Altermagnetism denotes a compensated, collinear spin structure, which can be seen as a special form antiferromagnetism in real space, and at the same time shares interesting aspects with ferromagnetism, especially time-reversal symmetry-breaking and spin-polarized bands. It is believed that altermagnets can be used in future spintronics applications. While ab initio calculations predict many materials to have an altermagnetic phase, only in very few materials, the altermagnetic spin structure has been proved experimentally. In this context, I propose a new type of experiment, namely circular dichroism in resonant photoelectron diffraction (CD-RPED) for the magnetic characterization of altermagnetic materials, especially thin films and surfaces [1]. I show that the RPED pattern of an altermagnet contains a large CD, which is partially of structural and partially of magnetic origin. It is easy to disentangle the two components. The angular distribution of the purely magnetic CD is given by the difference of the RPED patterns between the two sublattices, i.e. a quantity independent of the magnetic state. The amplitude of the magnetic CD is approximately proportional to the X-ray magnetic CD in absorption. As a consequence, the direction and magnitude of the sublattice magnetization can be measured by CD-RPED, with minimal input from calculations. I also present results on the CD-RPED of ferromagnetic [2] and non- magnetic materials, where the angular momentum transfer from the absorbed photon to the emitted electron has a complex binding energy dependence and gives insight in the atomic multiplet structure of the system.

[1] P. Krüger, Phys. Rev. Letters 135, 196703 (2025)

[2] R. Sagehashi, G. Park and P. Krüger, Phys. Rev. B 107, 075407 (2023)