Voltage-Driven High-Speed Skyrmion Motion in a Skyrmion-Shift Device

Magnetic skyrmions are promising information carriers for building future high-density and high-speed spintronic devices. However, to achieve a current-driven high-speed skyrmion motion, the required driving-current density is usually very large, which could be energy inefficient and even destroy the device due to Joule heating. The mechanism of voltage-driven skyrmion motion through equidistant identical electrodes on a uniformly thick nanowire is studied. The high-speed skyrmion motion is realized by utilizing a voltage shift, and the average skyrmion velocity reaches up to 259 m/s under a 0.45-V applied voltage. In comparison with the widely studied vertical current-driven model, the energy dissipation is three orders of magnitude lower in our voltage-driven model for the same speed motion of skyrmions. Our approach uncovers valuable opportunities for building skyrmion racetrack memories and logic devices with both ultra-low power consumption and ultra-high processing speed, which are appealing features for future spintronic applications.

Figure 1

(a) Sketch of the proposed device structure. Uniform width contacts on a 100-nm-wide nanowire serve as voltage gates with equidistant spacing. The insulating layer acts as a dielectric layer for the VCMA effect and the magnetic layer is for the generation of skyrmions. The nanowire is along the x axis. (b) Different skyrmion locations and the corresponding total energy of the system. The energy profile is continuously evaluated when skyrmions are located at different positions along the x axis. (c) The variation of the total energy when a skyrmion is centered at different locations along a nanowire under voltages +V, ground (G), −V, and ground (G) applied to contacts 1 to 4, successively. The potential barrier and well are formed at contacts 1 and 3, the skyrmion located at contact 2 falls into the energy well at contact 3. The potential well is 0.98 eV when a 0.3-V voltage is applied.

Figure 2

(a) Top view of the nanowire and typical trajectory of the skyrmion shift. +V and −V are applied to the left and the right contacts, respectively, to create a positive and a negative anisotropy constant change $\mathrm{\Delta }K$, respectively. The regions with red color (blue color) represent $m_z=+1(m_z=-1)$ of the magnetic layer. The skyrmion size increases at different locations during one shift. Note that the skyrmion size is compressed in order to clearly show the evolution of its size and trajectory. (b) Evolution of instantaneous velocity during the shift. v_x and v_y are the longitudinal and transverse components of velocity obtained with anisotropic constant $K=360\mathrm{kJ}/{\mathrm{m}}^3$ and Dzyaloshinskii-Moriya interaction constant $D=1.2\mathrm{mJ}/{\mathrm{m}}^2$.

Figure 3

Long-range motion trajectory and corresponding energy profiles for each shift. Energy wells move from left to right as the applied voltages switch from shift 1 to shift 4. To avoid the time-consuming procession, voltages are switched at the moments once the skyrmions pass the centers of the contacts. These moments are marked as $t_1$ to $t_4$. $v_0$, $v_1$, $v_2$, and $v_3$ are the average velocities of each shift, respectively. For the last shift, the voltage is on until the skyrmion is stabilized.

Figure 4

(a) Skyrmion trajectories with different anisotropic constant K. $K_1=310\mathrm{kJ}/{\mathrm{m}}^3$, $K_2=335\mathrm{kJ}/{\mathrm{m}}^3$, and $K_3=360\mathrm{kJ}/{\mathrm{m}}^3$. DMI constant $D=1.2\mathrm{mJ}/{\mathrm{m}}^2$. (b) Skyrmion shift trajectories with different DMI constants D. $D_1=0.9\mathrm{mJ}/{\mathrm{m}}^2$, $D_2=1.05\mathrm{mJ}/{\mathrm{m}}^2$, and $D_3=1.2\mathrm{mJ}/{\mathrm{m}}^2$. Anisotropic constant $K=360\mathrm{kJ}/{\mathrm{m}}^3$. The shape and size of skyrmions at different moments during the shifts are simulated and presented by the insets in panels (a) and (b). The applied voltages for the left and right parts are ground and −V, respectively. (c) Side view and different energy profiles when skyrmion radius $(R_{\mathrm{sk}})$ changes with a fixed contact width l. Energy platforms appear when $R_{\mathrm{sk}}$ is much less than l/2, and it disappears when $R_{\mathrm{sk}}\approx l/2$. (d) The initial average velocity $v_0$ vs $R_{\mathrm{sk}}/l$ with different l. To realize the $R_{\mathrm{sk}}/l$ modulation, we change l, $D$_, or K [49].

Figure 5

(a) The initial average velocity $v_0$. (b) The maximum limit of long-range motion average velocity $v_L$ vs applied voltage V under different Gilbert damping constants α. Inserted images are simulated results of skyrmions indicating their size and shape at the moments when the center of the skyrmions passes the center of the contacts. For open symbols, the skyrmion diameter is calculated to be larger than contact with, i.e., $2R_{\mathrm{sk}}/l>1$, which is inapplicable to our device.

Figure 6

The normalized energy profile along the wire when a uniform square spin texture moves into a low-K region (dark gray). Red and blue arrows stand for spin-up and spin-down moments, respectively.

Figure 7

The normalized energy profile along the wire when a uniform circular shape spin down moves into a low-K region (dark gray).

Figure 8

The normalized energy profile along the wire when a skyrmion moves into a low-K region (Dark gray).

Figure 9

The energy profile changes with different skyrmion radii. The contact width is fixed at 60 nm.

Figure 10

The normalized average velocity changes with different contact widths l.