The nature of domain walls in ultrathin ferromagnets revealed by scanning nanomagnetometry

Abstract: The recent observation of current-induced domain wall (DW) motion with large velocity in ultrathin magnetic wires has opened new opportunities for spintronic devices. However, there is still no consensus on the underlying mechanisms of DW motion. Key to this debate is the DW structure, which can be of Bloch or N\'eel type, and dramatically affects the efficiency of the different proposed mechanisms. To date, most experiments aiming to address this question have relied on deducing the DW structure and chirality from its motion under additional in-plane applied fields, which is indirect and involves strong assumptions on its dynamics. Here we introduce a general method enabling direct, in situ, determination of the DW structure in ultrathin ferromagnets. It relies on local measurements of the stray field distribution above the DW using a scanning nanomagnetometer based on the Nitrogen-Vacancy defect in diamond. We first apply the method to a Ta/Co40Fe40B20(1 nm)/MgO magnetic wire and find clear signature of pure Bloch DWs. In contrast, we observe left-handed N\'eel DWs in a Pt/Co(0.6 nm)/AlOx wire, providing direct evidence for the presence of a sizable Dzyaloshinskii-Moriya interaction (DMI) at the Pt/Co interface. This method offers a new path for exploring interfacial DMI in ultrathin ferromagnets and elucidating the physics of DW motion under current.

Direct Observation of Domain Wall Structures in Ultrathin Ferromagnets via Scanning Nanomagnetometry

The study titled "The nature of domain walls in ultrathin ferromagnets revealed by scanning nanomagnetometry" presents an innovative approach to directly determine the structure of domain walls (DWs) in ultrathin ferromagnetic materials. The research leverages scanning nanomagnetometry with nitrogen-vacancy (NV) centers in diamond to achieve nanoscale resolution of magnetic characteristics, potentially influencing spintronic device engineering.

The authors focus on the unresolved debate concerning the mechanisms governing current-induced DW motion in ultrathin ferromagnets with perpendicular magnetic anisotropy (PMA). Two primary types of DW structures—Bloch and N{é}el—are considered, with their dynamics influenced by the presence of the Dzyaloshinskii-Moriya interaction (DMI). Direct imaging of these structures has historically been challenging; however, this study introduces a method utilizing the NV center's sensitivity to magnetic fields to directly image and identify DW types without requiring assumptions about their dynamics.

Significantly, the research demonstrates two contrasting examples: a Ta/Co${40}$Fe${40}$B$_{20}$/MgO trilayer, which primarily exhibits Bloch-type DWs, and a Pt/Co/AlO$_x$ trilayer, where left-handed N{é}el DWs are observed, suggesting a strong DMI at the Pt/Co interface. These observations provide direct evidence of the DMI's role in stabilizing specific DW configurations and emphasize the technique's potential for exploring other ferromagnetic systems similarly influenced by DMI.

The paper's methodological advancement lies in its application of nanometer-scale magnetometry leveraging NV defects, which allows for in situ determinations without significant disturbance to the material. This technique facilitates precise DW imaging, improving our understanding of DW behaviors and their potential manipulation via electrical currents. It also opens new avenues for designing spintronic devices that exploit magnetization dynamics for data storage and processing.

From a theoretical perspective, the implications extend to enhancing models of spin-torque effects and the role of interface-induced interactions in ferromagnetic systems. Experimentally, this work enables exploration into the fine-tuning of ferromagnetic material properties through compositional engineering, potentially allowing for the controlled introduction of DMI to favor specific DW chirality and dynamic behavior.

In conclusion, the introduction of scanning NV magnetometry to study ultrathin ferromagnets highlights a significant methodological shift, enabling direct characterization of DW structures at previously inaccessible scales. This study not only refines the understanding of DW physics but also balances theoretical predictions with empirical results, bringing spintronic device development a step closer to practical application. Looking forward, future research could extend these methods to explore other magnetic systems and further exploit the technological prospects of spin-based computational architectures.