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$_{2,3}$ 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 $m=\pm 1$ into beams closely resembling Hermite-Gaussian HG$_{10}$ and HG$_{01}$ 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 $m=\pm 1$. This combination serves as a generator of electron vortex beams of high brilliance