Electron Transport and Magnetism of EuN, GdN and DyN

Abstract

This thesis reports the growth, electrical transport and magnetism of europium nitride and the electrical transport properties of gadolinium nitride and dysprosium nitride. EuN films were grown by thermal evaporation in an ultra-high vacuum system on pre-deposited AlN or GaN layers on sapphire and silicon substrates at three different growth temperatures; room temperature, 680°C and 800°C. The structural and compositional properties of the films were investigated by different techniques. Films grown at room temperature are polycrystalline while films fabricated at higher temperature (680°C and 800°C) are epitaxial. The main charge state of europium in EuN is 3+ but there is still an existence of Eu²+ ions in the films due to the presence of nitrogen vacancies (VN), the concentration of which is strongly dependent on the growth conditions. The existence of Eu²+ ions causes EuN to have interesting magnetic properties. The 680°C grown films with a few percent of Eu²+ ions have a van Vleck paramagnetic signal corresponding to Eu³+ ions and a Brillouin paramagnetic response at the lowest temperature from the small amount of Eu²+. More surprisingly, the films grown at room temperature have a much higher concentration of Eu²+ ions and these show a ferromagnetic phase, for which the Curie temperature and saturation moment can approach more than 100K and 1 μB/Eu, respectively. The 800°C grown samples show only a paramagnetic phase. The temperature dependent Hall effect and resistivity of the EuN films grown at 680°C and room temperature show that the films are doped semiconductors. The small amount of Eu²+ ions in 680°C grown films not only acts as local moments for interaction with conduction electrons through the Kondo effect but also causes a negative magnetoresistance due to the suppression of spin flip scattering by an applied field. The influence of the magnetic state on the electron transport of the ferromagnetic films grown at room temperature can be seen through the anomalous Hall effect and the negative magnetoresistance. The films grown at 800°C and high growth rate (0.2nm/s) exhibit metallic behaviour and have a linear and positive magnetoresistance. The experimental results demonstrate the electrical and magnetic properties of EuN to be strongly dependent on the growth conditions and show the potential to fabricate ferromagnetic EuN by the dilution of Eu²+ ions through the control of nitrogen vacancies. Such ferromagnetic EuN films represent a new class of dilute magnetic semiconductor. The transport properties of the rare earth nitrides GdN and DyN were analysed through measurements of the Hall effect, resistivity and magnetoresistance. The results show the films are moderately doped semiconductors for which the Curie temperature of GdN and DyN is about 65K and 22-25K, respectively. Both films have negative magnetoresistance with the strongest value around the Curie temperature TC. The magnetoresistance is interpreted in terms of contributions from magnetic scattering and band structure effects.