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Time- and layer-resolved magnetization dynamics

The operating speed of devices based on magnetoresistive effects is determined by the time it takes to reverse the magnetization direction in one or more ferromagnetic layers in ultrathin films and multilayers. A thorough understanding and control of the relevant mechanisms linking the processes occurring on ultrashort timescales to the magnetization dynamics follow­ing on longer timescales until the system settles back into a stable and addressable state is inevitable for establishing faster switching schemes.

In order to gain a comprehensive insight into the interplay of ultrafast demagnetization, spin currents, and thermal effects, and to describe the dynamic behavior on all timescales, we conduct pump–probe experiments with laser pump and x-ray magnetic circular dichroism (XMCD) or magnetic linear dichroism (XMLD) probe. The sample is excited by ultrashort (< 100 fs) laser pulses and its response stroboscopically recorded by XMCD or by magnetic microscopy using photoelectron emission microscopy (PEEM) with XMCD magnetic contrast. The focus is on multilayered samples in which materials with different spin structures are in contact to each other. This research is part of project A07 of the Collaborative Research Center/Transregio 227 "Ultrafast spin dynamics", Berlin–Halle.

Example 1: Moving magnetic domain walls by single laser pulses

The evolution of the magnetization as a function of time and space following elementary excitations is a topic of central interest in current magnetism research. We take advantage of the lateral resolution from photoelectron emission microscopy (PEEM) with XMCD and XMLD as contrast mechanism to study the lateral propagation of excitations of the spin system and its influ­ence on and interaction with magnetic domain walls. 

In the example presented here, static PEEM images show the effect of ultrashort laser excitation on a Co/Cu/Ni trilayer on Cu(001). The figure presents in the left column XMCD-PEEM images taken at the same place of a Co/Cu/Ni trilayer on Cu(001), but each after the application of one individual laser pulse of 800 nm wavelength and 14.4 mJ/cm2 fluence. Clear changes in the magnetic domain pattern are observed. They are highlighted in the images shown in the right column, which display the differences between each two successive images of the left column. Dark and white contrast indicates areas in which the local magnetization direction has changed by the laser pulse, while intermediate grey contrast indicates areas with no effect. Laser-induced domain-wall motion on distances up to the order of microns is observed, although the duration of the laser pulses is orders of magnitude shorter than the time needed for the domain walls to travel these distances under realistic assumptions for the domain-wall velocity. XMCD-PEEM pump–probe experiments on the same sample showed that the sample does not demagnetize by the laser pulses. The dependence of the effect on temperature and laser fluence revealed that the laser-induced motion of domain walls cannot be explained by a simple thermal excitation of domain-wall fluctuations, but has to be a two-step process. We suggest laser-induced depinning of a domain wall, followed by thermally activated domain wall motion, as the underlying mechanism.

These experiments have been conducted in collaboration with Jan Vogel of Institut Néel, CNRS Grenoble (France), and Sergio Valencia and Florian Kronast of Helmholtz-Zentrum Berlin für für Materialien und Energie, Berlin (Germany).

Example 2: Unidirectional steering of magnetic domain walls

The possibility to move magnetic information by ultrashort laser pulses is very attractive for future spin-based information technologies. Using XMCD-PEEM, we have found evidence for a propagation of domain walls of close to a micron in the photon-flux-density gradient of single 100 fs laser pulses away from the center of the laser spot. Ultrashort laser pulses can thus be used to steer domain walls along a defined direction.

The figure depicts on the left XMCD-PEEM images of two near-parallel domain walls in an FeGd film with small domain wall pinning. Arrows indicate the local magnetization directions. Red ellipses mark the different positions of the laser footprints on the sample set in the experiment, labeled “1”, “2, and “3”. Panel (a) shows the starting configuration. Applying one, two, and three pulses at position “1”, in the middle of the black domain, shifts the two domain walls away from the laser spot, shown in panels (b) through (d), respectively. Colored straight lines mark identical positions in each image to better see the motion of the domain walls. The result of nine laser pulses applied to position “2” is shown in panel (e). The left domain wall has now moved back to the right, even further than the position it had started. Placing the laser spot on position “3”, right of the right domain wall, did not result in further changes to the domain pattern, probably because of some stronger pinning of the domain wall at that place. The evaluation of the positions of the two domain walls along the blue axis shown in panel (a) after each laser pulse is depicted in the graph on the right hand side. The amount traveled by the domain walls decreases with the distance from the laser pulse. The maximum distance is nearly one micron. Since the excitation by the laser pulse is much shorter than the time it takes for the domain wall to travel such distances, we attribute the unidirectional steering of domain walls by the laser pulse to the inhomogeneous temperature distribution on the sample after the laser pulse, which decays much more slowly. Magnonic spin currents, induced by the spin Seebeck effect, could then in principle lead to the observed domain wall motion if reflection of magnons at the domain walls is taken into account.

These experiments have been conducted in collaboration with Mustafa Erkovan, INESC-Microsystems and Nanotechnologies, Lisboa (Portugal), Florian Kronast and Ahmet A. Ünal, Helmholtz-Zentrum Berlin für für Materialien und Energie, Berlin (Germany), Umut Parlak, Gebze Technical University, Kocaeli (Turkey), and Jan Vogel, Institut Néel, CNRS Grenoble (France).

Publications about this research:

O. Sandig, Y. A. Shokr, J. Vogel, S. Valencia, F. Kronast, and W. Kuch
Movement of magnetic domain walls induced by single femtosecond laser pulses
Phys. Rev. B 94, 054414 (2016).
DOI: 10.1103/PhysRevB.94.054414

Y. A. Shokr, O. Sandig, M. Erkovan, B. Zhang, M. Bernien, A. A. Ünal, F. Kronast, U. Parlak, J. Vogel, and W. Kuch
Steering of magnetic domain walls by single ultrashort laser pulses
Phys. Rev. B 99, 214404 (2019).
DOI: 10.1103/PhysRevB.99.214404