Hydrogen toggling between Yoshimori spin spirals and elliptical Dzyaloshinskii-Moriya skyrmions in Fe on Ir(110)

TL;DR

The paper addresses non-volatile control of spin textures in ultrathin magnetic films by hydrogen-induced tuning of the competition between exchange interactions and Dzyaloshinskii-Moriya interaction (DMI). Using 2 ML Fe on Ir(110), density functional theory shows hydrogen adsorption shifts band filling and reduces certain AFM exchanges, enabling a long-period, DMI-dominated spin spiral distinct from the pristine Yoshimori spiral. Spin-polarized STM experiments reveal a dense hydrogen phase where the 8.5 nm spiral can be transformed under moderate out-of-plane fields into elongated skyrmions, which persist up to several tesla and become field-polarized at higher fields. The work demonstrates a non-volatile, reversible hydrogenation strategy to toggle between spiral and skyrmion states, broadening the material landscape for controllable spin textures and potentially enabling antiskyrmions in low-symmetry surfaces.

Abstract

Skyrmions are particle-like spin textures that arise from spin spiral states in the presence of an external magnetic field. These spirals can originate from either frustrated Heisenberg exchange interactions or the interplay between exchange interactions and the relativistic Dzyaloshinskii-Moriya interaction, leading to atomic- and mesoscale textures, respectively. However, the conversion of exchange-stabilized spin spirals into skyrmions typically requires magnetic fields that exceed practical laboratory limits. Here, we demonstrate a strategy leveraging hydrogen adsorption to expand the range of magnetic films capable of hosting stable or metastable skyrmions. In a structurally open and anisotropic system of two pseudomorphic Fe layers on Ir(110), spin-polarized scanning tunneling microscopy combined with ab initio calculations reveals that a right-handed, exchange-stabilized Néel-type spin spiral propagating along the [$\overline{1}10$] direction with a $1.3$~nm period transitions upon hydrogen adsorption to a Dzyaloshinskii-Moriya type spiral with a sevenfold longer period of $8.5$~nm. This transition enables elliptical skyrmions to form at moderate magnetic fields. Hydrogenation thus provides a non-volatile mechanism to toggle between distinct magnetic states, offering a versatile platform for controlling spin textures.

Figures (3)

• Figure 1: DFT calculated energetics of spin spirals. (a) Energy dispersion $E(q)$ of clean 2 ML Fe/Ir(110) along two orthogonal directions and with virtually additional charge of 0.1 e, 0.2 e and 0.3 e. The calculations were performed scalar-relativistically with LDA using the structure optimized in GGA shown in Note 1 in the SI. (b) Energy dispersions $E(q)$ with SOC relative to the FM state of homogeneous, flat cycloidal spin spirals as a function of the wave vector $q$ for 2 ML Fe/Ir(110). Hydrogenated films for different H concentration (given in the graph). The inset displays a ball model with the adsorbed H at full coverage as white spheres.
• Figure 2: Field induced unwinding of spin spirals in the dense H adsorption phase on 2 ML Fe/Ir(110). Spin-polarized STM data obtained using a magnetically soft Fe-coated W tip with $\textbf{m}_\mathrm{tip}$ normal to surface. (a)-(f) Constant-current STM topographs after H$_2$ exposure at 100 K with indicated increasing out-of-plane magnetic fields. Images with: $V_\mathrm{b} = 100$ mV, $I_\mathrm{set} = 1$ nA, and image size 40 nm $\times$ 40 nm. (g) Averaged line profile (black circles and line) taken within the white rectangle in (a); Red curve: sinusoidal curves with periods given in the graph.
• Figure 3: Identification of magnetic skyrmions. (a), (b) $\mathrm{d}I/\mathrm{d}V$ maps of 2 ML Fe/Ir(110) taken with a bulk Cr tip with fixed in-plane magnetization as indicated. The external magnetic field is normal to the surface into the sample. $B = 1.0$ T in (a) and $B = 3.0$ T in (b). The magnetic contrast is visualized with a red to blue color scale indicating parallel to anti-parallel alignment between $\textbf{m}_\mathrm{tip}$ and $\textbf{m}_\mathrm{s}$, respectively. The double lobes surrounding the magnetic stripes or skyrmions show their in-plane spin texture. Inset of (b) is the spin model simulation colored only along the tip magnetization experimental axis. $V_\mathrm{b} = 100$ mV, $I_\mathrm{set} = 1$ nA, and image size $50~\mathrm{nm} \times 50~\mathrm{nm}$. Spin model map: $6~\mathrm{nm} \times 12~\mathrm{nm}$ (c) Differential conductance linescan signal (black circles) averaged within the black rectangle in (b). The red line is a fit assuming the characteristic double domain wall of the skyrmion texture with Eq. \ref{['ddw']}.