Current-driven dynamics of chiral ferromagnetic domain walls (1302.2257v1)

Abstract: In most ferromagnets the magnetization rotates from one domain to the next with no preferred handedness. However, broken inversion symmetry can lift the chiral degeneracy, leading to topologically-rich spin textures such as spin-spirals and skyrmions via the Dzyaloshinskii-Moriya interaction (DMI). Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy metal and an oxide, the DMI stabilizes chiral domain walls (DWs) whose spin texture enables extremely efficient current-driven motion. We show that spin torque from the spin Hall effect drives DWs in opposite directions in Pt/CoFe/MgO and Ta/CoFe/MgO, which can be explained only if the DWs assume a N\'eel configuration with left-handed chirality. We directly confirm the DW chirality and rigidity by examining current-driven DW dynamics with magnetic fields applied perpendicular and parallel to the spin spiral. This work resolves the origin of controversial experimental results and highlights a new path towards interfacial design of spintronic devices.

• The paper demonstrates that chiral ferromagnetic domain walls achieve efficient current-driven motion via spin Hall-effect torques.
• The paper quantifies domain wall motion with spin-torque efficiencies of 120 Oe/10¹¹ A/m² in Pt/CoFe/MgO and 170 Oe/10¹¹ A/m² in Ta/CoFe/MgO stacks.
• The paper employs harmonic and Slonczewski-like torque measurements to uncover material-dependent differences, informing future spintronic device design.

Analysis of Current-driven Dynamics in Chiral Ferromagnetic Domain Walls

The paper "Current-driven dynamics of chiral ferromagnetic domain walls," authored by Satoru Emori et al., presents a nuanced paper of the underlying dynamics of domain walls (DWs) in ultrathin metallic ferromagnets. These ferromagnets are characterized by their chiral features due to broken inversion symmetry, leading to spin textures stabilized by Dzyaloshinskii-Moriya Interaction (DMI). The authors investigate the efficiency of current-driven motion of chiral DWs in stacks comprising Pt/CoFe/MgO and Ta/CoFe/MgO, thereby exploring both theoretical and practical implications for spintronic device engineering.

Key Findings and Methodology

1. Domain Wall Chirality and Spin Torque: The paper unequivocally proves that in Pt/CoFe/MgO and Ta/CoFe/MgO stacks, DWs exhibit a Néel configuration with a consistent left-handed chirality. This unique configuration steers DW motion by exploiting spin torques generated by the spin Hall effect (SHE) in adjacent heavy metal layers. The quantitative symmetry and the efficiency of these DW motions are confirmed through rigorous experimental data involving varying magnetic field orientations.
2. Current-induced Domain Wall Motion: The authors provide empirical evidence indicating that DWs, influenced by current-induced spin torques, travel in opposite directions in Pt and Ta underlayers. In precise terms, the spin-torque efficiency measured was notably high: 120 Oe/10^11 A m^−2 for Pt/CoFe/MgO and 170 Oe/10^11 A m^−2 for Ta/CoFe/MgO. This observation is explained by the interplay between the spin-orbit torques and the DMI, successfully demonstrating that these elements can drive chiral DWs with exceptional efficiency.
3. Slonczewski-like Torque Analysis: Through harmonic measurements, the authors quantify the Slonczewski-like torques, finding distinct differences in sign and magnitude between Pt/CoFe/MgO and Ta/CoFe/MgO systems. Notably, SHE-driven torque was found sufficient to switch the magnetization direction uniformly within these ultrathin films, paving a path for novel spintronic applications.

Implications and Future Research

The theoretical and experimental insights presented signify profound implications for the advancement of spintronic devices, particularly those geared towards memory and logic applications. By elucidating the role of SHE and DMI in enhancing spin-torque efficiencies and DW velocities, this paper facilitates strategic material selection and structural design in optimizing device performance.

Looking forward, the work invites further research into understanding the nuances of interfacial phenomena affecting DW dynamics. Moreover, the findings suggest potentially transformative impacts on the development of low-power devices leveraging magnetic domain wall motion for data storage and processing. The exploration of alternative materials and configurations that can replicate or improve upon the results in this paper could yield further innovations in the practical applications of spintronic technology.

In conclusion, this paper effectively bridges theoretical predictions with experimental validation in the field of chiral DW dynamics, laying a robust groundwork for future explorations and technological developments in spintronics.