Generation of single skyrmions by picosecond magnetic field pulses

We numerically demonstrate an ultrafast method to create single skyrmions in a collinear ferromagnetic sample by applying a picosecond (effective) magnetic field pulse in the presence of Dzyaloshinskii-Moriya interaction. For small samples the applied magnetic field pulse could be either spatially uniform or nonuniform while for large samples a nonuniform and localized field is more effective. We examine the phase diagram of pulse width and amplitude for the nucleation. Our finding could ultimately be used to design future skyrmion-based devices.

Figure 1

Schematic representation of the simulated system. We consider a nanodisc of radius $R$ and thickness $N$ atomic layers, with an underlying square lattice structure. Direction of the Gaussian magnetic field pulse is defined by the polar angle $\theta$ and azimuthal angle $\varphi$.

Figure 2

Time evolution snapshots of the magnetic spin configuration induced by a single 20 ps Gaussian magnetic field pulse normal to a circular nanodisc of $100a\times 3a$. (a) The initial state is a ferromagnetic state in the upward direction. (b) The maximum peak of a Gaussian magnetic field pulse arrives at $t=0$, and a domain with in-plane spin direction emerges from the edge. (c) Remaining domain in the center has circular form, while the edge area shows reversed magnetization. (d)–(i) After passing of the pulse the shape of the center domain gradually changes from a circular to a cross shape, as a result of interference of spin waves. (j) Ultimately one chiral Bloch type skyrmion is stabilized in the center of the nanodisc.

Figure 3

Time dependent energy contributions and topological charge $Q$, in a circular nanodisc of $100a\times 3a$, during a spatially uniform magnetic field pulse with 3.5 T amplitude and 20 ps pulse width. (a) The Zeeman energy (blue dotted line) varies together with the changing of the magnetic field pulse until the pulse has completely passed the system around 50 ps. The green line indicates the variation of the DMI energy. The red line shows the change of the anisotropy energy. (b) Temporal evolution of the exchange energy and the total energy. (c) $Q$ changes from $Q\sim 0$ to $Q\sim +1$ as the maximum peak of the Gaussian magnetic field pulse arrives at the system (vertical dashed line). Black line indicates the Gaussian magnetic field pulse.

Figure 4

Phase diagram for the skyrmion nucleation induced by a magnetic field pulse: (a) Atomistic simulation of single skyrmion generation by applying a uniform and perpendicular $\theta =0^{\circ }$ magnetic field pulse on a nanodisc with a size of $100a\times 3a$. Two vertical dashed lines indicate the minimum and maximum values of $B_p$ for the nucleation of a skyrmion. b) Micromagnetic simulation of single (multiple) skyrmion generation, region I (II), by applying a localized and tilted magnetic field $\theta ={45}^{\circ }$ to a CoPt disk of 500 nm. See also movies in the Supplemental Material [15].