Interfacial Dzyaloshinskii-Moriya interaction in perpendicularly-magnetized Pt/Co/AlO$_x$ ultrathin films measured by Brillouin light spectroscopy

Abstract: Spin waves in perpendicularly-magnetized Pt/Co/AlO$_x$/Pt ultrathin films with varying Co thicknesses (0.6-1.2 nm) have been studied with Brillouin light spectroscopy in the Damon-Eshbach geometry. The measurements reveal a pronounced nonreciprocal propagation, which increases with decreasing Co thicknesses. This nonreciprocity is attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI), which is significantly stronger than asymmetries resulting from surface anisotropies for such modes. Results are consistent with an interfacial DMI constant $D_s = -1.7 \pm 0.11$ pJ/m, which favors left-handed chiral spin structures. This suggests that such films below 1 nm in thickness should support novel chiral states like skyrmions.

• The paper demonstrates that interfacial DMI causes pronounced nonreciprocal spin wave propagation in ultrathin Pt/Co/AlOₓ films as cobalt thickness decreases.
• The paper employs Brillouin light spectroscopy in a Damon-Eshbach configuration to quantify DMI by analyzing frequency shifts between Stokes and anti-Stokes lines.
• The paper finds that a DMI constant of approximately -1.7 pJ/m favors left-handed chiral spin structures, indicating potential for skyrmion-based spintronic devices.

Interfacial Dzyaloshinskii-Moriya Interaction in Perpendicularly Magnetized Pt/Co/AlO_x Ultrathin Films

The study presented in this paper provides a comprehensive analysis of the interfacial Dzyaloshinskii-Moriya interaction (DMI) in Pt/Co/AlO_x ultrathin films, utilizing Brillouin light spectroscopy (BLS) to measure spin wave dynamics. With the growing importance of spintronic applications, understanding the role of DMI in these films is critical for the development of magnetic memory and logic devices.

Summary of Key Findings

The authors investigated ultrathin films with varying cobalt (Co) thicknesses ranging from 0.6 nm to 1.2 nm. They observed a pronounced nonreciprocal propagation of spin waves, which increased as the Co thickness decreased. This effect is strongly attributed to the interfacial DMI, quantified by a constant $D_s$ with a value of approximately -1.7 pJ/m. This indicates a preference for left-handed chiral spin structures, providing potential for the stabilization of skyrmions in such films.

Technical Insights

1. Measurement Methodology: The study applied BLS in the Damon-Eshbach geometry to directly quantify the DMI. The analysis of frequency shifts between Stokes and anti-Stokes lines in the spectra elucidated the nonreciprocal nature of spin wave propagation.
2. Modeling and Analysis: The frequency difference between spin wave modes, a direct indicator of DMI, was fitted using a theoretical model. The model incorporates several parameters including saturation magnetization and anisotropy fields, ensuring accurate estimation of $D_s$.
3. Relation to MOKE Data: Magneto-optical Kerr effect (MOKE) measurements were used to corroborate the saturation magnetization and anisotropy fields, confirming the DMI's significant contribution relative to surface anisotropies in these ultrathin films.

Implications and Future Directions

The pronounced DMI in Pt/Co/AlO_x films reveals potential for designing devices that exploit chiral spin textures. The presence of left-handed spin structures like skyrmions could lead to the development of novel magnetic storage solutions with improved stability and efficiency. Furthermore, the ability to directly measure DMI offers insights into the design of more complex layered systems where spin-orbit interactions play a crucial role.

Future research could expand on this foundational work by exploring the effects of different capping and substrate materials on DMI. Additionally, integrating these films into device architectures could ascertain the practical feasibility of employing DMI in industrial applications. Understanding the interplay between DMI and other magnetic interactions might also help tune material properties for specific spintronic functionalities. This study provides essential data that could underpin such technological advancements, paving the way for more sophisticated magnetic devices.