Electron Beams with Angular Momentum and Their Application to Magnetic Imaging

Electron vortex beams, composed of helical electron wavefunctions that carry quantized orbital angular momentum (OAM), are analogous to optical vortices in beams of light, and they can be produced in similar ways. For example, nanofabricated diffraction gratings can be used to holographically imprint a phase vortex onto free electron matter waves in a transmission electron microscope (TEM). We use this technique to generate free electron vortex beams with selectable amounts quantized OAM, and also to place free electrons in coherent superpositions of orbital states. These beams can interact with surfaces and materials in unique ways. For example, electron vortex beams can transfer quantized OAM to an atom through inelastic scattering, inducing preferred atomic transitions in which the atom’s magnetic quantum number (mj) changes – precisely the same phenomenon that is accessed optically in X-ray magnetic circular dichroism (XMCD) measurements of material magnetization. This OAM-dependent scattering provides a “dichroic” signal that can be measured in electron energy loss spectra from magnetic samples in a TEM. In an effort to use this effect to provide high resolution magnetic images, we are modifying a scanning TEM instrument to use OAM-carrying electron vortex probe beams, with the ultimate goal of analyzing magnetic materials with atomic resolution.

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