties.