Single-crystalline YIG nanoflakes with uniaxial in-plane anisotropy and diverse crystallographic orientations

We study Y3Fe5O12 (YIG) nanoflakes that we produce via mechanical cleaving and exfoliation of YIG single crystals. By characterizing their structural and magnetic properties, we find that these YIG nanoflakes have surfaces oriented along unusual crystallographic axes and uniaxial in-plane magnetic anisotropy due to their shape, both of which are not commonly available in YIG thin films. These physical properties, combined with the possibility of picking up the YIG nanoflakes and stacking them onto nanoflakes of other van der Waals materials or pre-patterned electrodes or waveguides, open unexplored possibilities for magnonics and for the realization of novel YIG-based heterostructures and devices.Comment: 13 pages, 4 figure

Suitability of ZnO for future spin-based applications

During the last decades development in semiconductor technology was based on the miniaturization of already existing devices. This approach will soon reach its limits since further miniaturization will effectively lead to the observation of quantum effects even at room temperature. Future technology demands newdevices which avail themselves of these effects. A property suitable as control mechanism is the spin of charge carriers rather than their charge itself since devices based on spin, e.g., consume less energy. The field of research which investigates the controlled manipulation of spins is called spintronics and has gained much attention during the last years. Spintronics does not only demand new devices, but also new classes of materials.This thesis is devoted to the investigation of (Zn,Mn)O, (Zn,Co)O and ZnO quantum dots as new materials for future spintronic applications.Since a few years researchers have tried to establish diluted magnetic semiconductors by doping ZnO with transition metal ions. First attempts by various research groups were promising. However, it soon became evident that the results were only poorly reproducible. Furthermore, the mechanism which should mediate ferromagnetic coupling in transition metal doped ZnO remains under debate up to now.In this thesis (Zn,Mn)O and (Zn,Co)O mixed oxides have been prepared by radio-frequency magnetron sputtering. Characterization of structural properties was accomplished by X-ray diffraction and various techniques based on transmission electron microscopy. The latter was carried out at the laboratory for transmission electron microscopy at the Karlsruhe Institute of Technology in the group of Prof. Dagmar Gerthsen. To some extent, both mixed oxides showed formation of secondary phases in the form of transition metal monoxides, depending on thepreparation conditions and the composition of the samples.Magnetic properties were charaterized by SQUID measurements and—in case of (Zn,Co)O—X-ray magnetic circular dichroism. Magnetic behavior of the (Zn,Co)O samples which did not show phase segregation was hardly reproducible. Both paramagnetism as well as weak ferromagnetism was observed for different samples prepared under the same conditions. The (Zn,Co)O samples containing a secondary phase of CoO were purely paramagnetic. Despite reports of ferromagnetism in CoO nanoclusters, this behavior was not observed for samples prepared in the course of this work. In contrast, strong ferromagnetic behavior was found for (Zn,Mn)O samples containing MnO which could be linked to the secondary phase.In addition to the mixed oxides, sputtered ZnO quantum dots have been fabricated and investigated. A preparation routine was established to produce ZnO quantum dots at room temperature. Preparation without additional heating is important with regard to future integration into devices, especially optical resonators which could be destroyed by the exposure to heat. In a first step, ZnO nanocrystals with a large size distribution of 1-16 nm were prepared which did not yet show quantum effects. Later a smaller size distribution of 2-5 nm and a blue-shift of the bandgap with respect to bulk ZnO was achieved. The quantum dots were embedded into optical resonators composed of two Bragg mirrors. Their optical properties were characterized by transmittance measurements

X-Ray Spectroscopic Investigations of Zn 0.94 Co0.06 O Thin Films

We investigated Zn0.94Co0.06O thin films on sapphire (0001) substrates with respect to their structural and magnetic properties. X-ray diffraction shows a axis oriented growth and no secondary phases within its resolution. A clear improvement of the crystalline quality was obtained by post annealing under vacuum conditions. Further information about the local electronic structure is obtained by X-ray absorption spectroscopy at the Co L2,3 and the O edge. Magnetic properties were investigated with a superconducting quantum interference device (SQUID) and by X-ray magnetic circular dichroism at the Co L edg2,3e. Both techniques yield mainly paramagnetic behavior of the samples. For low temperatures, an additional small ferromagnetic contribution was observed in SQUID measurements. Several possible origins of this ferromagnetic contribution are discussed

Organic Ligands Made Porous: Magnetic and Catalytic Properties of Transition Metals Coordinated to the Surfaces of Mesoporous Organosilica

Inorganic solids with porosity on the mesoscale possess a high internal surface area and a well-accessible pore system. Therefore, it is a relevant task to equip the surfaces of such materials with a maximum density of various organic functional groups. Among these functions it is the capability of coordinating to metal species as a ligand that is of extraordinary importance in many areas, for example, in catalysis. This paper describes how prominent ligands containing donor functions such as carboxylic, thio, chelating, or amine groups can be obtained in the form of nanoporous organosilica materials. The coordination of metal centers such as CoII, MnII, VIV, or PtIV is studied in detail. The magnetic properties of the corresponding materials and some applications in catalysis are reported. A quantitative determination of the surface density of donor atoms by distance measurements using EPR spectroscopy is shown

The structure and optical properties of ZnO nanocrystals embedded in SiO2 fabricated by radio-frequency sputtering

Zinc oxide (ZnO) nanocrystals (NCs) with high crystalline quality were prepared via radio-frequency magnetron sputtering as a SiO2/ZnO/SiO2 trilayer on Si(100) and Al2O3(0001) substrates with an intermediate in situ annealing step. Transmission electron microscopy reveals a uniform dispersion of ZnO NCs in the amorphous SiO2 matrix with typical sizes up to 16 nm with a larger fraction of smaller crystals. The size distribution analysis yields a mean grain size of 5 nm for small particles. Individual ZnO NCs show a well-defined hexagonal close packed wurtzite structure and lattice parameters close to those of bulk ZnO, confirming their high crystalline quality. Mapping of the Zn distribution by means of energy-filtered ransmission electron microscopy reveals a strongly non-uniform distribution of Zn within the SiO2 matrix, corroborating the chemical separation of ZnO NCs from surrounding SiO2. Optical transmittance measurements confirm the findings of the electron microscopy analysis. The fabrication technique described opens up new possibilities in the preparation of ZnO NCs with high crystalline quality, including growth in monolithic optical cavities without intermediate ex situ fabrication steps

Ultraviolet photoluminescence of ZnO quantum dots sputtered at room-temperature

We observe ultraviolet photoluminescence from sputtered ZnO quantum dots which are fabricated with no annealing steps. The nanocrystals are embedded in amorphous SiO2 and exhibit a narrow size distribution of 3.5 ± 0.6 nm. Photoluminescence and transmittance measurements show a shift of ultraviolet emission and absorption of the dots compared to bulk ZnO material. This work paves the way for cheap nanooptical devices in the ultraviolet which are fabricated in a single sputtering run

Defect-induced ferromagnetism in Co-doped ZnO thin films

We present a study on the structural, magnetic, and optical properties, as well as the electronic structure of Co-doped ZnO films prepared by magnetron sputtering.Magnetization measurements performed at different temperatures indicate ferromagnetic and paramagnetic behavior for the samples prepared at oxygen-poor conditions whereas the samples prepared at oxygen-rich conditions show only paramagnetic behavior corroborating that the presence of oxygen-related defects is essential for ferromagnetism in Zn1−xCoxO. Xray absorption spectroscopy (XAS) at the Co L2,3 edge together with optical transmittance measurements show that Co ions are present in the high-spin Co2+ (d7) state under tetrahedral symmetry indicating a proper incorporation in the ZnO host lattice. Comparison of the O K edge XAS spectra of the samples prepared at different conditions show substantial changes in the spectral line shape which are attributed to the presence of lattice defects such as oxygen vacancies in the ferromagnetic oxygen-poor Co-doped ZnO samples. Our findings indicate that the ferromagnetic properties of Co-doped ZnO samples are strongly correlated with the presence of oxygen vacancies in the ZnO lattice supporting the spin-split impurity band model

Spin-on Spintronics : Ultrafast Electron Spin Dynamics in ZnO and Zn1-xCoxO Sol-Gel Films

We use time-resolved Faraday rotation spectroscopy to probe the electron spin dynamics in ZnO and magnetically doped Zn1-xCoxO sol-gel thin films. In undoped ZnO, we observe an anomalous temperature dependence of the ensemble spin dephasing time T2*, i.e., longer coherence times at higher temperatures, reaching T2* ∼ 1.2 ns at room temperature. Time-resolved transmission measurements suggest that this effect arises from hole trapping at grain surfaces. Deliberate addition of Co2+ to ZnO increases the effective electron Landé g factor, providing the first direct determination of the mean-field electron-Co2+ exchange energy in Zn1-xCoxO (N0alpha = +0.25 (0.02 eV). In Zn1-xCoxO, T2* also increases with increasing temperature,allowing spin precession to be observed even at room temperature

Personal identity An animalist response to Parfit's revisionism

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