3.2 Solids and Nanostructures: Electrons, Spins, and Phonons
Project coordinator(s): C. von Korff Schmising, M. Wörner

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.


CRC/TRR 227 Ultrafast Spin Dynamics

The DFG-funded Collaborative Research Center / Transregio 227 of Freie Universität Berlin (FUB) and Martin-Luther-Universität Halle-Wittenberg (MLU) together with non-university institutions Fritz-Haber-Institut (FHI), Helmhotz-Zentrum Berlin (HZB), Max-Born-Institut (MBI) and Max-Planck-Institut (MPI) für Mikrostrukturphysik has just started in January 2018.

Our project A02, "Ultrafast spin dynamics in heterogeneous magnetic systems" will investigate ultrafast, non-equilibrium spin dynamics triggered by femtosecond optical pulses and to exploit tailored nanoscale heterogeneities in magnetic systems to control their magnetic response on an ultrafast time scale.

The heterogeneities are of three different types:

Our main experimental tools will be ultrafast, broadband extreme ultraviolet (XUV) spectroscopy and resonant small angle scattering to capture a microscopic picture of the element-specific magnetization dynamics on the femtosecond time and nanometer length scale. These will be complemented by all-optical Kerr spectroscopy and microscopy.

For more information regarding project A02 as well as on a vacant PhD position please also refer to the webpage of the TRR 227 .

New lab for ultrafast magnetism

Our new laboratory for investigations of ultrafast processes in magnetic systems has been setup. We use intense and ultrashort laser pulses (λ = 800 nm, E = 4.5 mJ at 3 kHz, Δ t <25 fs) to generate high harmonic radiation in the extreme ultraviolet (XUV) spectral range up to approximately 100 eV. In a pump-probe measurement scheme we then employ magnetic dichroism to probe the femtosecond magnetic response after optical excitation. This allows simultaneous access to the element-specific magnetization dynamics in complex and multi-element magnetic systems. A clever normalization scheme yields an excellent signal to noise ratio within a short measurement time. The sample environment includes an external magnetic field as well as a closed cycle cryostat with a temperture range between <10 K and 700 K.

Processes on the nanoscale will be explored in small angle X-ray scattering as well as coherent imaging experiments.

Additonally, we have commissioned a setup for ultrafast time resolved Kerr spectroscopy and microscopy.

Figure 3: a) Photograph of the XUV beamline. Details of the closed cycle cryostat: b) heat exchange and c) heat shield. d) Hitachi abberation-corrected concave grating for our flat-field spectrograph. e) Gas inlet and pressure sensor of the higher harmonic generation chamber.

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 4: a) Artistic depiction of the generation of circularly polarized radiation at FLASH. b) Photograph of the the four-mirror polarizer. c) 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)