Efficient creation of electron vortex beams for high resolution STEM imaging

Abstract

The recent discovery of electron vortex beams carrying quantised angular momentum in the TEM has led to an active field of research, exploring a variety of potential applications including the possibility of mapping magnetic states at the atomic scale. A prerequisite for this is the availability of atomic sized electron vortex beams at high beam current and mode purity. In this paper we present recent progress showing that by making use of the Aharonov-Bohm effect near the tip of a long single domain ferromagnetic Nickel needle, a very efficient aperture for the production of electron vortex beams can be realised. The aperture transmits more than 99% of all electrons and provides a vortex mode purity of up to 92%. Placing this aperture in the condenser plane of a state of the art Cs corrected microscope allows us to demonstrate atomic resolution HAADF STEM images with spatial resolution better than 1 Angström, in agreement with theoretical expectations and only slightly inferior to the performance of a non-vortex probe on the same instrument.

Electron vortices: Beams with orbital angular momentum

“…This method is potentially versatile, since the phase change of the beam depends on the magnetic flux contained within the needle, so that higher and noninteger vortex states may also be created (Blackburn and Loudon, 2014). When such a needle is placed in the middle of the aperture for the vortex forming lens, over 90% incident beam can be converted (Béché et al, 2016). This high e ciency is a very useful property for many applications.…”

“…The main challenges in the use of monopole-like fields to create vortices will be the control of the fields themselves -the field geometry and strength are significantly a↵ected by the thickness, width and magnetisation of the needle. Slight artifacts of the fabrication process may substantially a↵ect the shape of the fields, as discussed in detail in (Blackburn and Loudon, 2014), resulting in some mixing of di↵erent vortex states (Béché et al, 2016).…”

“…With a microscope capable of 0.5Å non-vortex beam, a singly charged vortex beam of subAnstrom size should be possible. Experimentally, most studied electron vortex beams can be e↵ectively obtained as some forms of bandwidthlimited beams with wavefunction truncation by a hard aperture at the plane of the focusing lens (Béché et al, 2016;Schattschneider et al, 2012b;Verbeeck et al, 2011a). The scalar di↵raction theory of such a case has been thoroughly investigated in the course of optical vortex studies (Kotlyar et al, 2005) and this compares favorably with the theoretical investigations when finite source size e↵ects or even spherical aberration, is taken into account, where appropriate (Schattschneider et al,FIG.…”

“…Incoherent broadening plays an increasingly important role in atomic size electron vortex beam experiments as a sizelimiting factor (Löfgren et al, 2016), but its influence can be taken into account in the same manner as done routinely in describing focused non-vortex beams commonly used in scanning transmission electron microscopy (Kirkland, 2010). For an electron vortex beam generated inside a 300kV scanning transmission electron microscope a probe size of the order of 0.87Å has been demonstrated (Béché et al, 2016), invalidating the early too pessimistic prediction (Idrobo and Pennycook, 2011). As the current resolution of electron microscopes is not yet wavelength limited, there is still scope to reduce the vortex beam size further.…”

Towards topological spectroscopy in the electron microscope with atomic resolution

“…Different approaches have been pursued, such as utilizing holographic masks with dislocations, [2,3] aberration correctors [4,5], and magnetic needles [6]. Images with atomic resolution utilizing vortex beams have been produced [7,8], but the quality of the images indicates that the coherence of the electron beams is not yet optimal, particularly when trying to perform atomically resolved electron magnetic chiral dichroism (EMCD) spectroscopy measurements.In this talk, I will discuss how and why vortex beams can be utilized to perform novel spectroscopy associated with broken symmetries, such as magnetism. Additionally, I will demonstrate how to produce atomically resolved EMCD measurements utilizing aberrated probes formed by a linear combination of vortex beams (shown schematically in Figure 1) [9]).…”