A Magnon Scattering Platform

Scattering experiments have revolutionized our understanding of nature. Examples include the discovery of the nucleus, crystallography, and the discovery of the double helix structure of DNA. Scattering techniques differ by the type of the particles used, the interaction these particles have with target materials and the range of wavelengths used. Here, we demonstrate a new 2-dimensional table-top scattering platform for exploring magnetic properties of materials on mesoscopic length scales. Long lived, coherent magnonic excitations are generated in a thin film of YIG and scattered off a magnetic target deposited on its surface. The scattered waves are then recorded using a scanning NV center magnetometer that allows sub-wavelength imaging and operation under conditions ranging from cryogenic to ambient environment. While most scattering platforms measure only the intensity of the scattered waves, our imaging method allows for spatial determination of both amplitude and phase of the scattered waves thereby allowing for a systematic reconstruction of the target scattering potential. Our experimental results are consistent with theoretical predictions for such a geometry and reveal several unusual features of the magnetic response of the target, including suppression near the target edges and gradient in the direction perpendicular to the direction of surface wave propagation. Our results establish magnon scattering experiments as a new platform for studying correlated many-body systems

Defect-Related Magnetic and Electronic Properties of Graphene

A fundamental study of the electronic and magnetic properties of graphene modified by defects is presented. This work includes both theoretical and experimental investigations of graphene, graphene-metal composites and related structures, together with edge effects. The theoretical model employed for the description of p-electrons in graphene is based on the tight-binding Hamiltonian. On the experimental side we place special emphasis on the electron spin resonance technique (ESR). After describing the theoretical and experimental methods, we first investigate the origin of paramagnetism in graphene nanoribbons (GNRs) using a combination of ESR and other characterisation techniques, corroborated by a theoretical model. We find two paramagnetic species in our GNRs related to structural defects. Subsequently, interactions between magnetic species, introduced in GNRs as impurities are investigated. These are RKKY-type indirect exchange interactions involving the p-electrons in graphene. The influence of zigzag edges on the RKKY interaction is looked at in detail by using a Green\u27s function method for the bulk and edge modes of GNRs. We also study the influence of metal nanoparticles on the electronic properties of graphene thin films. Specifically, we focus on copper nanoparticles (Cu-NPs), which are expected to form weak bonds with graphene preserving its electronic properties. We introduce a doping mechanism for this system related to the electron-hole symmetry breaking of the electronic band in graphene when its surface is decorated with Cu-NPs. Additional discussion on the paramagnetic nature of other materials, including Au25+ molecular nanoclusters and defects in silicon epilayers are provided. These investigations focus on the ESR techniques and relevant theoretical models for interpreting the data

Physical properties of nanostructures induced by irradiation in diamond

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in ful llment of the requirements for the degree of Doctor of Philosophy in Physics. Johannesburg, 10 August 2017.We investigate the interaction of slow highly charged ions (SHCIs) with insulating type-Ib diamond (111) surfaces. Bismuth and Xenon SHCI beams produced using an Electron Beam Ion Trap (EBIT) and an Electron Cyclotron Resonance source (ECR) respectively, are accelerated onto type Ib diamond (111) surfaces with impact velocities up to 0.4 Bohr. SHCIs with charge states corresponding to potential energies between 4.5 keV and 110 keV are produced for this purpose. Atomic Force Microscopy analysis (AFM) of the diamond surfaces following SHCI impact reveals surface morphological modi cations characterized as nanoscale craters (nano-craters). To interpret the results from Tapping Mode AFM analysis of the irradiated diamond surfaces we discuss the interplay between kinetic and potential energy in nanocrater formation using empirical data together with Stopping and Range of Ions in Matter (SRIM) Monte Carlo Simulations. In the case of irradiation induced magnetic e ects in diamond, we investigate the magnetic properties of ultra-pure type-IIa diamond following irradiation with proton beams of 1-2 MeV energy. SQUID magnetometry of proton irradiated non-annealed diamond indicates formation of Curie type paramagnetism according to the Curie law. Raman and Photoluminescence spectroscopy measurements show that the primary structural features created by proton irradiation are the centers: GR1, ND1, TR12 and 3H. The Stopping and Range of Ions in Matter (SRIM) Monte Carlo simulations together with ii iii SQUID observations show a strong correlation between vacancy production, proton uence and the paramagnetic factor. At an average surface vacancy spacing of 1-1.6 nm and bulk (peak) vacancy spacing of 0.3-0.5 nm Curie paramagnetism is induced by formation of ND1 centres with an e ective magnetic moment eff (0.1-0.2) B. Post annealing SQUID analysis of proton irradiated diamond shows formation temperature independent magnetism with magnetic moment 6-7 emu superimposed to Curie-type paramagnetism. The response of ultra-pure type-IIa single crystal CVD diamond following 2.2 MeV proton micro-irradiation is further investigated using Atomic Force, Magnetic Force and Electrostatic Force Microscopy (AFM, MFM and EFM) under ambient conditions. Analysis of the phase shift signals using probe polarization dependent magnetization measurements and comparison of the MFM and EFM signals at zero electrical bias, show that measured force gradients originate from a radiation induced magnetic response in the micro-irradiated regions in diamond.LG201

Magnetic properties of nitrogen- doped carbon nanospheres

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science Johannesburg September 2012Electron spin resonance (ESR) was used to characterize a suite of carbon nanospheres (CNS) samples with varying nitrogen concentrations at room temperature. The CNS were produced using two different reactors (vertical and horizontal) under different preparatory conditions. Resonance spectra of samples produced from the vertical reactor showed resonance lines- a narrow paramagnetic component, and broader component. They were attributed to nitrogen paramagnetic impurities and carrier spins, respectively. Samples produced in the horizontal reactor revealed stronger line spectra that were narrower and Dysonian in shape. The nitrogen content of the samples produced by the horizontal reactor was determined through ESR analysis which involves integration of the resonance peak, and normalizing to the mass of the sample. The relative g-shift was also measured by using a DPPH reference sample. Room temperature power saturation experiments were performed on samples produced from the horizontal reactor with the aim of estimating the spin relaxation times. Two samples from the horizontal reactor were further investigated at low temperatures (4 K- 320 K) at a constant microwave power. The resonance parameters investigated were linewidth, asymmetry ratio and amplitude, and possible spin-lattice relaxation mechanisms were investigated. The variation of the amplitude with temperature was investigated using two models: (1) a model based on lattice vibrations, and (2) a model based on nanographites assembly (considered interaction between carrier and localized spins). At low temperatures both models have amplitude that changes inversely with temperature in accordance with Curie law. At high temperatures (T > 200 K) a model based on nanographites assembly provide an alternative; it describes the rise in the signal amplitude in terms of thermally activated paramagnetic electrons from non-magnetic ground state to excited state at energy . Analysis of linewidth and asymmetry ratio data confirmed that the spin-lattice relaxation governed by thermal activated electrons is a dominant relaxation mechanism at high temperatures