Towards atomic magnetic measurements with single electron vortex beams on nanocubes

Abstract

X‐ray magnetic circular dichroism is a well‐established method to study element specific magnetic properties of a material, while electron energy‐loss magnetic chiral dichroism (EMCD), which is the electron wave analogue to XMCD, is scarcely used today. Recently discovered electron vortex beams, which carry quantized orbital angular momenta (OAM) L, promise to also reveal magnetic signals [1]. Since electron beams can be easily focused down to sub‐nanometer diameters, this novel technique provides the possibility to quantitatively determine local magnetic properties with unrivalled lateral resolution. In order to generate the spiralling wave front of an electron vortex beam with an azimuthally growing phase shift of up to 2p and a phase singularity in its axial centre, specially designed apertures are needed [2,3]. Dichroic signals on the L 2 and L 3 edge are expected to be of the order of 5% [4,5]. The generation of EVBs in the double aberration‐corrected FEI Titan 3 80‐300 transmission electron microscope (TEM) is achieved by the implementation of a dislocation‐type apertures into the condenser lens system. The setup allows for scanning TEM investigations (STEM) with vortex beams, whose OAM is selected by means of an additional discriminator aperture. New FIB cutting strategies facilitate the production for 50 µm wide and 1 µm thick high quality vortex apertures (see fig. 1a). However, in the case of a fork‐type aperture, the EVB are dispersed in the x‐y plane resulting in mixed probe that interacts with the magnetic sample. We have recently devised an escape route to this problem by blocking any partial beams that carry other but the desired OAM prior to the interaction of the beam with the ferromagnetic sample. This is achieved by using a special condensor aperture in combination with a fork‐type aperture to select a single partial beam with the chosen OAM (s. fig 1b). This approach allows to generate atom‐sized EVB with angstrom‐sized probes and a well‐defined OAM by which atomic resolution HR‐STEM is achieved (see. fig 2). Although this discretization results in an increased signal‐to‐noise ratio, this novel technique is capable of atomic resolution EELS measurements which is the prerequisite for atomic EMCD measurements. First experiments using this new optical setup show very promising EMCD results on ferromagnetic FePt nanoparticles.