Influence of magnetic fields on ultrafast laser-induced switching dynamics in Co/Gd bilayers

M. J. G. Peeters, Y. M. van Ballegooie, and B. Koopmans

Phys. Rev. B 105, 014429 – Published 26 January 2022

Abstract

Recently it has been shown that not only GdFeCo alloys exhibit single-pulse helicity-independent all-optical switching, but that this effect is also seen in Co/Gd bilayers. However, there have been no reports on the explicit time dynamics of the switching process in these bilayers as of yet. Furthermore, time-resolved measurements of switching of other materials are typically done with a constant applied field to reset the magnetization between consecutive pulses and thus ensure repeatable behavior. In this paper we experimentally resolve the explicit dynamics of the switching process in Co/Gd, and the influence of applied magnetic fields on the switching process. We observe that after a switch within several picoseconds, the magnetization switches back at a time scale of hundreds of picoseconds. This backswitch includes a strong dependence on the magnetic field strength even at subtesla fields, significantly smaller than the exchange fields that govern the switching dynamics. This surprising behavior is explained by a combination of longitudinal switching (on a picosecond timescale), precessional switching (on a nanosecond time scale), and domain-wall motion (on a timescale of $10 n s$ and beyond). We discuss these different switching regimes and their relative importance using simple model calculations.

Received 31 May 2021
Revised 5 November 2021
Accepted 20 December 2021

DOI:https://doi.org/10.1103/PhysRevB.105.014429

Physics Subject Headings (PhySH)

Magnetization switching Ultrafast magnetization dynamics

Magnetic multilayers

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

M. J. G. Peeters^*, Y. M. van Ballegooie, and B. Koopmans

Department of Applied Physics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

^*m.j.g.peeters@tue.nl

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Vol. 105, Iss. 1 — 1 January 2022

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Images

Figure 1
(a) Kerr microscope images of AOS in Co/Gd. Dark and light contrast corresponds to up and down magnetization, the dashed line indicates the region where the local fluence is high enough to reverse the magnetization. A switched domain is observed depending on whether the total number of pump pulses is even or odd. The labels indicate the (approximate) number of pulses for each image. The measurements are done with a repetition rate of $125 k Hz$ . (b) Field-free time-resolved measurement of AOS in Co/Gd, by measuring only half the pulses, at $100 k Hz$ . Using only the odd or even pulses corresponds to measuring the up-down or down-up switch, respectively. Note the axis break on the $x$ axis.
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Figure 2
(a) Time-resolved measurements of AOS with an applied magnetic field. The constant field is applied in the positive $z$ direction. (b) Hysteresis loops with and without pump. The black line shows the hysteresis loop without pump, the red and blue lines show the hysteresis loops with pump and the delay fixed at $- 5$ and $10 p s$ , respectively. (c) Schematic depiction of the three switching mechanisms examined in this paper: longitudinal switching, precessional switching, and switching via domain-wall motion.
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Figure 3
M3TM calculations. (a) $m_{z}$ after laser pulse excitation with a fluence below the threshold fluence. (b) $m_{z}$ after excitation with a laser pulse above the threshold fluence. (c) $m_{z}$ after excitation with a laser pulse above the threshold fluence and a magnetic field in the positive $z$ direction strong enough to result in no switch. (d) Phase plot of the magnetization direction after $100 p s$ as a function of applied field and the laser fluence. The three regions correspond to the traces shown in (a), (b), and (c).
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Figure 4
LLG calculations. (a) $z$ magnetization as a function of delay for various magnetic fields, pointing in the positive $z$ direction. The angle between the applied field and the $z$ axis is $θ しーた = 0.05 rad$ . (b) $z$ magnetization as a function of delay for various values of $θ しーた$ . The applied field is ${μ みゅー}_{0} H = 1200 m T$ .
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Figure 5
Schematic depiction of the cycles that can appear in the switching process due to domain-wall motion and AOS, with time going from left to right. The black arrows indicate the direction of DW motion due to the applied field. (a) Cycle for an applied field strong enough to fully reverse the magnetization between two pulses. (b) Cycle for a smaller field, resulting in a longer cycle.
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