Field-free deterministic ultra fast creation of skyrmions by spin orbit torques (1705.01927v1)

Abstract: Magnetic skyrmions are currently the most promising option to realize current-driven magnetic shift registers. A variety of concepts to create skyrmions were proposed and demonstrated. However, none of the reported experiments show controlled creation of single skyrmions using integrated designs. Here, we demonstrate that skyrmions can be generated deterministically on subnanosecond timescales in magnetic racetracks at artificial or natural defects using spin orbit torque (SOT) pulses. The mechanism is largely similar to SOT-induced switching of uniformly magnetized elements, but due to the effect of the Dzyaloshinskii-Moriya interaction (DMI), external fields are not required. Our observations provide a simple and reliable means for skyrmion writing that can be readily integrated into racetrack devices.

• The paper demonstrates the deterministic creation of skyrmions with subnanosecond SOT pulses targeted at defects in racetrack devices.
• It employs micromagnetic simulations and X-ray holography to validate skyrmion nucleation at current densities exceeding 6.5×10^11 A/m² and velocities up to 8 m/s.
• The findings pave the way for advanced racetrack memory technology by eliminating external magnetic fields and enabling precise control through engineered defects.

Overview of Field-Free Deterministic Ultra-Fast Creation of Skyrmions by Spin Orbit Torques

The paper presents a comprehensive paper on the deterministic and ultra-fast generation of magnetic skyrmions utilizing spin orbit torques (SOT) in an applied-field-free environment. This research addresses a significant technological need for controlled skyrmion writing in racetrack memory devices, marking a departure from external field dependencies.

Summary of Findings

The researchers demonstrate the deterministic creation of individual skyrmions using subnanosecond SOT pulses, precisely targeted at defects within magnetic racetracks. The defects can be either naturally occurring or artificially induced such as constrictions, making the method highly adaptable across different device configurations. The central mechanism leverages the Dzyaloshinskii-Moriya interaction (DMI), which enables the generation of skyrmions without the need for external magnetic fields. This is akin to the SOT-induced switching seen in uniformly magnetized elements, but with enhanced control and reliability.

The paper elucidates on micromagnetic simulations demonstrating that SOT pulses can nucleate skyrmions at predefined positions and then shift them using the same current path. Specifically, the research shows that skyrmions can be generated in the presence of pre-existing pinning sites, exhibiting that the pulse amplitudes and durations can be finely tuned to dictate the nucleation and movement characteristics. These results were corroborated through experimental setups utilizing X-ray holography, which revealed that high current densities are adept at producing skyrmions in multilayer stacks with engineered DMI at interfaces like Pt/CoFeB.

Key Insights and Strong Numerical Results

• The experiment confirms that skyrmions with deterministic placements can be nucleated reliably at high current densities, exceeding approximately $6.5 \times 10^{11} \text{A/m}^2$.
• Simulations forecast the skyrmion generation and traverse pathways, with a demonstrated velocity of up to \SI{8}{m/s} using current pulses, reflecting an effective Hall effect without significant deviations under standard conditions.
• The paper identifies a critical pulse width ($\tau$) and current density (j) relationship, mapping out the switching regimes via angular momentum integrations consistent with SOT switching paradigms.

Implications and Future Prospects

Practically, the ability to localize skyrmion creation without external magnetic fields is a substantial advancement for racetrack memory technologies, enhancing their feasibility and integration into existing computer architectures. The achievement simplifies device architecture since skyrmion generators can be embedded as simple microstructural features like notches or constrictions.

Theoretically, the findings open pathways for more profound explorations into topological magnetic states and their manipulation under highly controlled conditions. The enhanced understanding of DMI and SOT dynamics in skyrmion behavior can catalyze advancements in spintronics, refining concepts for high-density, ultrafast non-volatile memory applications.

Prospects include optimizing material engineering to further lower the energetic requirements for skyrmion manipulation and enhancing stability against naturally occurring defects. Moreover, continued refinement of model parameters against experimental constraints, such as temperature variations affecting current requirements, will strengthen the precision of skyrmion operations in practical deployment.

Research is recommended to extend these findings to complex geometries and to develop integrated systems for real-world applications, focusing on scalability and robustness against environmental perturbations. Future explorations might also explore multi-dimensional racetrack configurations and attempt to achieve asymmetry-controlled skyrmion trajectorial configurations without relying on fixed constraints.