Observation of Hydrogen-Induced Dzyaloshinskii-Moriya Interaction and Reversible Switching of Magnetic Chirality

The Dzyaloshinskii-Moriya interaction (DMI) has drawn much attention, as it stabilizes magnetic chirality, with important implications in fundamental and applied research. This antisymmetric exchange interaction is induced by the broken inversion symmetry at interfaces or in noncentrosymmetric lattices. Significant interfacial DMIs are often found at magnetic/heavy-metal interfaces with large spin-orbit coupling. Recent studies have shown promise for induced DMI at interfaces involving light elements such as carbon (graphene) or oxygen. Here, we report direct observation of induced DMI by chemisorption of the lightest element, hydrogen, on a ferromagnetic layer at room temperature, which is supported by density functional theory calculations. We further demonstrate a reversible chirality transition of the magnetic domain walls due to the induced DMI via hydrogen chemisorption and desorption. These results shed new light on the understanding of DMI in low atomic number materials and the design of novel chiral spintronics and magneto-ionic devices.

Popular Summary

Chiral magnets have the potential to act as energy-efficient information carriers. In a chiral magnet, the neighboring magnetic moments adopt a winding arrangement, as opposed to the linear arrangements seen in ferromagnets and antiferromagnets. In very thin films, this chiral structure arises from an antisymmetric interaction among adjacent spins known as the Dzyaloshinskii-Moriya interaction, or DMI. Here, we demonstrate a fundamentally new way to induce the DMI: Adsorption of hydrogen onto a ferromagnetic surface not only instigates the interaction but also allows us to dial its strength up or down on demand, a powerful ability for certain types of magnetic memories and logic devices.

In our work, we detect the role of the DMI on magnetic domain walls, which are boundaries between regions with different magnetic architectures. Using spin-polarized low-energy electron microscopy, we can directly visualize the arrangement of magnetic moments on the domain walls. In each experiment, we can monitor these structures in real time, which allows us to track changes of the DMI via hydrogen adsorption and desorption. We are also able to quantify hydrogen-triggered changes in the system’s work function, which match the hydrogen-induced switching of the magnetic chirality.

With hydrogen control of the DMI now demonstrated, we hope to quantify other hydrogen-induced changes to magnetic behavior. These new findings advance the understanding of DMI in materials with low atomic numbers and the design of novel chiral spintronics and magnetoionic devices.

Figure 1

Room-temperature observation of reversible chemisorption/desorption of atomic hydrogen on metal surfaces. (a) LEEM IV spectra on bare 1 ML $\mathrm{Ni}/3\mathrm{ML}$ Co (initial) and the same surface before and after the hydrogen exposure. Measuring the energy at which the reflectivity drops allows quantification of the work function. (b)--(d) Work-function response on the surface of metals during the presence and absence of hydrogen at room temperature. Red and black triangles indicate the ON and OFF control of the hydrogen leak valve. (b) 6 ML Ni, (c) 6 ML Co, (d) 1 ML Ni/3 ML Co. (e) Work-function response of 1 ML $\mathrm{Ni}/3\mathrm{ML}$ Co at about $90°\mathrm{C}$, indicating about 90% chemisorption/desorption ratio.

Figure 2

Exploring chemisorbed-hydrogen-induced Dzyaloshinskii-Moriya interaction. (a,b) Observation of hydrogen-induced domain-wall chirality switching in compound SPLEEM images of 1 ML $\mathrm{Ni}/3\mathrm{ML}$ $\mathrm{Co}/2.09\mathrm{ML}\mathrm{P}\mathrm{d}\mathrm{/W}(110)$. (a) As-grown (sketch shows left-handed walls in magnetic layers); (b) with hydrogen exposure at $5\times {10}^{-9}\mathrm{torr}$ (sketch shows right-handed walls upon hydrogen chemisorption). Black and gray areas indicate perpendicularly magnetized up and down domains; colors indicate the in-plane orientation of magnetization in the domain-wall region. (c,d) $\alpha$ histogram of the SPLEEM image before (c) and after (d) the hydrogen exposure; $\alpha$ is the angle between domain-wall magnetization $\mathbf{m}$ and the domain-wall normal vector $\mathbf{n}$ (insert). (e) Hydrogen-exposure-dependent evolution of Néel-type chirality at various Pd thicknesses, where the chirality is calculated as $({\sum }_{-{90}^{°}}^{{90}^{°}}\mathrm{c}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}\mathrm{s}-{\sum }_{{90}^{°}}^{{270}^{°}}\mathrm{c}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}\mathrm{s})/({\sum }_{-{90}^{°}}^{{90}^{°}}\mathrm{c}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}\mathrm{s}+{\sum }_{{90}^{°}}^{{270}^{°}}\mathrm{c}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}\mathrm{s})$. (f) Summarized values of DMI strength $D$ induced by various elements adjacent to Ni, all measured by the same SPLEEM-based method. (g) DFT calculation of Pd-layer $d_{\mathrm{Pd}}$-dependent DMI strength $D$ from various contributions: $\mathrm{Ni}/\mathrm{Co}$ layers in the 250 pm structure model (solid squares), $\mathrm{Pd}/\mathrm{W}$ in the 275 pm model (hollow squares), the sum of both contributions (red dots), and the total contribution including the chemisorbed hydrogen ($\mathrm{H}/\mathrm{Ni}/\mathrm{Co}$ in 250 pm model $+\mathrm{Pd}/\mathrm{W}$ in 275 pm model) (green dots). Lines are guides to the eye.

Figure 3

Reversible switching of magnetic chirality via hydrogen at room temperature. (a) Time sequence of SPLEEM images of a domain wall in a $\mathrm{Ni}(1\mathrm{ML})\mathrm{/Co}(3\mathrm{ML})\mathrm{/Pd}(2.09\mathrm{ML})\mathrm{/W}(110)$ system. Hydrogen status is labeled above and below the images. The in-plane magnetization in the domain-wall region is rendered in gray according to the scale bar (right). Domains left and right of the domain wall are perpendicularly magnetized. The magnetization in the left and right regions points up and down, respectively. Magnetic chirality is highlighted by red and cyan arrows (see sketch). The field of view is $2\mu \mathrm{m}\times 4\mu \mathrm{m}$. (b) Evolution of magnetic chirality (derived from the average of the wall contrast). Gray diamonds indicate the timing of the images in panel (a).