Current-induced skyrmion generation and dynamics in symmetric bilayers

Abstract: Magnetic skyrmions are textures behaving as quasiparticles which are topologically different from other states. Their discovery in systems with broken inversion symmetry sparked the search for materials containing such magnetic phase at room temperature. Their topological properties combined with the chirality-related spin-orbit torques make them interesting objects to control the magnetization at nanoscale. Here we show that a pair of coupled skyrmions with the same topological charge and opposite chiralities can be stabilized in a symmetric magnetic bilayer system by combining Dzyaloshinskii-Moriya interaction (DMI) and dipolar coupling effects. This effect opens a new path for skyrmion stabilization with much lower DMI. We then demonstrate in a single device with two different electrodes that such skyrmions can be efficiently and independently written and shifted by electric current at large velocities. The skyrmionic nature of the observed quasiparticles is further confirmed by using the gyrotropic force as a topological filter. These results set the ground for emerging spintronic technologies where issues concerning skyrmion stability, nucleation, and propagation are paramount.

• The paper shows that skyrmions can be generated and stabilized at room temperature using a balance of Dzyaloshinskii–Moriya interaction and dipolar coupling.
• Magnetic force microscopy and energy profile calculations confirm the transition from worm-like magnetic domains to well-defined skyrmions.
• Current pulses drive skyrmion motion at speeds up to 60 m/s, highlighting their potential for innovative memory and spintronic devices.

Current-Induced Skyrmion Generation and Dynamics in Symmetric Bilayers: An Overview

The paper presents a thorough investigation into the generation, stabilization, and dynamics of skyrmions in a symmetric bilayer system characterized by specific magnetic properties and interactions. The study focuses on skyrmions, which are fascinating topological magnetic structures occurring in materials with broken inversion symmetry. The emphasis is on leveraging these structures for potential data storage and spintronic applications due to their unique properties.

Key Findings and Methods

This research demonstrates that skyrmions can be generated and manipulated at large velocities using electric currents in a symmetric bilayer system comprising two ferromagnetic layers separated by a non-magnetic spacer. The core aspects of the study are:

1. Skyrmion Stabilization: The paper addresses a significant challenge in skyrmion research—ensuring stability at room temperature—by utilizing a combination of Dzyaloshinskii-Moriya interaction (DMI) and dipolar coupling effects. The authors show that these interactions, particularly the dipolar coupling in symmetrically coupled layers, can stabilize coupled skyrmions with opposite chiralities and identical topological charges without necessitating a large DMI.
2. Material Setup: The experiments use a stack of Pt/FM/Au/FM/Pt layers, where FM indicates specific ferromagnetic materials. The careful selection and arrangement of these layers allow the dipolar interactions to thrive, optimizing skyrmion stability without demanding exceptionally large DMI values.
3. Experimental Verification: The study employs magnetic force microscopy (MFM) and calculated energy profiles to confirm the presence and stability of skyrmion structures. These insights into skyrmion formation illustrate the transition from worm-like magnetic domains to isolated skyrmions under applied magnetic fields.
4. Skyrmion Dynamics: The dynamics of skyrmions are explored through their controlled motion within a device using current pulses. The authors report skyrmion velocities of up to 60 m/s, indicating efficient manipulation using the spin Hall effect. Additionally, the work illustrates the gyrotropic forces experienced by skyrmions due to their topological nature, influencing their directional motion even within confining geometries.
5. Independent Writing and Shifting: The innovative setup permits independent writing and shifting of skyrmions, an essential capability for developing memory storage technologies.

Implications and Future Directions

This study's implications for spintronics are profound, offering an elegant solution to the challenge of skyrmion stabilization and controllability in practical devices. The method proposed provides material flexibility, circumventing the limitation of needing a high DMI, thus expanding the pool of potential materials applicable for skyrmion-based technologies.

The work paves the way for practical device implementations, utilizing the combination of DMI and dipolar interactions in bilayer systems. As skyrmion-based applications progress, such devices could play a crucial role in spintronic memories and logic applications, promising higher efficiency and miniaturization.

Future research may focus on optimizing material characteristics and device architectures to further enhance skyrmion stabilization and kinetic control. Additionally, exploring interactions within more complex multilayer systems might unravel new dynamics and stabilization mechanisms, enriching the theoretical and practical understanding of skyrmion physics in nanoscale systems.

Overall, this paper provides significant insights and contributions to the ongoing exploration of skyrmions in spintronic applications, presenting a viable path for future technological advancements.