160 research outputs found
Convergent-Beam EMCD: Benefits, Pitfalls, and Applications
Energy-loss magnetic chiral dichroism (EMCD) is a versatile method for
studying magnetic properties on the nanoscale. However, the classical EMCD
technique is notorious for its low signal to noise ratio (SNR). Here, we study
the theoretical possibilities of using a convergent beam for EMCD. In
particular, we study the influence of detector positioning as well as
convergence and collection angles on the detectable EMCD signal. In addition,
we analyze the expected SNR and give guidelines for achieving optimal EMCD
results
Sub-nanometer free electrons with topological charge
The holographic mask technique is used to create freely moving electrons with
quantized angular momentum. With electron optical elements they can be focused
to vortices with diameters below the nanometer range. The understanding of
these vortex beams is important for many applications. Here we present a theory
of focused free electron vortices. The agreement with experimental data is
excellent. As an immediate application, fundamental experimental parameters
like spherical aberration and partial coherence are determined.Comment: 4 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
Is magnetic chiral dichroism feasible with electron vortices?
We discuss the feasibility of detecting magnetic transitions with focused
electron vortex probes, suggested by selection rules for the magnetic quantum
number. We theoretically estimate the dichroic signal strength in the L
edge of the ferromagnetic d metals. It is shown that under realistic
conditions, the dichroic signal is undetectable for nanoparticles larger than
~1 nm. This is confirmed by a key experiment with nanometer sized vortices
A novel vortex generator and mode converter for electrons
A mode converter for electron vortex beams is described. Numerical
simulations, confirmed by experiment, show that the converter transforms a
vortex beam with topological charge into beams closely resembling
Hermite-Gaussian HG and HG modes. The converter can be used as a
mode discriminator or filter for electron vortex beams. Combining the converter
with a phase plate turns a plane wave into modes with topological charge . This combination serves as a generator of electron vortex beams of high
brilliance
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