Ferromagnetism in oxide and dichalcogenide based diluted magnetic semiconductors

Abstract

Over the past decades, the world has witnessed the rapid development of semiconductor industry according to Moore's law. However, the pace of advancement has slowed in recent years and is already nearing its end due to the limits of miniaturization. The industry is in urgent need for new materials and device designs, for example, spintronics. Spintronics is a multifunctional device that utilizes spin degrees of freedom of electrons. The key issue in spintronics is the manipulation of spins. Diluted magnetic semiconductors are able to combine both semiconducting and magnetic properties into spintronics. Room-temperature ferromagnetism is expected in transition metal oxides like TiO2 and transition metal dichalcogenides such as MoS2 and WS2, which are the subjects of this dissertation. In the first part of this dissertation, 5 at% Co-doped TiO2 thin films were grown on LaAlO3 and SrTiO3 substrates by pulsed laser deposition. The growth mechanism and magnetic properties of the thin films were investigated in detail. It was found that although only anatase phase was observed in X-ray diffraction, the epitaxial films can be either fully strained or fully relaxed based on the choice of substrates. The Co dopants tend to segregate near the interface, but the size and distribution of the nanoclusters depend on the deposition parameters. Higher temperatures and slow growth rate lead to bigger cluster size and greater distance to the interface. The magnetic moments can reach 3.5 μB/Co at room temperature, supported by the first-principles calculations considering spin-orbit coupling. Polarised neutron reflectometry measurement reveals that Co clusters and possible surrounding defects are the main magnetic sources. The second part of this dissertation focuses on defect engineering in WS2 and MoS2. Both WS2 annealed in reducing gas and Nd-doped MoS2 show enhanced room-temperature ferromagnetism. The magnetic properties of the annealed WS2 powders are governed by the sulfur vacancies, which significantly increase in density with increased annealing temperature. In the Nd-doped MoS2 crystals, the exchange coupling of Nd3+ ions and sulfur vacancies introduced by ion implantation can form magnetic polarons that determine the magnetic properties.