Enhancement of perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction in thin ferromagnetic films by atomic-scale modulation of interfaces

The study addresses how interfacial structure at atomic length scales influences spin–orbit coupling-driven phenomena in ultrathin ferromagnet/heavy-metal heterostructures, notably perpendicular magnetic anisotropy (PMA) and interfacial Dzyaloshinskii–Moriya interaction (IDMI). While ab initio theories often assume ideal atomically smooth interfaces and constant IDMI, realistic interfaces exhibit roughness, intermixing, and local strain that cause spatial fluctuations in SOC, IDMI, and surface anisotropy, thereby affecting chiral spin textures and their dynamics. Stabilization of Néel walls and skyrmions requires IDMI above critical values. Breaking structural inversion symmetry and optimizing interface quality are essential; moreover, inducing opposite-sign IDMIs at top and bottom FM interfaces (using different HMs) can yield additive enhancement. The authors propose that achieving correlated roughness of both interfaces minimizes effective FM thickness variation, maximizes IDMI, and enhances PMA. They investigate this hypothesis experimentally by engineering atomic-scale surface modulation via an epitaxial Pd seed layer beneath Pt/CoFeSiB with top capping layers (Ru, Ta, or MgO).

Prior work shows: (i) IDMI depends on SOC strength, broken structural inversion symmetry, and 3d–5d(4d) orbital hybridization near the Fermi level; (ii) theoretical calculations typically assume atomically smooth interfaces with uniform IDMI; (iii) real interfaces possess roughness and intermixing, leading to fluctuations in SOC, IDMI, and surface magnetic anisotropy; (iv) identical interfaces in symmetric stacks (e.g., Pt/Co/Pt, Ru/Co/Ru) lead to vanishing IDMI unless asymmetry is introduced (e.g., high-temperature deposition or ultrathin interlayers); (v) IDMI shows strong, often nonlinear, dependence on FM thickness (∼1/t_FM) at small t_FM due to interface degradation; (vi) theory suggests correlated roughness of top and bottom interfaces can enhance PMA. This work experimentally examines correlated roughness to enhance both PMA and IDMI in asymmetric stacks.

Sample preparation: Epitaxial Si(111) substrates (misoriented 0.1° toward [11-2]) were cleaned (isopropyl alcohol, distilled water), degassed at 500 °C for 12 h, flash-heated to 1200 °C (3×, 10 s each), and cooled to 50 °C. Using MBE (Omicron Nanotechnology) in UHV, a Cu buffer (10 ML; 1 ML Cu = 2.09 Å; rate 4.3 ML/min) and a Pd seed layer (0–56 ML; 1 ML Pd = 2.25 Å; rate 0.75 ML/min) were grown at 75 °C to achieve epitaxial fcc-Pd(111) and to prevent Pd–Si intermixing. RHEED was monitored in situ during growth. Pd surface topography was characterized in situ by STM; ex situ AFM quantified roughness over 5×5 µm² and 2×2 µm² areas. After MBE, three series were deposited by magnetron sputtering atop Pd: MgO-series: Pt(2)/CoFeSiB(1.5)/MgO(2)/Ta(5); Ta-series: Pt(2)/CoFeSiB(1.5)/Ta(5); Ru-series: Pt(2)/CoFeSiB(1.5)/Ru(3)/Ta(5). Thicknesses in nm; FM composition Co70.5Fe4.5Si11B10 (at.%). Atomic structure observation: Cross-sectional HAADF-STEM and HRTEM (Cs-corrected Titan 80–300, 300 kV; Gatan Quantum 966 spectrometer; convergence 24.9 mrad; collection 24.7 mrad). TEM lamellae prepared by PIPS (Gatan 691), 3.5 keV ions, 6° milling angle, double sector mode; specimen cooled to −165 °C. EELS acquired for elemental profiling; EDX elemental mapping performed. SIMS (TOF.SIMS 5) with Cs+ ions used for depth profiling. Surface and interface roughness: STM and AFM measured morphology. XRD/XRR (Rigaku SmartLab, Cu Kα, λ=1.54 Å) characterized crystal structure and interfaces (roughness, intermixing, thickness variations). XRR spectra (θ=0–6°) fitted using GlobalFit to extract top-interface roughness of Pt, CoFeSiB, MgO, Ru. Magnetic characterization: Magneto-optical Kerr effect (NanoMOKE II), vibrating sample magnetometry (LakeShore 7410 VSM), magnetic force microscopy (Ntegra Aura) with low-moment tips for domain imaging under applied field, and Kerr microscopy (Evico Magnetics) for magnetization reversal studies. Growth-mode and roughness control: Pd seed thickness t_pd varied from 0 to 12.6 nm to induce controlled surface modulation. STM and RHEED identified growth regimes: (i) 2D growth up to ~0.6 nm; (ii) layer-by-layer from 0.6–2.9 nm; (iii) 3D island growth for t_pd>2.9 nm, producing long-wavelength roughness with average amplitudes 0.2–1.5 nm and periods 0.75–57.0 nm. Subsequent sputtered layers inherited and correlated this roughness.

- Atomic-scale interface modulation via an epitaxial Pd(111) seed layer enables controlled roughness (Rq from ~0.15 to 1.0 nm) and periodic surface morphology (amplitude 0.2–1.5 nm; period 0.75–57.0 nm) as a function of Pd thickness t_pd. - Three sample series were realized: Pt/CoFeSiB/MgO, Pt/CoFeSiB/Ta, and Pt/CoFeSiB/Ru, all with CoFeSiB thickness of 1.5 nm and Pt underlayer of 2 nm. - Morphological correlation of the top and bottom interfaces occurs at t_pd = 10.35 nm, where the Rq values of Pd, Pt, CoFeSiB, and capping layers converge (from XRR fits and AFM/STM). At this correlated-roughness condition, the IDMI reaches peak values for all three series, indicating IDMI maximization when interface quality asymmetry parameter tends to zero through correlated roughness. - Structural characterization shows: Pt adopts fcc(111) texture coherent with Pd(111) (small lattice mismatch ~1–8%); CoFeSiB, typically amorphous, exhibits crystallinity with fcc-like lattice and replicates the Pd/Pt surface morphology (quasi-epitaxial behavior); Ru and (partial) Ta capping layers display pronounced fcc(111) structure. - Intermixing at interfaces and boron diffusion were detected by SIMS and EDX; Ta capping shows top oxidation (~3.5 nm TaOx) over ~1.5 nm metallic Ta. - RHEED evolution corroborates growth-mode transitions with increasing t_pd (appearance of transmission-diffraction spots indicating developed surface topography) while maintaining overall Pd crystalline quality (stable background except at initial stages). - Magnetic anisotropy: Ta-series samples (t_pd ~1.125–12.6 nm) exhibit fourfold out-of-plane anisotropy with easy axes tilted ~45° (225°) to the plane; Ru-series samples show in-plane anisotropy with easy-axis disorientation within −30° to 30° relative to the plane. (MgO-series note truncated in provided text.)

The results demonstrate that engineering correlated roughness at both interfaces of an ultrathin ferromagnet effectively cancels local FM thickness variations and stabilizes the interfacial DMI, leading to enhanced IDMI and PMA. Since IDMI is highly sensitive to FM thickness and interface quality, matching the morphology (amplitude and period) of bottom and top interfaces minimizes spatial fluctuations in SOC-mediated interactions and anisotropy, yielding maximal IDMI at t_pd = 10.35 nm where roughness metrics converge. The coherent Pd(111)/Pt(111) stack propagates the designed modulation into the FM, while quasi-epitaxial ordering in CoFeSiB further preserves interface correlation. The observation of distinct anisotropy symmetries across different capping layers (Ta, Ru, MgO) indicates that both material choice (sign/magnitude of interfacial contributions) and morphological correlation co-govern the net chiral and anisotropic responses, supporting the strategy of simultaneous PMA and IDMI control for stabilizing chiral textures and enabling robust spin–orbitronic device operation.

This work introduces an interface roughness engineering approach at the atomic scale to simultaneously enhance perpendicular magnetic anisotropy and interfacial DMI in ultrathin HM/FM/HM(oxide) heterostructures. By using an epitaxial Pd seed to imprint controlled periodic roughness and achieving correlated top/bottom interface morphology (optimized at t_pd ≈ 10.35 nm), the IDMI attains peak values in Pt/CoFeSiB/MgO, Pt/CoFeSiB/Ta, and Pt/CoFeSiB/Ru stacks. Structural analyses confirm coherent fcc(111) growth (Pd→Pt), quasi-epitaxial CoFeSiB crystallinity, and replication of interface modulation through the stack. This strategy offers a practical route for precise tuning of chiral interactions and anisotropy in devices targeting skyrmions, chiral domain walls, and SOT functionalities. Future work could systematically map the parameter space (roughness amplitude/period, FM thickness, and choice of top HM/oxide) and explore dynamic properties (domain-wall motion, skyrmion stability) under electrical stimuli to optimize device performance.

- The provided data indicate intermixing at interfaces and boron diffusion, which may influence magnetic properties and complicate isolation of pure roughness effects. - Partial oxidation of Ta capping (TaOx ~3.5 nm) could modify interface properties relative to the intended metallic Ta interface. - IDMI’s strong sensitivity to FM thickness (t_FM) and interface quality implies potential variability at very small t_FM due to degradation. - The optimization of IDMI is demonstrated at a specific Pd thickness (t_pd = 10.35 nm); generalization to other material systems and a broader range of parameters is not detailed in the provided text.