T4: Magnetism and transient electronic structure

We aim to understand how fundamental interactions in magnetism like spin-orbit coupling, exchange interaction, and spin dependent electron transport govern the ultrafast response of magnetic order after optical excitation. Currently, we focus on ferromagnetic and ferrimagnetic metallic thin films and multilayers and explore the influence of magnetic and chemical nanoscale inhomogeneities on ultrafast spin dynamics and ultrafast magnetic switching. Furthermore, we design and manufacture plasmonic structures for subwavelength confinement of the optical excitation to achieve control of magnetism on the nanoscale.

In addition to time resolved all-optical spectroscopy we use novel light sources like free electron X-ray lasers and high harmonic sources in the extreme ultraviolet spectral range to probe the transient magnetic state via element-specific magnetic dichroic spectroscopy and small angle scattering. These experimental methods give us detailed information on the distinct dynamics in multicomponent magnetic systems with a femtosecond temporal and nanometer spatial resolution. Our experimental efforts are supported by ab-initio density functional theory (DFT) developed in the condensed matter theory group.

A static high-order harmonic spectrum for two opposite magnetization directions (B+, B−) of a Pt/Co/Pt sample shows pronounced dichroic absorption (MCD) at the M2,3 edge of Co (60.8 eV). The inset shows X-ray absorption (red line) and MCD (black line) spectra measured with circularly polarized synchrotron radiation. In these measurements, we can clearly identify the absorption peaks and the magnetic asymmetry at the M2,3 edge. Additionally, the Pt/Co/Pt sample exhibits MCD signals at the 5p (O) and 4f (N) transitions of Pt, allowing to access processes related to interface magnetism.

(F. Willems et al., PRB, 92, 220405(R) (2015))