223 research outputs found
Electron vortices in crystals
The propagation of electron beams carrying angular momentum in crystals is
studied using a multislice approach for the model system Fe. It is found that
the vortex beam is distorted strongly due to elastic scattering. Consequently,
the expectation value of the angular momentum as well as the local vortex
components change with the initial position of the vortex and the propagation
depth, making numerical simulations indispensable when analyzing experiments
Comment on "Quantized Orbital Angular Momentum Transfer and Magnetic Dichroism in the Interaction of Electron Vortices with Matter"
It was claimed (Lloyd et al., PRL 108 (2012) 074802) that energy loss
magnetic chiral dichroism (EMCD) with electron vortex beams is feasible, and
has even advantages over the standard setup with Bragg diffracted waves. In
this Comment, we show that Lloyd et al. ignored an important constraint on the
proposed selection rule for the transfer of angular momentum in the
interaction, namely that it is only valid for an atom located in the very
center of the vortex. As an experimental consequence, the EMCD signal will only
be strong for extremely small nanoparticles of 1 to 2 nm diameter.Comment: Submitted to Physical Review Letters 11 July 2012. Accepted for
publication 3 April 2013. "Copyright (2013) by the American Physical
Society." http://prl.aps.org
Observation of the Larmor and Gouy Rotations with Electron Vortex Beams
Electron vortex beams carrying intrinsic orbital angular momentum (OAM) are
produced in electron microscopes where they are controlled and focused using
magnetic lenses. We observe various rotational phenomena arising from the
interaction between the OAM and magnetic lenses. First, the Zeeman coupling,
proportional to the OAM and magnetic field strength, produces an
OAM-independent Larmor rotation of a mode superposition inside the lens.
Second, hen passing through the focal plane, the electron beam acquires an
additional Gouy phase dependent on the absolute value of the OAM. This brings
about the Gouy rotation of the superposition image proportional to the sign of
the OAM. A combination of the Larmor and Gouy effects can result in the
addition (or subtraction) of rotations, depending on the OAM sign. This
behaviour is unique to electron vortex beams and has no optical counterpart, as
Larmor rotation occurs only for charged particles. Our experimental results are
in agreement with recent theoretical predictions.Comment: 5 pages, 5 figure
Magnetic circular dichroism in EELS: Towards 10 nm resolution
We describe a new experimental setup for the detection of magnetic circular
dichroism with fast electrons (EMCD). As compared to earlier findings the
signal is an order of magnitude higher, while the probed area could be
significantly reduced, allowing a spatial resolution of the order of 30 nm. A
simplified analysis of the experimental results is based on the decomposition
of the Mixed Dynamic Form Factor S(q,q',E) into a real part related to the
scalar product and an imaginary part related to the vector product of the
scattering vectors q and q'. Following the recent detection of chiral
electronic transitions in the electron microscope the present experiment is a
crucial demonstration of the potential of EMCD for nanoscale investigations.Comment: 12 pages, 6 figures, submitted to Ultramicroscop
Magnetic properties of single nanomagnets: EMCD on FePt nanoparticles
Energy-loss magnetic chiral dichroism (EMCD) allows for the quantification of
magnetic properties of materials at the nanometer scale. It is shown that with
the support of simulations that help to identify the optimal conditions for a
successful experiment and upon implementing measurement routines that
effectively reduce the noise floor, EMCD measurements can be pushed towards
quantitative magnetic measurements even on individual nanoparticles. With this
approach, the ratio of orbital to spin magnetic moments for the Fe atoms in a
single L ordered FePt nanoparticle is determined to be . This finding is in good quantitative agreement with the results of
XMCD ensemble measurements.Comment: 35 pages, 10 figure
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