Circular motion of non-collinear spin textures in Corbino disks: Dynamics of Néel- versus Bloch-type skyrmions and skyrmioniums

TL;DR

The paper tackles circular skyrmion dynamics in Corbino disks under spin-orbit torques and spin-transfer torques, contrasting Néel and Bloch textures and extending to skyrmioniums. It combines analytical Thiele-equation modeling with micromagnetic simulations to reveal helicity-dependent antagonistic dynamics under SOT, where Bloch skyrmions rotate with radial currents and Néel skyrmions are edge-bound, while the opposite occurs for tangential current. Skyrmioniums, being topologically trivial, move along the SOT force and can reach higher rotation frequencies with reduced edge interactions, presenting an attractive path for high-frequency skyrmion-based nano-oscillators. The STT case shows helicity-insensitive circular motion for skyrmions but edge trapping for skyrmioniums, underscoring the nuanced roles of topology and current type in device design. Overall, the work provides a comprehensive framework for selecting textures and current injection schemes in Corbino-disk spintronic applications, including MHz-frequency skyrmion-based oscillators and memory elements.

Abstract

Magnetic skyrmions are nano-scale magnetic whirls that can be driven by currents via spin torques. They are promising candidates for spintronic devices such as the racetrack memory, where a motion along the uniform current is typically desired. However, for spin torque nano-oscillators in Corbino disks, the goal is to achieve a circular motion, perpendicular to the radially applied current. As we show, based on analytical calculations and micromagnetic simulations, Bloch skyrmions engage in a circular motion with frequencies in the MHz range when driven by spin-orbit torques. In contrast, Néel skyrmions get stuck at the edges of the disk. Our analysis reveals that the antagonistic dynamics between Bloch- and Néel-type magnetic textures arise from their helicity. Furthermore, we find that skyrmioniums, which are topologically trivial variations of skyrmions, move even faster and allow an increase in the current density without being pushed toward the edges of the disk. When driven by spin-transfer torques instead, Bloch and Néel skyrmions no longer exhibit different dynamics. Instead, they move along a circular trajectory due to the skyrmion Hall effect caused by their topological charge. Consequently, the topologically trivial skyrmioniums inevitably become trapped at the disk edge in this scenario. To provide a comprehensive understanding, our study also examines currents applied tangentially, further enriching our insights into skyrmion dynamics and appropriate current injection methods for skyrmion-based devices.

Figures (7)

• Figure 1: Technologically relevant scenarios of skyrmion motion driven by spin torques in Corbino disk geometries, as discussed in this work. (a) Spin-orbit torque scenario: This panel schematically illustrates the interface between ferromagnetic (FM) and heavy metal (HM) layers. As shown in the inset, a radial charge current (blue arrow) in the HM layer induces a spin current directed towards the FM layer, resulting from the spin Hall effect. Consequently, a tangential spin polarization (orange arrows) arises in the FM layer. The spin current induces a torque, allowing a skyrmion to engage in a circular motion along the disk's edges at MHz frequency, as detailed in Sec. \ref{['section:SOTskyrmion']}. (b) Spin transfer torque scenario: This panel schematically displays the charge current (blue arrows) flowing radially through the FM layer and through the skyrmion, as shown in the inset. The radial flow of conduction electrons induces a torque that facilitates the circular motion of skyrmions, as elaborated in Sec. \ref{['subsection:STT']}.
• Figure 2: Illustrative comparison of two types of magnetic skyrmions. Schematic representation of (a) Néel skyrmion and (b) Bloch skyrmion. Arrows correspond to the orientation of magnetic moments, which is also encoded by the color. Out-of-plane: Black and white. In-plane: color spectrum depending on the in-plane angle.
• Figure 3: Dynamics of Néel and Bloch skyrmions under radial currents giving rise to a tangential spin polarization. (a) Schematic representation of Néel and Bloch skyrmions subjected to spin currents with tangential polarization. Néel skyrmions are influenced by a radial SOT force (green arrow), while Bloch skyrmions experience a tangential SOT force (red arrow). (b) and (c) show the trajectories of Néel and Bloch skyrmions, respectively, obtained by micromagnetic simulations for 20 ns for (b) and 200 ns for (c). The marker at the lower left corner of subplot (c) indicates the time duration for completing one loop around the disk with $\Delta t \approx 131.6$ ns for the Bloch skyrmion.
• Figure 4: Dynamics of Néel and Bloch skyrmions under tangential currents generating a radial spin polarization. (a) Schematic representation of Néel and Bloch skyrmions subjected to spin currents with radial polarization. Néel skyrmions experience a tangential SOT force (green arrow), while Bloch skyrmions are influenced by a radial SOT force (red arrow). (b) and (c) show the trajectories of Néel and Bloch skyrmions, respectively, obtained by micromagnetic simulations for 100 ns for (b) and 60 ns for (c). The marker at the lower left corner of subplot (b) indicates the time duration for completing one loop around the disk with $\Delta t \approx 25.81$ ns for the Néel skyrmion.
• Figure 5: Illustrative comparison of two types of magnetic skyrmioniums. Schematic representation of (a) Néel-type skyrmionium and (b) Bloch-type skyrmionium. Arrows correspond to the orientation of magnetic moments like in Fig. \ref{['fig1:skyrmions']}.