Ultrafast Nanomagnetic Toggle Switching of Vortex Cores

We present an ultrafast route for a controlled, toggle switching of magnetic vortex cores with ultrashort unipolar magnetic field pulses. The switching process is found to be largely insensitive to extrinsic parameters, like sample size and shape, and it is faster than any field-driven magnetization reversal process previously known from micromagnetic theory. Micromagnetic simulations demonstrate that the vortex core reversal is mediated by a rapid sequence of vortex-antivortex pair creation and annihilation subprocesses. Specific combinations of field-pulse strength and duration are required to obtain a controlled vortex core reversal. The operational range of this reversal mechanism is summarized in a switching diagram for a 200 nm Permalloy disk.

Figure 1

Schematics of a field-pulse driven vortex core switching. The vortex core magnetization can be switched by a short magnetic field pulse applied in the film plane. This switching process requires only 40–50 ps.

Figure 2

Pair-creation mediated vortex core reversal in a Py disk of 100 nm radius and 20 nm thickness. A Gaussian field pulse is applied in the disk plane, parallel to the $y$ axis. The top row shows the $x$ component of the magnetization $m_x$ in the initial state, followed by the state of the sample at the pulse maximum (“0 ps”) and then at two different times after the pulse maximum has been reached. The blue and red ribbons represent the $m_x=0$ and $m_y=0$ isosurfaces, respectively. The bottom row shows a magnification of the region where these ribbons intersect, marking the cores of the original vortex, then at 12 ps after the pulse maximum, of the newly created vortex-antivortex pair, and finally of the remaining vortex core. This core has opposite polarization with respect to the initial vortex core, indicated by the yellow color on the underlying cut plane representing the $z$ component of the magnetization $m_z$. The green and orange cylindrical ribbons are the isosurfaces where $m_z=0.8$ and $m_z=-0.8$, respectively.

Figure 3

Topography of $m_z$ during the core switching process. (a) Static equilibrium. The cutline on the upper right shows the small negative dip in $m_z$ near the core. (b) Formation of a large negative dip of $m_z$. The cutline goes along the diameter through the vortex core and the adjacent dip. (c) Pair creation leads to two separate points with $m_z=-1$. (d) The original vortex core annihilates with the new antivortex. (e) Spin waves are emitted after the annihilation. Finally, a vortex core with opposite polarization remains (f). The insets on the lower left show the $x$ component of the magnetization (red: $m_x=1$, blue: $m_x=-1$). The times are relative to the pulse maximum.

Figure 4

Switching diagram for the pulse parameters leading to a toggle switch of the vortex core in a 100 nm radius and 20 nm thick Py disk. The pulse duration is quantified by its width $\sigma$ and the strength by its maximum value. The cell size was 3 nm. It was varied by $\pm 0.7\mathrm{nm}$ at $80\mathrm{mT}/20\mathrm{ps}$ (diagram edge), giving a variation in the field required to switch the core of $+5.0\mathrm{mT}$ for 2.3 nm cells and $-5.6\mathrm{mT}$ for 3.7 nm cells.