Spin Hall torque magnetometry of Dzyaloshinskii domain walls (1308.1432v2)

Abstract: Current-induced domain wall motion in the presence of the Dzyaloshinskii-Moriya interaction (DMI) is experimentally and theoretically investigated in heavy-metal/ferromagnet bilayers. The angular dependence of the current-induced torque and the magnetization structure of Dzyaloshinskii domain walls are described and quantified simultaneously in the presence of in-plane fields. We show that the DMI strength depends strongly on the heavy metal, varying by a factor of 20 between Ta and Pa, and that strong DMI leads to wall distortions not seen in conventional materials. These findings provide essential insights for understanding and exploiting chiral magnetism for emerging spintronics applications.

• The paper demonstrates that DMI strength varies up to twentyfold between Ta and Pt, directly affecting domain wall distortion.
• The paper distinguishes that SHE and DMI arise from separate spin–orbit coupling mechanisms, enabling independent tuning of these effects.
• The paper employs analytical and micromagnetic modeling to quantitatively extract key parameters for advancing spintronic device design.

Analysis of Spin Hall Torque Magnetometry of Dzyaloshinskii Domain Walls

The research paper focuses on the experimental and theoretical exploration of current-induced domain wall (DW) motion in heavy-metal/ferromagnet (HM/FM) bilayers, emphasizing the role of the Dzyaloshinskii-Moriya interaction (DMI). This paper aims to elucidate the complexities of spin-orbit-driven phenomena at HM/FM interfaces, especially addressing the poorly understood Dzyaloshinskii domain walls (DWs). By using the angular dependence of the SHE-induced torque and through comprehensive experimentation and modeling, the paper provides valuable insights into the characteristics and mechanisms underlying DW dynamics influenced by DMI.

Key Findings and Numerical Results

1. Dzyaloshinskii-Moriya Interaction (DMI) Strength Variability: The paper finds that the DMI strength varies significantly with the choice of heavy metal, exhibiting a twentyfold difference between Ta and Pt. Such diversity in DMI impacts the domain wall structures, resulting in distortions not typically observed in conventional materials.
2. Spin Hall Effect (SHE) and DMI Distinction: The results show that while both SHE and DMI stem from spin-orbit coupling (SOC), they originate from distinct mechanisms within these materials. Importantly, the DMI exchange constant maintains the same sign across different heavy metals, yet the SHE occurs with opposite signs for Ta and Pt. This finding suggests that the spin Hall angle ($\theta_{SH}$) and DMI can be independently engineered—a crucial insight for spintronic applications.
3. Analytical and Micromagnetic Modeling: The paper introduces unique behavior exhibited by DWs with strong DMI, where applied torque can tilt the entire DW line profile rather than simply rotating the DW moment. This behavior is analytically and micromagnetically modeled to accurately represent experimental observations, allowing for quantitative extraction of DMI parameters.
4. Quantitative Analysis of Micromagnetic Parameters: Utilization of spin Hall torque magnetometry in conjunction with modeling permits the extraction of specific micromagnetic parameters. These parameters include the effective DMI constant, spin Hall angle, saturation magnetization, and ferromagnetic exchange constants, thereby offering a comprehensive layer of quantitative understanding previously unattainable.
5. Domain Canting and Wall Tilting: The paper also provides a robust framework to describe the effects of domain canting and wall tilting. It describes the thresholds where the tilting becomes significant, illustrating its dependency on field strengths, showing how the model qualitatively and quantitatively aligns with micromagnetic simulations.

Implications for Spintronics

The paper's findings offer substantial implications for advancing the design of spintronics devices. Given the distinct origins of DMI and Slonczewski-like SOT, the demonstrated ability to independently control these parameters in HM/FM interfaces opens avenues for tailoring high-performance, low-power magnetic memory and logic devices. Moreover, the identification of strong DMI in Pt/CoFe/MgO comparable to ultrathin epitaxial films on single crystal substrates suggests the potential for achieving exotic spin textures, such as skyrmions, in conventional thin-film heterostructures. This capability could lead to advancements in the integration of these materials into practical devices, further pushing the boundaries of spintronic technology.

Prospective Developments

Looking forward, the research community would benefit from exploring the implications of the discovered behaviors for scalable memory technologies. Understanding the conditions that stabilize complex spin configurations, such as skyrmions, through controlled manipulation of DMI and SHE could enable energy-efficient memory storage solutions. Future work might also investigate the role of temperature, disorder, and real-world material imperfections in the context of the theoretical models developed, thereby bridging gaps between experimental observations and technological applications in broader operational scenarios.

This research marks a significant step towards a granular understanding of chiral magnetism at interfaces and sets the stage for further exploration into the potential of engineered SOC effects in advanced material systems.