Spin polarisation with electron Bessel beams

The theoretical possibility to use an electron microscope as a spin polarizer is studied. It turns out that a Bessel beam passing a standard magnetic objective lens is intrinsically spin polarized when post-selected on-axis. In the limit of infinitely small detectors, the spin polarisation tends to 100%. Increasing the detector size, the polarisation decreases rapidly, dropping below 10−4 for standard settings of medium voltage microscopes. For extremely low voltages, the Figure of Merit increases by two orders of magnitude, approaching that of existing Mott detectors. Our findings may lead to new desings of spin filters, an attractive option in view of its inherent combination with the electron microscope, especially at low voltage. •TEM round magnetic lenses can act as spin polarizers when a Bessel beam is sent through.•This is found on theoretical grounds and demonstrated numerically for a few cases.•The effect is small, but can reach a Figure of Merit similar to existing Mott detectors.•This opens the possibility to construct nanometer-sized spin filters or detectors.