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Eugenie Kirk

Publications and source records attributed to Eugenie Kirk.

10 recordsLinked to original sources
arXiv2020

Soft x-ray absorption of thin films detected using substrate luminescence: A performance analysis

X-ray absorption spectroscopy of thin films is central to a broad range of scientific fields, and is typically detected using indirect techniques. X-ray excited optical luminescence (XEOL) from the sample's substrate is one such detection method, in which the luminescence signal acts as an effective transmission measurement through the film. This detection method has several advantages that make it versatile compared to others, in particular for insulating samples or when a probing depth larger than 10nm is required. In this work we present a systematic performance analysis of this method with the aim of providing guidelines for its advantages and pitfalls, enabling a wider use of this method by the thin film community. We compare and quantify the efficiency of XEOL from a range of commonly used substrates. Our measurements demonstrate the equivalence between XEOL and x-ray transmission measurements for thin films. Moreover, we show the applicability of XEOL to magnetic studies by employing XMCD sum rules with XEOL-generated data. Lastly, we demonstrate that above a certain thickness XEOL shows a saturation-like effect, which can be modelled and corrected for.

physics.app-ph↗
arXiv2018

Exploiting atomic layer deposition for fabricating sub-10 nm X-ray lenses

Moving towards significantly smaller nanostructures, direct structuring techniques such as electron beam lithography approach fundamental limitations in feature size and aspect ratios. Application of nanostructures like diffractive X-ray lenses requires feature sizes of below 10 nm to enter a new regime in high resolution X-raymicroscopy. As such dimensions are difficult to obtain using conventional electron beam lithography, we pursue a line-doubling approach. We demonstrate that thismethod yields structure sizes as small as 6.4 nm. X-ray lenses fabricated in this way are tested for their efficiency and microscopic resolution. In addition, the line-doubling technique is successfully extended to a six-fold scheme, where each line in a template structure written by electron beam lithography evolves into six metal lines.

physics.app-ph↗
arXiv2017

Control of the gyration dynamics of magnetic vortices by the magnetoelastic effect

The influence of a strain-induced uniaxial magnetoelastic anisotropy on the magnetic vortex core dynamics in microstructured magnetostrictive Co$_{40}$Fe$_{40}$B$_{20}$ elements was investigated with time-resolved scanning transmission x-ray microscopy. The measurements revealed a monotonically decreasing eigenfrequency of the vortex core gyration with the increasing magnetoelastic anisotropy, which follows closely the predictions from micromagnetic modeling.

cond-mat.mes-hall↗
arXiv2016

Switching field distribution of exchange coupled ferri-/ferromagnetic composite bit patterned media

We investigate the switching field distribution and the resulting bit error rate of exchange coupled ferri-/ferromagnetic bilayer island arrays by micromagnetic simulations. Using islands with varying microstructure and anisotropic properties, the intrinsic switching field distribution is computed. The dipolar contribution to the switching field distribution is obtained separately by using a model of a triangular patterned island array resembling $1.4\,\mathrm{Tb/in}^2$ bit patterned media. Both contributions are computed for different thickness of the soft exchange coupled ferrimagnet and also for ferromagnetic single phase FePt islands. A bit patterned media with a bilayer structure of FeGd($5\,\mathrm{nm}$)/FePt($5\,\mathrm{nm}$) shows a bit error rate of $10^{-4}$ with a write field of $1.16\,\mathrm{T}$.

physics.comp-ph↗
arXiv2016

Micromagnetic simulation of exchange coupled ferri-/ferromagnetic composite in bit patterned media

Ferri-/ferromagnetic exchange coupled composites are promising candidates for bit patterned media because of the ability to control the magnetic properties of the ferrimagnet by its composition. A micromagnetic model for the bilayer system is presented where we also incorporate the microstructural features of both layers. Micromagnetic finite element simulations are performed to investigate the magnetization reversal behaviour of such media. By adding the exchange coupled ferrimagnet to the ferromagnet, the switching field could be reduced by up to $40\,\%$ and also the switching field distribution is narrowed. To reach these significant improvements, an interface exchange coupling strength of $2\,\mathrm{mJ/m^2}$ is required.

physics.comp-ph↗
arXiv2006

Imaging of single infrared, optical, and ultraviolet photons using distributed tunnel junction readout on superconducting absorbers

Single-photon imaging spectrometers of high quantum efficiency in the infrared to ultraviolet wavelength range, with good timing resolution and with a vanishing dark count rate are on top of the wish list in earth-bound astronomy, material and medical sciences, or quantum information technologies. We review and present improved operation of a cryogenic detector system potentially offering all these qualities. It is based on a superconducting absorber strip read out with superconducting tunnel junctions. The detector performance is discussed in terms of responsivity, noise properties, energy and position resolution. Dynamic processes involved in the signal creation and detection are investigated for a basic understanding of the physics, and for possible application-specific modifications of device characteristics.

physics.ins-det↗
arXiv2005

Distributed readout detectors using superconducting tunnel junctions

Superconducting tunnel junctions (STJs) are able to measure the energy of single photons in the range from near infrared to X-rays. They provide simultaneous information of the impact time and wavelength of an absorbed photon. The main difficulty of these detectors compared with conventional imaging detectors lies in their limited pixel number. Each STJ has to be connected independently and therefore the wiring becomes technologically more demanding as the number of STJs increases. One approach to solving this problem is to use a single large absorber and to distribute STJs for position sensitive signal readout. This configuration is able to detect single optical photons with an energy resolution close to that of a single STJ pixel. We have produced a Ta absorber strip with Ta/Al/AlOx/Al/Nb/Ta junctions at either end. The energy and position of single photons were measured simultaneously. The energy resolving power approaches the theoretical limit. We will present a simple Monte Carlo simulation which reproduces the measurement exactly.

physics.ins-det↗
arXiv2005

Self-cooling cryogenic microcalorimeters made of SINIS junctions

High quality low leakage SINIS devices made of Al-AlMn-Al layers were fabricated for energy dispersive single photon detection. Information on different heat flow channels was extracted from the measured dynamics of detector signals due to X-ray events. At the optimum operation point, the extracted effective electron temperature decreased from 88 mK down to 43 mK due to self-cooling, roughly doubling the detector sensitivity.

physics.ins-det↗
arXiv2005

New frequency-modulation readout based on relaxation oscillations

Scaling of multi-pixel cryogenic detectors for imaging becomes increasingly difficult with size due to complexity of readout circuitry and cryogenic constraints (thermal load from wiring). We propose and demonstrate a new readout scheme based on a highly stable RF oscillator composed of a superconducting tunnel junction which exhibits relaxation oscillations. The oscillation frequency is almost linear with the analog bias signal over a wide operation range. The frequency signals from different detectors can be combined into one single readout line. The current noise of an optimized circuit is about 5 pA/sqrt(Hz), which is comparable to standard SQUID amplifiers. We show experimental data from `stand-alone' operation as well as response to microcalorimeter X-ray signals.

physics.ins-det↗