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 allow the optical manipulation of magnetic order on an ultrafast time scale. 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 all-optical spectroscopy we use novel light sources like free electron laser 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 nanometre spatial resolution.
Figure 1: Schematic of the experimental setups for magnetic small angle scattering and magnetic dichroism spectroscopy.
The extreme ultraviolett probe pules are derived by free-electron laser or high harmonic radiation.
Polarization Control of extreme ultraviolet radiation
We have recently commissioned a device for the generation of circularly polarized radiation at FLASH1 at DESY in the extreme ultraviolet spectral range between 30 eV (41.3 nm) to 90 eV (13.7 nm).
We present three independent measurements based on polarimetry, electron time-of-flight (eTOF) spectroscopy and magnetic dichroism which are in excellent agreement with calculations and demonstrate the efficient
and broadband functionality of the device.
With the unique properties of FEL radiation, we envision new types of experiments in femto-magnetism ranging from magnetic circular dichroism spectroscopy to time-resolved coherent imaging
of magnetic nanostructures. Enabling element specific spectroscopy of chiral light-matter interaction in (bio-)molecules makes this new device also attractive for a wider range of users from physics,
chemistry and life science.
Figure 3: a) Artistic depiction of the four-mirror polarizer. b) Performance of the device: the data points of an XUV polarimeter measurement (round and square symbols) and of an electron time-of-flight (eTOF)
analysis (diamond symbol) show a very good agreement with the theoretical predictions of both the degree of circular polarization, PC, (red solid line) and of the total reflection,
R, (green dash-dot line).
Review of Scientific Instruments 88, 053903 (2017)
Ultrafast Interface Magnetism of Co/Pt Heterostructures
Interface effects have very recently been identified to play a decisive role in ultrafast spin dynamics and all-optical magnetic switching.
Currently discussed mechanism include interface exchange interaction, enhanced spin-orbit coupling, spin-polarized electron transport across an
interface and more generally phenomena arising from symmetry breaking at the boundary.
However, progress in understanding and, hence, in the successful design of layered multicomponent magnetic systems for ultrafast applications
has been challenged by the difficulty to experimentally access the underlying complex microscopic processes. Our novel approach of time-
resolved magnetic circular dichroism combines femtosecond time resolution with element-specificity, both a prerequisite for extracting
information about contributions of the different materials and interacting spin systems.
In the present work, we demonstrated for the first time time resolved magnetic dichroic measurements with broad-band high harmonic radiation
and show simultaneous sensitivity to a Co film and the Co/Pt interface in a single experiment.
Figure 2: (a) Inset: x-ray absorption (XAS) and magnetic circular dichroism (MCD) of a Pt/Co/Pt trilayer. Pronounced magnetic signals are
found at the Co M2,3 edge and O2,3 and N6,7 edge of Pt. High harmonic spectrum showing resonant magnetic
asymmetry at 60.8 eV and 54.6 eV, which we can attribute to Co and Pt, respectively. (b) Ultrafast evolution of Co and Pt suggesting that the
Co/Pt interface directly follows the magnetization dynamics of the Co film.
Collaboration with N. Zhavoronkov, O. Kornilov and M.J.J. Vrakking.
Physical Review B 92, 220405(R) (2015)