Sharath Kumar Manjeshwar Sathyanath: Electron magnetic circular dichroism of thin films and at single interfaces at atomic lattice plane resolution

Abstract

The magnetic properties of thin films, nanostructures and interfaces differ from their bulk counterparts. Transmission electron microscopy (TEM) with its unique capability to analyse buried interfaces at atomic resolution is uniquely suited for studying magnetic properties at high resolution. Though atomic resolution structural analysis in TEM is well established, resolving magnetic fields on single atomic lattice planes remains a challenging.

Using electron magnetic circular dichroism (EMCD), local changes of magnetic properties such as the magnetization and the orbital-to-spin magnetic moment ratio ml/ms can be detected. While prior studies have achieved near atomic lattice plane resolution by averaging across several lattice planes, we present a systematic study developing the EMCD technique in scanning TEM (STEM) mode to achieve single atomic lattice plane resolution with quantitative precision. Scatter angle dependent EELS spectra were acquired where dichroic signals are strongest, with the EMCD signal extracted from their difference spectrum. By using custom-built spectrometer apertures, we found that the EMCD signal remains highly stable under slight misorientations.

Using custom-built apertures for the acquisition of the dichroic spectra of Fe samples, we established the dependence of the EMCD signal on the semi-convergence angle   of incoming electrons. We confirmed the quantitative measurements of the ml/ms ratio with   that yields atomic-resolution probes. Our results show that the EMCD signal oscillates with the atomic lattice plane periodicity, thereby achieving single atomic plane 2 Å resolution of the magnetic signal. Applying this technique to a Fe/MgAl2O4 interface, we found that the magnetic interface is 2 Å narrower than the structural one and observed oscillation of the EMCD signal across planes with a 1.4 Å d-spacing. Near the interface, a high ml/ms ratio as well as a split L3 difference signal were detected.

This work enables EMCD as a powerful technique for quantitatively resolving magnetic signals at atomic scale, opening new possibilities for studies of magnetism in fields such as spintronics, van-der-Waals materials, quantum computing and even in orbital magnetism.