Ultrafast chiral domain wall dynamics probed by time-resolved resonant magnetic scattering

The subpicosecond demagnetization of ferromagnets induced by an optical pulse was first demonstrated in 1996 in the seminal study by Beaurepaire. While several local spin-flip based mechanisms are considered to explain these ultrafast processes, only in the last years, the central role of spin dependent transport of hot electrons has been clearly evidenced. The hot electrons contribution to ultrafast demagnetization has also been tested in transmission on single magnetic layers with a heterogeneous magnetization configuration, i.e., containing a large density of magnetic domains and DWs. However, the effects of such ultrafast spin current on topological stable spin structure is still an uncharted realm, in particular the effects of spin torque that a moving electron induce on the surrounded spin texture and its dynamics has never been observed so far.
In this study, we use circular dichroism in x-ray resonant magnetic scattering (CD-XRMS) that gives insights on the internal spin texture of the domain walls. An asymmetric magnetic multilayer sample [Pt(3 nm)|Co(1.5 nm)|Al(1.4 nm)]x5 was grown by sputtering on a thermally oxidized Si wafer buffered by Ta(5)|Pt(5) in order to stabilized chiral Néel type domain wall. By pumping with an optical laser, we induced on such magnetic structure a spin current across the domain wall. The spin dynamics was monitored by the evolution of the magnetic diffraction pattern recorded on a bidimensional CCD detector.
The experiment has been performed on the DiProI endstation at the FERMI free electron laser and realized in reflectivity geometry illuminating the sample at 45° of incidence with both circularly left (CL) and right (CR) x-ray polarization. The use of variable circular polarization allows to disentangle the behavior of spin dynamics from chiral contribution. Indeed, the sum of the two polarizations (CL + CR) gives insight on the domain magnetic order Fig. 1(a,b) while the difference image (CL - CR) indicates the type of domain wall and their chirality (here clockwise Néel) Fig 1(c). 

 figure 1

Figure 1.  (a) Magnetic force microscopy image of the sample after out of plane demagnetization. The scale bar is 2 µm. (b) Magnetic diffraction pattern obtained by summing CL and CR at 45°. The acquired CCD image is corrected for the geometrical projection. (c) The polarization difference image (CL-CR) exhibiting a clockwise Néel domain walls signature. Each polarization image accumulates the signal from 500 XFEL pulses.

To exploit the dynamics of spin and texture contribution after optical pump stimulus, we perform the azimuthal integration of the images 1(b) and 1(c) absolute value, fit the peak by a gaussian function and take its amplitude. As shown in Fig. 2, the ultrafast difference signal, carrying information on chiral order, demagnetizes more than the sum. The effect is more evident from the ratio of the difference over the sum plot in Fig. 2(b), which provides information on the relative dynamics between domain walls and internal part of the domains. The results suggest that the spin structure at the border between domains with different orientation of magnetic momentum are influenced by the spin current flowing across the domain wall after optical stimulus. Indeed, computational simulation based on the assumption of a homogeneous (pink curve) demagnetization rate between the domain and the domain wall, or a domain wall expansion (blue curve) cannot reproduce the experimental data. To fully model the observed experimental behavior, the action of a torque exerted on the domain walls by hot electrons originating from the domains must be added to the contribution of the demagnetization across the domain wall (green curve). In a nutshell, the intense isotropic spin polarized hot electron flow induces an additional incoherent loss of angular momentum inside the domain walls with respect to the internal uniform part of the magnetic domain. Moreover, the model predicts the apparition of a transient Bloch/Néel/Bloch domain wall type in the first 2ps caused by a coherent effect from hot electrons coming perpendicularly to the domain walls. The coherent and incoherent torques demagnetize more the domain walls compared to the domains, which fit the ratio signal and explain the more pronounced demagnetization amplitude as well as the faster recovery of the domain wall signal. The understanding of the observed dynamics can be extremely important to model more complex spin texture structures such as skyrmions or magnetic bubble that are suppose to be the next generation data recording building block structures.

Figure 2. a) Signal evolution of the chiral (CL-CR) and sum (CL+CR) signal up to 5 ps. Both signals are normalized by the mean value of the negative time delays. It is the result of the azimuthal integration on the difference and sum images. (b) The asymmetry ratio (Chiral/Sum) normalized by the mean negative delays value from two different beamtime on the same sample. The simulations in coloured lines are discussed in the text.


This research was conducted by the following research team:

Cyril Léveillé1, Erick Burgos-Parra1,2, Yanis Sassi2, Fernando Ajejas2, Valentin Chardonnet3, Emanuele Pedersoli4, Flavio Capotondi4, Giovanni De Ninno4,5, Francesco Maccherozzi6, Sarnjeet Dhesi6, David M. Burn6, Gerrit van der Laan6, Oliver S. Latcham7, Andrey V. Shytov7, Volodymyr V. Kruglyak7, Emmanuelle Jal3, Vincent Cros2, Jean-Yves Chauleau8, Nicolas Reyren2, Michel Viret8 and Nicolas Jaouen1


1 Synchrotron SOLEIL, Saint-Aubin, Gif-sur-Yvette Cedex, France
2 Unité Mixte de Physique, CNRS, Thales, Université Paris-Saclay, Palaiseau, France
3 Sorbonne Université, CNRS, Laboratoire Chimie Physique – Matière et Rayonnement, Paris, France
4 Elettra-Sincrotrone Trieste, Basovizza, Trieste, Italy
5 University of Nova Gorica, Nova Gorica, Slovenia
6 Diamond Light Source, Didcot, United Kingdom.
7 University of Exeter, Stocker road, Exeter, United Kingdom.
8 SPEC, CEA, CNRS, Université Paris-Saclay, Gif-sur-Yvette, France

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C. Léveillé,  E. Burgos-Parra, Y. Sassi, et al. “Ultrafast time-evolution of chiral Néel magnetic domain walls probed by circular dichroism in x-ray resonant magnetic scattering”, Nat. Commun. 13, 1412 (2022). DOI: 10.1038/s41467-022-28899-0


Last Updated on Monday, 30 May 2022 11:07