Room-temperature current-induced generation and motion of sub-100nm skyrmions (1702.04616v1)

Abstract: Magnetic skyrmions are nanoscale windings of the spin configuration that hold great promise for technology due to their topology-related properties and extremely reduced sizes. After the recent observation at room temperature of sub-100 nm skyrmions stabilized by interfacial chiral interaction in magnetic multilayers, several pending questions remain to be solved, notably about the means to nucleate individual compact skyrmions or the exact nature of their motion. In this study, a method leading to the formation of magnetic skyrmions in a micrometer-sized nanotrack using homogeneous current injection is evidenced. Spin-transfer-induced motion of these small electricalcurrent-generated skyrmions is then demonstrated and the role of the out-of-plane magnetic field in the stabilization of the moving skyrmions is also analysed. The results of these experimental observations of spin torque induced motion are compared to micromagnetic simulations reproducing a granular type, non-uniform magnetic multilayer, in order to address the particularly important role of the magnetic inhomogeneities on the current-induced motion of sub-100 nm skyrmions, for which the material grains size is comparable to the skyrmion diameter.

• The paper demonstrates the nucleation and current-driven motion of sub-100nm skyrmions in Pt/Co/Ir multilayers at room temperature.
• It employs magnetic force microscopy and micromagnetic simulations to correlate skyrmion velocity with current density and out-of-plane magnetic fields.
• The study highlights grain-boundary pinning challenges and proposes material optimizations for enhanced low-power spintronic device performance.

Room-Temperature Current-Induced Generation and Motion of Sub-100nm Skyrmions

Magnetic skyrmions have emerged as entities of interest due to their unique topological properties and potential applications in spintronic devices. These nanoscale magnetic configurations, particularly those stabilized by Dzyaloshinskii–Moriya interaction (DMI) in non-centrosymmetric magnetic multilayers, offer promising paths for high-density storage and novel computational paradigms. The paper under review investigates methods to generate and manipulate sub-100nm skyrmions at room temperature using current-induced processes, analyzing the skyrmions' motion and interactions with material inhomogeneities.

Methodology and Findings

The paper demonstrates the nucleation of compact sub-100nm skyrmions through the injection of homogeneous current into micrometer-wide nanotracks, exploiting the inherent structural and magnetic characteristics of the Pt/Co/Ir multilayers. The paper effectively illustrates how spin-torque results in the mobility of skyrmions, supported by experimental observations using magnetic force microscopy (MFM) and accompanying micromagnetic simulations. Through a sequence of controlled current pulses, the authors map the dynamic behavior resultant from spin-orbit torques, analyzing the conditions under which skyrmions can be stable or mobile within these artificial confinements.

In addition to the application of currents, the paper explores the role of an out-of-plane magnetic field in stabilizing moving skyrmions, correlating the field's strength to changes in skyrmion morphology and movement patterns. Here, the authors quantitatively correlate skyrmion velocity and current density, noting distinct regime shifts influenced by skyrmion diameter relative to DMI and thermal effects induced by current pulses.

Implications and Comparisons

The implications for technological applications are significant, as skyrmions present avenues for energy-efficient non-volatile memory and logic devices due to their stability and low-power control traits. However, the paper identifies the pinning effects induced by grain boundaries in granular magnetic layers as potential challenges, highlighting the importance of minimizing material inhomogeneities for smoother skyrmion motion and efficient device operation.

The skyrmion velocities recorded in this paper tend to be lower compared to those reported using different material systems, a result of pronounced pinning effects and the specific multilayer construction. Such findings suggest material design enhancements targeted at reducing the variability of magnetic parameters such as DMI and anisotropy are crucial. Furthermore, the simulations effectively show that overcoming the pinning effects results in a recovery of skyrmion mobility, underscoring the significance of designing materials with optimally tuned granular features or alternative confinement strategies.

Future Directions

The results achieved open further discussion for exploring novel material stacks that potentially present less pinning and higher intrinsic skyrmion velocities. Incorporating alternative heavy metals or adapting interface designs could be crucial for tuning the effective spin Hall angles and damping traits in multilayers. Future computational work may explore the complex interplay of thermal gradients with skyrmion dynamics under varied environmental settings, offering potential insights into optimizing conditions for skyrmion logic and memory applications.

Conclusively, the paper adeptly furthers the understanding of skyrmion manipulation, providing a springboard for the continued exploration into robust skyrmion-based technologies. The interplay of experiment and simulation showcased within enables a detailed understanding of fundamental skyrmion dynamics within current-driven scenarios, marking a step forward in spintronic device engineering.