Acoustic-driven magnetic skyrmion motion

Using magnetic skyrmions as controllable information carriers offers potential for high-density, low-power spintronic memory and logic. Efficient manipulation of skyrmions is crucial for device concepts such as racetrack memory. Prior electrical manipulation has mainly used current-induced spin–orbit torques or thermal gradients. Electric-field control via magnetoelectric or magnetoelastic effects promises lower power by minimizing Joule heating. Dynamic strain from acoustic waves, particularly surface acoustic waves (SAWs), can couple strongly to magnetization via magnon–phonon interactions and has been used to generate skyrmions. Theory has suggested SAW-driven motion using counter-propagating waves, but experimental electric-field-induced static strain or acoustic-wave control of skyrmion motion had not been demonstrated. This study aims to experimentally demonstrate and elucidate SAW-driven motion of Néel-type skyrmions, identify effective acoustic modes, and characterize the resulting trajectories and efficiencies.

The paper situates its work within: (i) current-driven skyrmion manipulation in asymmetric multilayers via spin–orbit torques and thermal gradients; (ii) energy-efficient electric-field control through magnetoelectric and magnetoelastic effects, including static strain tuning of anisotropy and Dzyaloshinskii–Moriya interaction; (iii) dynamic strain using SAWs as long-range carriers that couple to magnetization, with prior demonstrations of SAW-induced skyrmion creation via spatiotemporal strain and inhomogeneous effective torques; and (iv) a theoretical model predicting skyrmion motion driven by counter-propagating SAWs. Despite SAWs’ success in manipulating particles, electrons, and qubits, efficacy for skyrmion motion had not been experimentally established.

Experimental device and materials: Ta (5 nm)/Co20Fe60B20 (1 nm)/MgO (1 nm)/Ta (2 nm) multilayers with perpendicular magnetic anisotropy were sputtered onto 64°Y-cut LiNbO3 piezoelectric substrates. Synchronous two-port SAW delay lines were patterned with Ti (5 nm)/Pt (150 nm) interdigital transducers (IDTs) via photolithography and lift-off. By choosing the SAW propagation direction relative to the LiNbO3 crystal orientation, the effective piezoelectric tensor was transformed to selectively excite either Rayleigh waves (with longitudinal and shear-vertical displacement) or shear-horizontal (SH) waves (with only horizontal shear displacement) at the same sample area. RF characterization and SAW excitation: Transmission spectra S12 between IDTs were measured using a vector network analyzer, revealing resonances at ~451 MHz (Rayleigh) and ~486 MHz (SH). The SH mode shows higher propagation attenuation, yielding a lower resonant peak than the Rayleigh mode. SAW pulses were generated by on–off keyed RF modulation at resonance frequencies using a signal generator and waveform generator. Typical pulse durations were 300 ms, with RF powers varied for skyrmion generation studies and set to 26 dBm for motion studies. SAW wavelengths of 8 µm (main) and 10 µm (additional test) were used. Magnetic imaging and preparation: Skyrmions were imaged by polar magneto-optic Kerr effect (p-MOKE) microscopy at room temperature. Initial magnetic states were prepared by applying out-of-plane magnetic fields (e.g., −0.8 mT) to remove maze domains. Skyrmions with topological charge Q = ±1 were created by applying SAW pulses under small positive or negative out-of-plane bias fields (±0.8 mT). Skyrmion sizes and densities were quantified from MOKE images. Micromagnetic simulations: Simulations (MuMax3) included exchange, interfacial Dzyaloshinskii–Moriya interaction, perpendicular anisotropy, magnetostatics, and magnetoelastic coupling. Simulation box: 256 × 256 × 1 nm3 with 1 × 1 × 0.5 nm3 cells. Skyrmion diameter and SAW wavelength were set to 30 nm and 240 nm, respectively, preserving the experimental diameter-to-wavelength ratio. Parameters: Aex = 1×10−11 J/m, Ms = 5.8×10^6 A/m, D = 3×10−3 J/m2, Ku = 7×10^5 J/m3, α = 0.1, B1 = B2 = −8.8×10^6 J/m3, density 8000 kg/m3, elastic constants C11 = 283 GPa, C12 = 166 GPa, C44 = 58 GPa. Simulations assumed ideal elastic wave propagation without attenuation. Analytical modeling using the Thiele equation was used to relate deflection angles to damping and the ratio of magnetoelastic driving forces (Fy/Fx).

- Mode selectivity: SH waves efficiently drive skyrmion motion, while Rayleigh waves generate skyrmions but do not move them, consistent with simulations. Despite Rayleigh mode delivering ~12 dBm higher received power at the IDT, no motion was observed under Rayleigh excitation. - Generation: Both Rayleigh and SH waves (300 ms pulses at resonance) create skyrmions of ~1 µm diameter. SH waves yield higher skyrmion densities than Rayleigh waves at RF powers > 20 dBm. - Motion characteristics under SH waves: With 300 ms pulses at 26 dBm and λ = 8 µm, skyrmions exhibit both longitudinal displacement along propagation (x) and transverse displacement (y) analogous to the skyrmion Hall effect, with sign set by topological charge. Typical net displacement per pulse d ≈ 3 µm (d = sqrt(dx^2 + dy^2)). About 32% of skyrmions show d > 1 µm. Motion also observed at λ = 10 µm. - Velocity: Estimated velocity ~10 µm/s under SH waves, comparable to current-driven motion at low current densities, indicating creep regime dynamics. - Deflection angles: Average θ = arctan(dy/dx) ≈ 49.5° ± 15.2° for Q = −1 and −34.2° ± 17.7° for Q = +1. Analytical fits using Thiele equation with Fy/Fx ≈ 0.3 indicate damping α in the range ~0.01–0.07. Large angle variability attributed to random pinning. - Simulation insights: For Rayleigh waves (dominant vertical displacement), skyrmion total energy density remains symmetric, yielding negligible net force. For SH waves, asymmetric total energy density along a diagonal induces motion toward lower energy. When SAW wavelength ≫ skyrmion diameter, magnetoelastic forces average out to near zero, consistent with prior analytical predictions. - Additional observations: Shorter SH pulses (200 ms) can deform some skyrmions into stripes due to inhomogeneous strain and pinning. Longer pulses generate more skyrmions, enhancing skyrmion–skyrmion repulsion and limiting motion.

The experiments confirm that dynamic in-plane shear strain from SH SAWs provides an effective magnetoelastic driving field for Néel skyrmions, producing both longitudinal motion and a transverse component whose sign depends on the skyrmion topological charge. This addresses the open question of whether electric-field-induced acoustic waves can directly and efficiently move skyrmions, extending prior SAW-based skyrmion generation to controlled motion. The observed velocities (~10 µm/s) and large, charge-dependent deflection angles in the creep regime reflect competition among magnetoelastic driving, the topological Magnus force, damping, and pinning. Analytical Thiele-equation modeling links the deflection to damping and the anisotropy of magnetoelastic forces (Fy/Fx), matching experimental angle distributions with α ~ 0.01–0.07. The lack of motion under Rayleigh waves highlights the importance of displacement symmetry: dominant vertical displacements yield symmetric energy landscapes and negligible net forces, whereas SH waves create asymmetric energy gradients that propel skyrmions. Enhancing magnetoelastic coupling, reducing damping, increasing RF power (without amplitude saturation), and using higher-frequency (shorter-wavelength) SAWs should transition dynamics from creep to flow, increasing velocities and enabling precise trajectory control (e.g., via phased arrays).

This work experimentally demonstrates SAW-driven directional motion of Néel-type skyrmions in Ta/CoFeB/MgO/Ta multilayers, with SH waves effectively propelling skyrmions while Rayleigh waves do not. The motion comprises longitudinal drift and a topological-charge-dependent transverse component, corroborated by micromagnetic simulations and Thiele analysis. These results establish acoustic waves as a viable, low-power route for skyrmion manipulation, opening avenues for current-free skyrmion memory, logic, and microwave devices. Future research should optimize materials for stronger magnetoelastic coupling and lower damping, employ higher-frequency SAWs and advanced acoustic beamforming for precise trajectory control, and explore regimes beyond creep for higher velocities.

- Measurements predominantly in the creep regime with notable pinning from defects, leading to broad variability in deflection angles and limited velocities (~10 µm/s). - Only ~32% of skyrmions showed displacements >1 µm per pulse; motion amplitude constrained by available RF power and SAW wavelength. - Wave amplitude saturation limits velocity gains from increasing RF power. - Skyrmion deformation occurs for shorter pulses (200 ms) due to inhomogeneous strain and pinning; for longer pulses, increased skyrmion density enhances repulsive interactions that can limit motion. - Simulations assume ideal conditions without elastic wave attenuation and use scaled geometries (nm-scale sizes and wavelengths), which may overestimate efficiency compared to real devices where attenuation and disorder reduce velocities.