Time- and spatially-resolved magnetization dynamics driven by spin-orbit torques

Abstract: Current-induced spin-orbit torques (SOTs) represent one of the most effective ways to manipulate the magnetization in spintronic devices. The orthogonal torque-magnetization geometry, the strong damping, and the large domain wall velocities inherent to materials with strong spin-orbit coupling make SOTs especially appealing for fast switching applications in nonvolatile memory and logic units. So far, however, the timescale and evolution of the magnetization during the switching process have remained undetected. Here, we report the direct observation of SOT-driven magnetization dynamics in Pt/Co/AlO$_x$ dots during current pulse injection. Time-resolved x-ray images with 25 nm spatial and 100 ps temporal resolution reveal that switching is achieved within the duration of a sub-ns current pulse by the fast nucleation of an inverted domain at the edge of the dot and propagation of a tilted domain wall across the dot. The nucleation point is deterministic and alternates between the four dot quadrants depending on the sign of the magnetization, current, and external field. Our measurements reveal how the magnetic symmetry is broken by the concerted action of both damping-like and field-like SOT and show that reproducible switching events can be obtained for over $10^{12}$ reversal cycles.

• The paper demonstrates that current-induced spin–orbit torques enable sub-nanosecond magnetization switching via deterministic edge-nucleated domain propagation.
• It employs time-resolved x-ray imaging on Pt/Co/AlOₓ dots to capture nanoscale dynamics and validate the roles of damping-like and field-like torques.
• The results show robust switching over 10^12 cycles, highlighting the potential for efficient, reliable spintronic device applications.

An Analysis of Time- and Spatially-Resolved Magnetization Dynamics via Spin-Orbit Torques

The paper under review focuses on the study of current-induced spin-orbit torques (SOTs) as a method for manipulating magnetization in spintronic devices. It presents a comprehensive set of experiments aimed at elucidating the time and spatial dynamics of magnetization switching processes. The significance of this study lies in its potential applications in nonvolatile memory and logic devices, where efficient and rapid control of magnetization is desired.

Methodology

The authors utilize Pt/Co/AlO$_x$ dots to explore the dynamics of SOT-driven magnetization. These dots are subjected to current pulses while being observed through time-resolved x-ray imaging with a nanometer-scale spatial resolution and picosecond temporal resolution. This approach allows direct observation of the processes involved in magnetization switching, providing insights that have been elusive in prior studies relying on indirect measurements like switching probabilities.

Key Findings

1. Switching Process: The study identifies that magnetization switching can complete within sub-nanosecond timeframes through a deterministic process. This occurs via an edge-nucleated domain that propagates across the dot. The nucleation's determinism is governed by factors such as the sign of the current and external magnetic fields.
2. Role of Damping and Field-like Torques: The damping-like and field-like components of SOTs, along with the Dzyaloshinskii-Moriya interaction (DMI), play crucial roles in breaking magnetic symmetry and influencing the deterministic nature of the switching.
3. Robustness: The switching process is tested for reliability across over 10$^{12}$ cycles, indicating the robustness necessary for practical applications.
4. Domain Wall Dynamics: Observations of tilted domain wall propagation are crucial for understanding how various torques influence the switching process. The authors perform micromagnetic simulations confirming that the field-like torque influences the observed domain wall dynamics significantly.

Implications

The findings of this paper are significant both for theoretical understanding and practical implementations in spintronic device applications. The understanding of magnetization dynamics on such fine scales could lead to more efficient designs of memory devices, such as MRAMs, where data can be stored through rapid and reliable magnetization state changes.

From a theoretical perspective, the paper elucidates the interplay between different torques and magnetic interactions. It shows the crucial role that both damping-like and field-like torques have in determining how domain walls behave under current influence, challenging some earlier perceptions about the minor role of field-like torques in such systems.

Prospects for Future Research

The insights gained about the dynamics of magnetization at the nanoscale open several avenues for future research. Potential directions include:

• Exploring Material Science: Investigating different material combinations to optimize the SOT efficiency.
• Interface Engineering: Tuning material interfaces to enhance specific torque effects.
• Advanced Simulation Techniques: Developing more complex simulations that can handle interactions far beyond current capabilities.
• Device Integration: Moving towards the integration of these insights into commercial device development, emphasizing energy efficiency and longevity.

In conclusion, this paper provides a pivotal step in understanding SOT-driven magnetization dynamics, offering both immediate applications in device engineering and contributing to theoretical models of magnetic switching processes. It bridges a gap in understanding that could propel further advancements in the field of spintronics.