Imaging Nanoscale Magnetic Structures With Polarized Soft X-Ray Photons

Abstract

Imaging nanoscale magnetic structures and their fast dynamics is scientifically interesting and technologically of highest relevance. The combination of circularly polarized soft X-ray photons, which provide a strong X-ray magnetic circular dichroism effect at characteristic X-ray absorption edges, with a high-resolution soft X-ray microscope utilizing Fresnel zone plate optics allows, in a unique way, the study of the stochastical behavior in the magnetization reversal process of thin films and the ultrafast dynamics of magnetic vortices and domain walls in confined ferromagnetic structures. Future sources of femtosecond-short and highly intense soft X-ray photon pulses hold the promise of magnetic imaging down to fundamental magnetic length and time scales.The magnetic properties of matter are one of the most vibrant research areas [1], [2], not only because the phenomenon of magnetism itself is scientifically very attractive, but it also has immense implications to modern magnetic storage and sensor device technologies. Nanomagnetism investigates magnetism approaching fundamental magnetic length scales which are given by material specific properties such as exchange lengths or anisotropy constants being in the fewnanometer regime for common materials.To minimize the competing magnetic interactions, e.g., exchange and anisotropy, in ferromagnetic systems, the ground state is often not the single domain state, where all spins are aligned parallel, but breaks up into multiple magnetic domains [3]. The transition region between domains is referred to as a domain wall (DW). In confined geometries, e.g., micrometer-sized disk structures, the spin configuration forms a magnetic vortex (MV) [4], [5], with a singularity occurring at the center, i.e., the MV core, which overcomes shape anisotropy and points perpendicular to the disk plane. The sizes of DWs and vortex cores are proportional to magnetic exchange lengths.With a typical bit size of only a few tens of nanometers in high-density magnetic storage media of 1 Tb/in 2 and the capability to artificially fabricate nanoscale magnetic structures either top-down or bottom-up, e.g., by state-of-the-art lithographical techniques [6], an abundance of analytical tools to characterize the magnetic behavior of these structures has been developed. Imaging methods are very attractive, since they give detailed and direct insight into the mechanisms involved.There are several interaction mechanisms to control and manipulate the spin structure on a nanoscale. For example, reversal of the magnetic moments can be achieved by applying an

Optical modeling of Fresnel zoneplate microscopes

“…Although the motivation presented here is in the realm of EUV mask inspection, the methods described and analysis results are broadly applicable to zoneplate microscopes in general including full field soft-x-ray microscopes routinely used in the synchrotron community [4][5][6].…”