Current-induced viscoelastic topological unwinding of metastable skyrmion strings

Abstract

In the MnSi bulk chiral magnet, magnetic skyrmion strings of 17 nm in diameter appear in the form of a lattice, penetrating the sample thickness, 10–1000 μm. Although such a bundle of skyrmion strings may exhibit complex soft-matter-like dynamics when starting to move under the influence of a random pinning potential, the details remain highly elusive. Here, we show that a metastable skyrmion-string lattice is subject to topological unwinding under the application of pulsed currents of 3–5 × 106 A m–2 rather than being transported, as evidenced by measurements of the topological Hall effect. The critical current density above which the topological unwinding occurs is larger for a shorter pulse width, reminiscent of the viscoelastic characteristics accompanying the pinning-creep transition observed in domain-wall motion. Numerical simulations reveal that current-induced depinning of already segmented skyrmion strings initiates the topological unwinding. Thus, the skyrmion-string length is an element to consider when studying current-induced motion.

Motion tracking of 80-nm-size skyrmions upon directional current injections

“…To understand the mechanism of extinction of metastable SkX with the current flows in the minus direction, the following should be taken into account: At lower temperatures, the nontopological state, such as helical/conical phase, is thermodynamically stable (see Fig. 1B); therefore, the metastable SkX as produced by pulsed current may be irreversibly destroyed by external stimuli (19). A numerical simulation of a skyrmion creation and extinction has been reported (7,12).…”

“…Thus, the skyrmion is a promising candidate for future application to energy-efficient electronic devices. In the context of currentdriven racetrack-type skyrmion memory, the isolated skyrmion, which can be manipulated in a metastable state (9,(16)(17)(18)(19), is of more importance than the skyrmion crystal (SkX) that is thermodynamically stabilized in chiral-/polar-lattice magnets with the intrinsic Dzyaloshinskii-Moriya interaction (DMI) (2,3,6). Thus, the interfaces between the heavy-metal layer and the ultrathin FM layer hosting isolated skyrmions caused by the interfacial DMI have been investigated extensively (10)(11)(12)(13)(14).…”

Theory of current-driven skyrmions in disordered magnets

“…In that case, the skyrmion becomes a string, the end points of which are monopole and anti-monopole. The dynamics of this string and (anti)monopole in the presence of disorder will offer a rich physics including the manipulation of metastable states 53 .…”

Increased lifetime of metastable skyrmions by controlled doping

“…These Bloch points act as sources or sinks of topological charge, allowing the skyrmion tubes to unwind into the competing conical or helical states [21]. It has been speculated that a key factor in the lifetime of the metastable state is that these Bloch points are strongly pinned by defects and disorder present in the underlying crystal lattice [12], propagating only by thermally-assisted creep motion [17], in a manner similar to the propagation of magnetic domain walls [22][23][24][25]. This effect offers a potential route to engineer the lifetime of metastable skyrmions via doping.…”