Dzyaloshinskii-Moriya interaction chirality reversal with ferromagnetic thickness

In ultrathin ferromagnetic films sandwiched between two distinct heavy metal layers or between a heavy metal and an oxide layer, the Dzyaloshinskii-Moriya interaction (DMI) is of interfacial origin. Its chirality and strength are determined by the properties of the adjacent heavy metals and the degree of oxidation at the interfaces. Here, we demonstrate that the DMI chirality can change solely with variations in the thickness of the ferromagnetic layer - an effect that has not been experimentally studied in details or explained until now. Our experimental observation in the trilayer system Ta/FeCoB/TaOx is supported by ab initio calculations: they reveal that variations in orbital filling and inter-atomic distances at the interface, driven by the structural relaxations in the ultrathin regime, lead to an inversion of DMI chirality. We hence propose a new degree of freedom to tune DMI chirality and the associated chiral spin textures by tailoring crystal structure e.g. using strain or surface acoustic waves.

💡 Research Summary

This work investigates a previously unexplored degree of freedom for tuning the interfacial Dzyaloshinskii‑Moriya interaction (DMI): the thickness of the ferromagnetic (FM) layer itself. Using a Ta/FeCoB/TaOx trilayer, the authors fabricate a “double‑wedge” sample in which the FeCoB thickness varies continuously along one axis while the oxidation state of the top Ta layer varies along the orthogonal axis. This geometry allows independent control of FM thickness and interface oxidation, enabling a clear separation of their respective contributions to DMI.

Experimentally, the chirality of current‑driven domain walls (DWs) is used as a direct probe of DMI sign. Because Ta possesses a negative spin‑Hall angle, clockwise (CW) DWs move with the electron flow, whereas counter‑clockwise (CCW) DWs move opposite to it. By recording DW motion across hundreds of positions on the wafer, the authors map the DMI sign as a function of FeCoB thickness and top‑layer oxidation. They find a clear DMI sign reversal line that runs diagonally across the thickness‑oxidation space. Strikingly, for a fixed oxidation state (i.e., a fixed top‑TaOx thickness), increasing the FeCoB thickness from ~1 nm to ~1.3 nm flips the DMI chirality, even though the bottom Ta/FeCoB interface remains unchanged. This thickness‑driven reversal is observed across multiple oxidation regimes, demonstrating that it is not an artifact of oxidation variations.

To uncover the microscopic origin, the authors perform first‑principles density‑functional calculations (VASP, PAW, GGA with SOC) on a simplified Fe/TaOx bilayer model. They vary the number of Fe monolayers (3–9 ML) and the oxidation percentage of the interfacial Ta layer (0–100 %). DMI is extracted using the constrained spin‑spiral supercell method, where the energy difference between clockwise and counter‑clockwise spirals yields the DMI constant. The calculations reveal two key trends: (1) for low oxidation (< 25 %), the DMI sign changes as the Fe thickness crosses 3–5 ML, reproducing the experimental thickness‑driven inversion; (2) increasing oxidation shifts the DMI magnitude and can suppress the sign change, consistent with the experimentally observed dependence on the top‑layer oxidation. The authors attribute the sign reversal to structural relaxation in the ultrathin regime: as the FM layer becomes thicker, the Fe‑Ta interlayer distance (z) and orbital hybridization (Fe d‑states with Ta/O p‑states) are modified, altering the symmetry of the spin‑orbit coupling and thus flipping the DMI vector.

The study therefore establishes FM thickness as a genuine tuning knob for DMI chirality, complementing previously known knobs such as heavy‑metal species, oxidation level, electric field, and strain. This insight opens new pathways for designing spin‑orbit‑torque devices: by patterning regions of different FM thickness, one could engineer adjacent domains with opposite DW chirality or skyrmion helicity, enabling deterministic control of chiral textures without changing material composition. Moreover, the authors suggest that external stimuli that affect interlayer spacing—such as strain, surface acoustic waves, or gate‑induced lattice expansion—could dynamically switch DMI sign in real time, offering a route to reconfigurable spin‑logic elements.

In summary, the paper (i) experimentally demonstrates a thickness‑induced DMI sign reversal in a weak‑DMI Ta/FeCoB/TaOx system, (ii) validates the observation with ab‑initio calculations that link the effect to orbital filling and inter‑atomic distance changes, and (iii) proposes ferromagnetic thickness as a new, practical degree of freedom for tailoring chiral spin textures in future spintronic technologies.