302 research outputs found
Ultrasensitive 3He magnetometer for measurements of high magnetic fields
We describe a 3He magnetometer capable to measure high magnetic fields (B >
0.1 Tesla) with a relative accuracy of better than 10^-12. Our approach is
based on the measurement of the free induction decay of gaseous, nuclear spin
polarized 3He following a resonant radio frequency pulse excitation. The
measurement sensitivity can be attributed to the long coherent spin precession
time T2* being of order minutes which is achieved for spherical sample cells in
the regime of motional narrowing where the disturbing influence of field
inhomogeneities is strongly suppressed. The 3He gas is spin polarized in-situ
using a new, non-standard variant of the metastability exchange optical
pumping. We show that miniaturization helps to increase T2* further and that
the measurement sensitivity is not significantly affected by temporal field
fluctuations of order 10^-4.Comment: 27 pages, 7 figure
Controlling shot noise in double-barrier magnetic tunnel junctions
We demonstrate that shot noise in Fe/MgO/Fe/MgO/Fe double-barrier magnetic
tunnel junctions is determined by the relative magnetic configuration of the
junction and also by the asymmetry of the barriers. The proposed theoretical
model, based on sequential tunneling through the system and including spin
relaxation, successfully accounts for the experimental observations for bias
voltages below 0.5V, where the influence of quantum well states is negligible.
A weak enhancement of conductance and shot noise, observed at some voltages
(especially above 0.5V), indicates the formation of quantum well states in the
middle magnetic layer. The observed results open up new perspectives for a
reliable magnetic control of the most fundamental noise in spintronic
structures.Comment: 8 pages, 4 figure
How do Cross-View and Cross-Modal Alignment Affect Representations in Contrastive Learning?
Various state-of-the-art self-supervised visual representation learning
approaches take advantage of data from multiple sensors by aligning the feature
representations across views and/or modalities. In this work, we investigate
how aligning representations affects the visual features obtained from
cross-view and cross-modal contrastive learning on images and point clouds. On
five real-world datasets and on five tasks, we train and evaluate 108 models
based on four pretraining variations. We find that cross-modal representation
alignment discards complementary visual information, such as color and texture,
and instead emphasizes redundant depth cues. The depth cues obtained from
pretraining improve downstream depth prediction performance. Also overall,
cross-modal alignment leads to more robust encoders than pre-training by
cross-view alignment, especially on depth prediction, instance segmentation,
and object detection
Two types of all-optical magnetization switching mechanisms using femtosecond laser pulses
Magnetization manipulation in the absence of an external magnetic field is a
topic of great interest, since many novel physical phenomena need to be
understood and promising new applications can be imagined. Cutting-edge
experiments have shown the capability to switch the magnetization of magnetic
thin films using ultrashort polarized laser pulses. In 2007, it was first
observed that the magnetization switching for GdFeCo alloy thin films was
helicity-dependent and later helicity-independent switching was also
demonstrated on the same material. Recently, all-optical switching has also
been discovered for a much larger variety of magnetic materials (ferrimagnetic,
ferromagnetic films and granular nanostructures), where the theoretical models
explaining the switching in GdFeCo films do not appear to apply, thus
questioning the uniqueness of the microscopic origin of all-optical switching.
Here, we show that two different all-optical switching mechanisms can be
distinguished; a "single pulse" switching and a "cumulative" switching process
whose rich microscopic origin is discussed. We demonstrate that the latter is a
two-step mechanism; a heat-driven demagnetization followed by a
helicity-dependent remagnetization. This is achieved by an all-electrical and
time-dependent investigation of the all-optical switching in ferrimagnetic and
ferromagnetic Hall crosses via the anomalous Hall effect, enabling to probe the
all-optical switching on different timescales.Comment: 1 page, LaTeX; classified reference number
Hearing What You Cannot See: Acoustic Vehicle Detection Around Corners
This work proposes to use passive acoustic perception as an additional
sensing modality for intelligent vehicles. We demonstrate that approaching
vehicles behind blind corners can be detected by sound before such vehicles
enter in line-of-sight. We have equipped a research vehicle with a roof-mounted
microphone array, and show on data collected with this sensor setup that wall
reflections provide information on the presence and direction of occluded
approaching vehicles. A novel method is presented to classify if and from what
direction a vehicle is approaching before it is visible, using as input
Direction-of-Arrival features that can be efficiently computed from the
streaming microphone array data. Since the local geometry around the
ego-vehicle affects the perceived patterns, we systematically study several
environment types, and investigate generalization across these environments.
With a static ego-vehicle, an accuracy of 0.92 is achieved on the hidden
vehicle classification task. Compared to a state-of-the-art visual detector,
Faster R-CNN, our pipeline achieves the same accuracy more than one second
ahead, providing crucial reaction time for the situations we study. While the
ego-vehicle is driving, we demonstrate positive results on acoustic detection,
still achieving an accuracy of 0.84 within one environment type. We further
study failure cases across environments to identify future research directions.Comment: Accepted to IEEE Robotics & Automation Letters (2021), DOI:
10.1109/LRA.2021.3062254. Code, Data & Video:
https://github.com/tudelft-iv/occluded_vehicle_acoustic_detectio
Artificial Kagome Arrays of Nanomagnets: A Frozen Dipolar Spin Ice
Magnetic frustration effects in artificial kagome arrays of nanomagnets are
investigated using x-ray photoemission electron microscopy and Monte Carlo
simulations. Spin configurations of demagnetized networks reveal unambiguous
signatures of long range, dipolar interaction between the nanomagnets. As soon
as the system enters the spin ice manifold, the kagome dipolar spin ice model
captures the observed physics, while the short range kagome spin ice model
fails.Comment: 4 pages, 4 figures, 1 tabl
Phase transition in ultrathin magnetic films with long-range interactions: Monte Carlo simulation of the anisotropic Heisenberg model
Ultrathin magnetic films can be modeled as an anisotropic Heisenberg model
with long-range dipolar interactions. It is believed that the phase diagram
presents three phases: An ordered ferromagnetic phase I, a phase characterized
by a change from out-of-plane to in-plane in the magnetization II, and a
high-temperature paramagnetic phase III. It is claimed that the border lines
from phase I to III and II to III are of second order and from I to II is first
order. In the present work we have performed a very careful Monte Carlo
simulation of the model. Our results strongly support that the line separating
phases II and III is of the BKT type.Comment: 7 page
Magnetoresistance, Micromagnetism, and Domain Wall Scattering in Epitaxial hcp Co Films
Large negative magnetoresistance (MR) observed in transport measurements of
hcp Co films with stripe domains were recently reported and interpreted in
terms of a novel domain wall (DW) scattering mechanism. Here detailed MR
measurements, magnetic force microscopy, and micromagnetic calculations are
combined to elucidate the origin of MR in this material. The large negative
room temperature MR reported previously is shown to be due to ferromagnetic
resistivity anisotropy. Measurements of the resistivity for currents parallel
(CIW) and perpendicular to DWs (CPW) have been conducted as a function of
temperature. Low temperature results show that any intrinsic effect of DWs
scattering on MR of this material is very small compared to the anisotropic MR.Comment: 5 pages, 5 Figures, submitted to PR
Magnetic properties of submicron Co islands and their use as artificial pinning centers
We report on the magnetic properties of elongated submicron magnetic islands
and their influence on a superconducting film. The magnetic properties were
studied by magnetization hysteresis loop measurements and scanning-force
microscopy. In the as-grown state, the islands have a magnetic structure
consisting of two antiparallel domains. This stable domain configuration has
been directly visualized as a 2x2-checkerboard pattern by magnetic-force
microscopy. In the remanent state, after magnetic saturation along the easy
axis, all islands have a single-domain structure with the magnetic moment
oriented along the magnetizing field direction. Periodic lattices of these Co
islands act as efficient artificial pinning arrays for the flux lines in a
superconducting Pb film deposited on top of the Co islands. The influence of
the magnetic state of the dots on their pinning efficiency is investigated in
these films, before and after the Co dots are magnetized.Comment: 6 pages including figure
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