110 research outputs found
NanoSQUIDs for Studies on the Magnetization Reversal of Individual Magnetic Nanoparticles
The subject of this thesis is the development, characterization and optimization of nanometer-sized superconducting quantum interference devices (nanoSQUIDs) for operation at cryogenic temperatures. This task is motivated by the need for convenient detectors for the investigation of individual magnetic nanoparticles, nanotubes, nanowires or molecular magnets. Two types of devices are considered in this thesis: (a) nanoSQUIDs based on Nb as a superconductor with Josephson junctions having normal metal HfTi barriers and (b) nanoSQUIDs based on Yttrium barium copper oxid (YBCO) as a superconductor with grain boundary Josephson junctions. The nanoSQUIDs have been investigated in terms of sensitivity to magnetic flux in low- and high-field environments. Numerical simulations based on the London and Maxwell equations have been deployed to determine the coupling between the nanoSQUID and a point-like magnetic moment. By using a hybrid magnetometer system consisting of an Nb nanoSQUID and a Si cantilever, individual ferromagnetic nanotubes have been investigated simultaneously by nanoSQUID and torque magnetometry, which yield complementary information of the magnetization reversal processes in the magnetic nanotube
Optimizing the spin sensitivity of grain boundary junction nanoSQUIDs -- towards detection of small spin systems with single-spin resolution
We present an optimization study of the spin sensitivity of nanoSQUIDs based
on resistively shunted grain boundary Josephson junctions. In addition the dc
SQUIDs contain a narrow constriction onto which a small magnetic particle can
be placed (with its magnetic moment in the plane of the SQUID loop and
perpendicular to the grain boundary) for efficient coupling of its stray
magnetic field to the SQUID loop. The separation of the location of optimum
coupling from the junctions allows for an independent optimization of the
coupling factor and junction properties. We present different
methods for calculating (for a magnetic nanoparticle placed 10\,nm
above the constriction) as a function of device geometry and show that those
yield consistent results. Furthermore, by numerical simulations we obtain a
general expression for the dependence of the SQUID inductance on geometrical
parameters of our devices, which allows to estimate their impact on the
spectral density of flux noise of the SQUIDs in the thermal white
noise regime. Our analysis of the dependence of and on the
geometric parameters of the SQUID layout yields a spin sensitivity
of a few
( is the Bohr magneton) for optimized parameters, respecting
technological constraints. However, by comparison with experimentally realized
devices we find significantly larger values for the measured white flux noise,
as compared to our theoretical predictions. Still, a spin sensitivity on the
order of for optimized devices seems to be
realistic.Comment: 10 pages, 5 figures, Superconductor Science and Technology
(submitted
Magnetization reversal of an individual exchange biased permalloy nanotube
We investigate the magnetization reversal mechanism in an individual
permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a
nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is
affixed to the tip of a Si cantilever and positioned in order to optimally
couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the
NT's volume magnetization by dynamic cantilever magnetometry and its stray flux
using the nanoSQUID. We observe a training effect and temperature dependence in
the magnetic hysteresis, suggesting an exchange bias. We find a low blocking
temperature K, indicating the presence of a thin
antiferromagnetic native oxide, as confirmed by X-ray absorption spectroscopy
on similar samples. Furthermore, we measure changes in the shape of the
magnetic hysteresis as a function of temperature and increased training. These
observations show that the presence of a thin exchange-coupled native oxide
modifies the magnetization reversal process at low temperatures. Complementary
information obtained via cantilever and nanoSQUID magnetometry allows us to
conclude that, in the absence of exchange coupling, this reversal process is
nucleated at the NT's ends and propagates along its length as predicted by
theory.Comment: 8 pages, 4 figure
Superconducting quantum interference devices with submicron Nb/HfTi/Nb junctions for investigation of small magnetic particles
We investigated, at temperature , electric transport, flux
noise and resulting spin sensitivity of miniaturized Nb direct current
superconducting quantum interference devices (SQUIDs) based on submicron
Josephson junctions with HfTi barriers. The SQUIDs are either of the
magnetometer-type or gradiometric in layout. In the white noise regime, for the
best magnetometer we obtain a flux noise
, corresponding to a
spin sensitivity . For the
gradiometer we find
and . The devices can still be
optimized with respect to flux noise and coupling between a magnetic particle
and the SQUID, leaving room for further improvement towards single spin
resolution.Comment: 4 pages, 3 figure
Quantitative nanoscale vortex-imaging using a cryogenic quantum magnetometer
Microscopic studies of superconductors and their vortices play a pivotal role
in our understanding of the mechanisms underlying superconductivity. Local
measurements of penetration depths or magnetic stray-fields enable access to
fundamental aspects of superconductors such as nanoscale variations of
superfluid densities or the symmetry of their order parameter. However,
experimental tools, which offer quantitative, nanoscale magnetometry and
operate over the large range of temperature and magnetic fields relevant to
address many outstanding questions in superconductivity, are still missing.
Here, we demonstrate quantitative, nanoscale magnetic imaging of Pearl vortices
in the cuprate superconductor YBCO, using a scanning quantum sensor in form of
a single Nitrogen-Vacancy (NV) electronic spin in diamond. The sensor-to-sample
distance of ~10nm we achieve allows us to observe striking deviations from the
prevalent monopole approximation in our vortex stray-field images, while we
find excellent quantitative agreement with Pearl's analytic model. Our
experiments yield a non-invasive and unambiguous determination of the system's
local London penetration depth, and are readily extended to higher temperatures
and magnetic fields. These results demonstrate the potential of quantitative
quantum sensors in benchmarking microscopic models of complex electronic
systems and open the door for further exploration of strongly correlated
electron physics using scanning NV magnetometry.Comment: Main text (5 pages, 4 figures) plus supplementary material (5 pages,
6 figures). Comments welcome. Further information under
http://www.quantum-sensing.c
Feasibility studies of time-like proton electromagnetic form factors at PANDA at FAIR
Simulation results for future measurements of electromagnetic proton form
factors at \PANDA (FAIR) within the PandaRoot software framework are reported.
The statistical precision with which the proton form factors can be determined
is estimated. The signal channel is studied on the basis
of two different but consistent procedures. The suppression of the main
background channel, , is studied.
Furthermore, the background versus signal efficiency, statistical and
systematical uncertainties on the extracted proton form factors are evaluated
using two different procedures. The results are consistent with those of a
previous simulation study using an older, simplified framework. However, a
slightly better precision is achieved in the PandaRoot study in a large range
of momentum transfer, assuming the nominal beam conditions and detector
performance
Can we identify patients at risk of life-threatening allergic reactions to food?
Anaphylaxis has been defined as a “severe, life-threatening generalized or systemic hypersensitivity reaction”. However, data indicate that the vast majority of food-triggered anaphylactic reactions are not life-threatening. Nonetheless, severe life-threatening reactions do occur, and are unpredictable. We discuss the concepts surrounding perceptions of severe, life-threatening allergic reactions to food by different stakeholders, with particular reference to the inclusion of clinical severity as a factor in allergy and allergen risk management. We review the evidence regarding factors which might be used to identify those at most risk of severe allergic reactions to food, and the consequences of misinformation in this regard. For example, a significant proportion of food-allergic children also have asthma, yet almost none will experience a fatal food-allergic reaction; asthma is not, in itself, a strong predictor for fatal anaphylaxis. The relationship between dose of allergen exposure and symptom severity is unclear. While dose appears to be a risk factor in at least a subgroup of patients, studies report that individuals with prior anaphylaxis do not have a lower eliciting dose than those reporting previous mild reactions. It is therefore important to consider severity and sensitivity as separate factors, as a highly sensitive individual will not necessarily experience severe symptoms during an allergic reaction. We identify the knowledge gaps which need to be addressed to improve our ability to better identify those most at risk of severe foodinduced allergic reactions
World allergy organization anaphylaxis guidance 2020
Anaphylaxis is the most severe clinical presentation of acute systemic allergic reactions. The occurrence of anaphylaxis has increased in recent years, and subsequently, there is a need to continue disseminating knowledge on the diagnosis and management, so every healthcare professional is prepared to deal with such emergencies. The rationale of this updated position document is the need to keep guidance aligned with the current state of the art of knowledge in anaphylaxis management. The World Allergy Organization (WAO) anaphylaxis guidelines were published in 2011, and the current guidance adopts their major indications, incorporating some novel changes. Intramuscular epinephrine (adrenaline) continues to be the first-line treatment for anaphylaxis. Nevertheless, its use remains suboptimal. After an anaphylaxis occurrence, patients should be referred to a specialist to assess the potential cause and to be educated on prevention of recurrences and self-management. The limited availability of epinephrine auto-injectors remains a major problem in many countries, as well as their affordability for some patients
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