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Disclosing the time evolution of magnetic chirality after an optical excitation

In the field of magnetism and spintronics, chiral magnetic structures, such as spin spirals, chiral domain walls and skyrmions, are intensively investigated due to their fascinating properties such as potentially enhanced stability and efficient spin-orbit torque driven dynamics. It has been shown that these structures are stabilized by the Dzyaloshinskii-Moriya interaction (DMI) that favours a chiral winding of the magnetisation. While the investigation of static structures and slow (ns) dynamics of chiral magnetic structures has been intensified recently, experimental studies addressing the ultimate fs-ps dynamics of the chirality have been elusive so far as ultrafast pump-probe experiments have concentrated on the collinear order found in most magnetic systems.
In this work we employ circularly polarized light pulses of the FERMI free-electron laser (FEL) and investigate at the DiProI beamline the X-ray magnetic scattering evolution of the chiral order of domain walls in magnetic thin film samples (Fig. 1a). Using samples with interfacial DMI and perpendicular magnetic anisotropy exhibiting labyrinth-like domain patterns (Fig. 1b), we measure in the same experiment both the contribution of ferromagnetic order (CL+CR) and the average chirality at the domain wall (CL-CR) (Fig. 1e).

Figure 1. Experimental setup and diffraction images. (a) Measurement geometry: A magnetic thin film sample is pumped by an optical infrared laser pulse and probed by a circularly polarized X-ray FEL pulse. Afterwards an IR-protected CCD detector records the magnetic SAXS pattern. (b) MFM image of a typical labyrinth domain pattern of the [Ta(5.3 nm)/Co20Fe60B20(0.93 nm)/Ta(0.08 nm)/MgO(2.0 nm)]x20/Ta(1.6 nm) sample. (d) The resulting sum = CL+CR of the diffraction pattern confirms that the diffraction corresponds to the magnetic domains observed by MFM. (e) The dichroic scattering signal = CL-CR and its azimuthal dependence (f) confirms the presence of (c) right-handed chiral Néel (cycloidal) domain walls.



In contrast to the (CL+CR) signal, phase information is preserved in the dichroic signal (CL-CR) allowing to determine the domain wall chirality winding direction (left/right-handed) and character (Bloch (helical), Néel (cycloidal)). Fig. 1f shows the orthoradial profile of the dichroic signal, which indeed confirms the prevalence of fully right-handed Néel-type domain walls. This provides a tool to individually probe the time resolved dynamics of the chiral magnetic order in the domain walls.
A key step of this experimental work is the comparison of the collinear ferromagnetic order dynamics and the chiral order dynamics. The corresponding temporal evolution of the sum and the difference signal normalized to the unpumped total intensity are shown in Fig.2.
We find an ultrafast intensity decrease of both signals in the sub-ps regime with similar time constants. However, a significantly faster recovery of the chiral signal in the sub-ns timescale is observed. 

 

Figure 1. Time dependence of total scattering intensities and second moments. Time evolution of the radially integrated intensity of the sum signal (ferromagnetic order) and the difference signal (chiral magnetic order).

 

In order to explain the key finding of the experimentally observed faster recovery of the chiral signal after laser excitation we envisage two different mechanisms: (1) a change in the size ratio between domain walls and domains caused by an increase of the domain wall width during the whole investigated time frame or (2) a faster recovery dynamics of the chiral order within the domain walls compared to the ferromagnetic order in the domains leading to a faster build-up of the chiral magnetisation.
We subsequently investigated the origin of the faster recovery of the chiral signal by performing numerical simulations of the scattering signal, which reproduce the experimental findings. We conclude from these that the main driver behind our experimental findings is likely mechanism (2) leading to a faster recovery of the chiral order in the domain walls in comparison to the ferromagnetic order in the domains.
Our study paves the way for future investigations of fundamental aspects such as, e.g., the dependence of the timescales of the chiral order build-up on the absolute strength of the DMI by varying the heavy metal layers. The control of the DMI and the chirality of spin structures on the ultrafast timescale can finally allow the controlled ultrafast manipulation of chiral magnetism, e.g., ultrafast writing of chiral topological objects such as skyrmions and pave the path to applications in the field of ultrafast chiral spintronics.


 

This research was conducted by the following research team:

Nico Kerber1,2, Dmitriy Ksenzov3, Frank Freimuth4, Flavio Capotondi5, Emanuele Pedersoli5, Ignacio Lopez-Quintas5, Boris Seng1,2,6, Joel Cramer1,2, Kai Litzius1,2, Daniel Lacour6, Hartmut Zabel1,2,7, Yuriy Mokrousov1,2,4, Mathias Kläui1,2 and Christian Gutt3


1 Institut für Physik, Johannes Gutenberg-Universität Mainz, Mainz, Germany.
2 Graduate School of Excellence Materials Science in Mainz, Germany.
3 Department Physik, Universität Siegen, Siegen, Germany.
4 Peter Grünberg Institute and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, Jülich, Germany.
5 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy.
6 Institut Jean Lamour, Université de Lorraine, Vandoeuvre-lès-Nancy, France.
7 Department of Physics, Ruhr-University Bochum, Bochum, Germany.


Contact persons:

Christian Gutt, email:
Mathias Kläui, email:


Reference

N. Kerber, D. Ksenzov, F. Freimuth, F. Capotondi, E. Pedersoli, I. Lopez-Quintas, B. Seng, J. Cramer, K. Litzius, D. Lacour, H. Zabel, Y. Mokrousov, M. Kläui, C. Gutt, “Faster chiral versus collinear magnetic order recovery after optical excitation revealed by femtosecond XUV scattering”, Nature Communications 11, 6304 (2020) DOI:10.1038/s41467-020-19613-z

 

Last Updated on Wednesday, 03 February 2021 14:08