8:25 AM - *NM04.01.03
Stability and Ultrafast Topological Switching of Magnetic Skyrmions
Magnetic skyrmions are long-lived topological excitations in a small subset of magnetic materials, such as some heavy-metal / ferromagnet multilayers with strong interfacial spin-orbit interactions. These materials can host a high density of skyrmions without any topological counterparts, which defines these states as topological phases with a large net topological charge. A transition from a topological trivial state to a skyrmion state is hence considered a topological phase transitions. Such topological phase transitions, which involve the nucleation or annihilation of magnetic skyrmions, are in principle allowed due to the discrete nature of the lattice. However, they are still suppressed by the same strong energy barriers that also allow skyrmions to exist at room temperature . Topological phase transitions are therefore expected to be of first order, with hysteresis and a transition dynamics characterized by slow and heterogeneous nucleation, as known, e.g., from freezing of water. In fact, we find that heterogeneous nucleation is the underlying principle of electrical skyrmion nucleation, which crucially relies on materials defects to break the translational symmetry .
Surprisingly, however, we find that picosecond homogeneous nucleation of an extended topological phase, comprising a dense array of nanometer-scale magnetic skyrmions, can be induced by a single femtosecond laser pulse.
In this talk, after giving an introduction to the stability  and the spin-orbit torque nucleation  of magnetic skyrmions, I will discuss the nucleation dynamics of this all-optical topological phase transition, which we were able to follow in real time during the early user operation of beamline SCS at the European XFEL . Using time-resolved small angle x-ray scattering, we discovered that rapid, homogeneous nucleation of the skyrmion phase is mediated by a previously undisclosed transient fluctuation state. This state, which is characterized by high spatial frequency magnetic fluctuations, persists for approximately 100 ps after exciting our magnetic multilayer with a femtosecond, infrared laser pulse. The topological phase emerges from these fluctuations by nucleation and coalescence, a mechanism that goes well beyond existing theories of topological phase transitions such as the Kibble–Zurek mechanism and the Berezinskii–Kosterlitz–Thouless transition. The process is completed on a time scale of 300 ps. Using atomistic spin dynamics simulations, we confirm that the fluctuation state is key to the ultrafast increase of the global topological charge, enabled by an almost complete elimination of the topological energy barrier in this transient state of matter.
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