Skyrmion Hall Effect Revealed by Direct Time-Resolved X-Ray Microscopy (1608.07216v2)

Abstract: Magnetic skyrmions are highly promising candidates for future spintronic applications such as skyrmion racetrack memories and logic devices. They exhibit exotic and complex dynamics governed by topology and are less influenced by defects, such as edge roughness, than conventionally used domain walls. In particular, their finite topological charge leads to a predicted "skyrmion Hall effect", in which current-driven skyrmions acquire a transverse velocity component analogous to charged particles in the conventional Hall effect. Here, we present nanoscale pump-probe imaging that for the first time reveals the real-time dynamics of skyrmions driven by current-induced spin orbit torque (SOT). We find that skyrmions move at a well-defined angle {\Theta}{SH} that can exceed 30{\deg} with respect to the current flow, but in contrast to theoretical expectations, {\Theta}{SH} increases linearly with velocity up to at least 100 m/s. We explain our observation based on internal mode excitations in combination with a field-like SOT, showing that one must go beyond the usual rigid skyrmion description to unravel the dynamics.

• The paper demonstrates that skyrmions exhibit a linearly increasing Hall angle with velocity, exceeding 30 degrees under current.
• It employs pump-probe X-ray microscopy on a Pt/CoFeB/MgO multilayer to visualize dynamic skyrmion behavior beyond static imaging methods.
• Incorporating both damping-like and field-like spin-orbit torques in simulations yields results that better align with experimental observations, informing future device design.

Insights into Skyrmion Hall Effect via Time-Resolved X-Ray Microscopy

The paper "Skyrmion Hall Effect Revealed by Direct Time-Resolved X-Ray Microscopy" presents a comprehensive paper into the dynamic behavior of magnetic skyrmions within a spintronic context, leveraging advanced X-ray microscopy techniques to achieve unprecedented temporal and spatial resolution. The primary focus is on elucidating the real-time dynamics of skyrmions, particularly addressing the influence of spin-orbit torques (SOT) and the Skyrmion Hall Effect within these nanoscale systems.

Magnetic skyrmions are promising candidates for future spintronic devices, serving both in memory and logic applications due to their robust topological properties and reduced sensitivity to material imperfections. The skyrmion Hall effect—analogous to the conventional Hall effect in charged particles—entails a transverse motion of skyrmions induced by current, which adds complexity to their incorporation into devices. This work breaks new ground by visualizing these dynamics in real time using pump-probe X-ray microscopy, offering insights beyond conventional static imaging methods.

The authors describe experiments performed with a Pt/CoFeB/MgO multilayer stack, chosen for its strong perpendicular magnetic anisotropy (PMA) and significant Dzyaloshinskii-Moriya interaction (DMI), which stabilize skyrmions under ambient conditions. The dynamic measurements of skyrmion behavior indicate that, contrary to earlier theoretical predictions, the skyrmion Hall angle increases linearly with velocity, reaching significant angles exceeding 30 degrees. This behavior could not be fully explained by existing micromagnetic simulations that include only damping-like SOT.

An intriguing aspect of the paper is the role of field-like (FL) SOTs, which the authors find to have a more substantial influence on skyrmion dynamics than previously thought. While models typically employ the rigid skyrmion approximation, incorporating FL-SOTs alongside deformation and excitation modes into simulations yielded a better alignment with experimental observations. The FL-SOTs, as detailed, increase the Hall angle as the skyrmion velocity rises, influencing their trajectory. The authors argue for the necessity of complex dynamic models that take these internal doses of freedom into account to accurately predict skyrmion behavior in devices.

The implications of these findings are substantial. The results highlight the nuanced interaction between skyrmion dynamics and the SOTs acting upon them, underscoring the importance of considering both DL and FL-SOT components in modeling. This understanding opens potential pathways for improved device designs that leverage skyrmion dynamics for high-density, efficient information processing applications, possibly even refining racetrack memory devices where the controllable motion of skyrmions is key.

Looking ahead, the paper suggests several directions for future work: further refinement of the theoretical models to incorporate temperature effects on skyrmion deformation; exploration of material systems where SOT and skyrmion interactions can be precisely controlled; and advancing imaging techniques to resolve even finer temporal and spatial skyrmionic features.

Overall, this research provides a critical step forward in our understanding of skyrmion dynamics under the influence of current-driven forces in magnetic materials, contributing fundamental insights that could facilitate their integration into future spintronic technologies.