Gyromagnetic ratio | EPFL Graph Search

In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic moment to its angular momentum, and it is often denoted by the symbol γ, gamma. Its SI unit is the radian per second per tesla (rad⋅s−1⋅T−1) or, equivalently, the coulomb per kilogram (C⋅kg−1). The term "gyromagnetic ratio" is often used as a synonym for a different but closely related quantity, the g-factor. The g-factor only differs from the gyromagnetic ratio in being dimensionless. For a classical rotating body Consider a nonconductive charged body rotating about an axis of symmetry. According to the laws of classical physics, it has both a magnetic dipole moment due to the movement of charge and an angular momentum due to the movement of mass arising from its rotation. It can be shown that as long as its charge and mass density and flow are distributed identi

Synthesis, structural and magnetic characterization of thin films of the chiral magnets CoZnMn and FeGe

Skyrmions are topologically protected nanometer-sized magnetic vortices that have sparked interest for future spintronics applications aiming at high-density, high-speed and low-power consuming magnetic memory devices. One type of skyrmions appears in non-centrosymmetric materials with spin-orbit coupling. Bulk materials such as FeGe and CoZnMn have been in the focus of recent research as they host skyrmion lattices near or above room temperature. The interest in thin-film materials has immensely increased as they promise better control of skyrmions. The main challenges are the synthesis of skyrmion-hosting materials with high critical temperatures and the compatibility with Si-based thin-film technologies. Currently most of the studied thin films do not exhibit the relevant properties. It has been argued that strain and defects related to the lattice mismatch inhibit the skyrmion phase.Here we present novel approaches to synthesize skyrmion-hosting thin films based on FeGe and CoZnMn: 1) the crystallization of an amorphous thin film on a vdW substrate (commercial graphene), 2) van der Waals (vdW) growth on graphene as a substrate to achieve a strain- and defect-free growth of the overlayer.Sputtered FeGe layers were obtained on graphene on oxidized silicon substrates and annealed via rapid thermal (RTA) and flash lamp annealing (FLA). The goal was to use these annealing techniques to guide the material to the temperature/composition window of the cubic phase. For this, we varied the thin film thickness, Fe:Ge ratio, pre-heating and annealing temperature. The thin film temperature during the FLA was studied through heat transfer simulations. The experiments uncovered the long annealing time via RTA with a capping layer leads to 4 types of morphology: islands, discs, individual crystals and crystals on discs. The sub-second annealing via FLA led to the polycrystalline thin film formation. The obtained materials contain hexagonal and cubic FeGe, Fe-rich phases, pure Ge and Fe oxides analysed by Raman spectroscopy and X-ray diffraction. We provide Raman vibrational modes and the symmetry description from first-principles calculations for FeGex with x=1, 1.5 and 2 to fast and locally characterize thin films and microstructures.Molecular beam epitaxy was used to grow CoZnMn thin films of different stoichiometries on graphene on oxidized Si. It has been found in bulk crystals that the critical temperature can be systematically engineered beyond room temperature by tuning the composition. We report on the structural, compositional and magnetic properties of Co10-xZn10-yZnx+y (0

EPFL2021

A method for the reliable and reproducible precipitation of phase pure high Ca/Si ratio (>1.5) synthetic calcium silicate hydrates (C-S-H)

Paul Bowen, Maya Nicole Harris, Karen Scrivener, Grace Simpson

Various methods have been developed to synthesize C-S-H; however, these methods have only allowed for the successful production of pure-phase C-S-H with Ca:Si of 1.5 or less. Attempts to form higher Ca:Si ratios by these methods gives C-S-H (Ca:Si 1.5 or less) in the presence of Ca(OH)(2). This paper focuses on a reliable method to produce single-phase C-S-H with Ca:Si molar ratios between 1 and 2. Reactant concentrations, temperature, pH, and mixing conditions are carefully controlled to ensure homogeneous precipitation of C-S-H. Guidelines on collection, drying, storage, and handling are also described.

PERGAMON-ELSEVIER SCIENCE LTD2022

Improving Sensitivity in Solid State NMR

Sensitivity in Nuclear Magnetic Resonance (NMR), especially in solid-state NMR, has always been a challenging and important issue and thus a motivation for new developments. The magnetic field B0, the gyromagnetic ratios of the observed nuclei, as well as the preparation of the samples are important for optimizing the sensitivity. Often only a small amount of sample is available and the nuclei of interest have low gyromagnetic ratios, which makes solid-state NMR extremely time consuming and, in some cases, practically impossible. In addition, strong interactions can cause extensive line-broadening, thus making it difficult to excite or detect high resolution spectra. In this thesis two main aspects concerning sensitivity in solid-state NMR are addressed. The first part reveals the difficulties of the excitation and detection of quadrupolar nuclei, in particular of the highly abundant nucleus nitrogen-14, and presents an indirect detection method combined with a more efficient excitation and reconversion scheme using Delays Alternating with Nutations for Tailored Excitation (DANTE). The second part presents a new experimental set-up for Dynamic Nuclear Polarization (DNP), which has evolved into a very successful method at the Ecole Polytechnique Fédérale de Lausanne. It is shown in this work that the DNP method combined with solid-state NMR allows one to enhance NMR signal intensities by one or two orders of magnitude, thus opening the doors towards a great variety of new exciting applications. In solids spinning at the magic angle, the indirect detection of single-quantum (SQ) and double-quantum (DQ) 14N spectra (I = 1) via spy nuclei S = 1/2 such as protons can be achieved in the manner of heteronuclear single- or multiple-quantum correlation (HSQC or HMQC) spectroscopy. The coherence transfer is achieved by recoupling the heteronuclear dipolar interactions, i.e., with symmetry-based sequences. The line broadening caused by the homogeneous decay of the transverse terms of the spy nuclei S can be reduced by implementing suitable dipolar decoupling schemes during the evolution period. The lack of efficiency of the excitation and reconversion can be overcome by replacing the conventional rectangular pulses applied to the nitrogen-14 nuclei by a train of short rotor-synchronized pulses in the manner of DANTE. These pulse trains excite a large number of crystallites in the sample uniformly and allow one to efficiently excite magnetization over large bandwidths, limited only by the quality factor of the probe. When combining N interleaved DANTE schemes, obtained by applying N pulses per rotor period, leading to so-called "DANTE-N" sequences, one obtains methods that greatly improve the uniformity of the excitation and lead to higher signal intensity in less rotor periods than basic DANTE. Applications to nitrogen-14 with fast magic angle spinning (typically νrot ≥ 60 kHz), using either direct or indirect detection, are backed up by simulations that provide insight into the properties of basic and interleaved DANTE sequences. DNP exploits the polarization transfer, driven by microwave irradiation, from unpaired electrons, usually carried by polarization agents such as TEMPO or TOTAPOL, to the target nuclei. This transfer results in enhancements of the signal-to-noise ratios and thus opens many new possibilities for challenging samples. In this thesis the experimental requirements are demonstrated and important parameters like the temperature dependence are investigated. Various sample preparation methods are proposed, e.g., by attaching the radical directly to the molecules under investigation, instead of preparing a solution that should forma glass upon freezing. The combination of indirect detection of 14N with DNP improves the signal-to-noise ratio dramatically and shortens the recovery delay between subsequent experiments. Besides samples of biological interest, it was also shown that the NMR signals of molecules on surfaces of various materials can be greatly enhanced by DNP. A characterization of the distribution of surface binding modes and interactions in a series of functionalized materials could be demonstrated. The bonding topology of functional groups in materials obtained via a sol-gel process and in materials prepared by post-grafting reactions could be identified and compared. Furthermore, the remarkable gain in time afforded by DNP allows the facile acquisition of two-dimensional correlation spectra. The surfaces were characterized by transferring the polarization from protons to other nuclei such as 13C, 29Si and 27Al by means of cross-polarization.

EPFL2012