Ultrafast internal dynamics of chiral domain walls probed by time-resolved XRMS

Abstract: We measure picosecond dynamics of labyrinthine stripe domains with chiral Néel-type domain walls using time-resolved x-ray resonant magnetic scattering (XRMS). At the stripe-domain wavevector, the helicity-summed signal shows ultrafast demagnetization, recovery, and a weak oscillatory contribution that we identify as a signature of laser-launched coherent surface phonons. In contrast, the dichroic signal, which is sensitive to the in-plane magnetization inside the Néel walls, shows a strong oscillatory response whose frequency decreases with increasing pump fluence. We attribute this softening to pump-induced changes in the effective anisotropy and saturation magnetization, which modify the restoring field of an internal domain-wall mode. Time-resolved XRMS thus isolates wall-specific dynamics and provides access to internal domain-wall motion in disordered stripe textures on picosecond time scales.

• The paper demonstrates that time-resolved XRMS can isolate internal chiral domain wall dynamics by detecting fluence-dependent oscillatory modes.
• The methodology combines femtosecond pump pulses with FEL probing at the Fe edge to differentiate between collective magnetization and wall-specific responses.
• The findings reveal that material composition and optical fluence modulate the wall precessional frequency, highlighting prospects for ultrafast spintronic devices.

Ultrafast Internal Dynamics of Chiral Domain Walls Probed by Time-Resolved XRMS

Introduction

The study investigates the picosecond-scale internal dynamics of chiral domain walls in perpendicularly magnetized multilayers using time-resolved x-ray resonant magnetic scattering (XRMS). Domain walls with chirality, stabilized by interfacial Dzyaloshinskii-Moriya interaction (i-DMI), are central to spintronic applications and emerging magnetic computing architectures, yet their ultrafast dynamics—critical for device performance—are not well-understood experimentally due to limitations of conventional probes. The paper demonstrates that time-resolved XRMS, with dichroic detection, discerns domain-wall–specific dynamics from texture-averaged magnetization, revealing internal wall modes and their fluence-dependent properties.

Experimental Approach

The methodology utilizes femtosecond infrared pump pulses to initiate dynamics in labyrinthine stripe-domain networks within [HM/CoFeB/MgO]$_n$/Ta multilayers (where HM denotes either Ir or W underlayers). The response is probed by circularly polarized free-electron laser (FEL) pulses at the Fe M$_{2,3}$ edge in a reflection geometry, extracting both helicity-summed (sensitive to total magnetization texture) and dichroic (sensitive to in-plane wall magnetization) scattering signals. Three samples with distinct domain periods and wall types (pure Néel vs mixed Bloch-Néel) were assessed, enabling a comparative analysis of internal wall dynamics against different multilayer compositions and domain architectures.

Demagnetization and Structural Signatures

The helicity-summed XRMS signal ($I_+$) captures ultrafast demagnetization and recovery of the labyrinthine stripe order following optical excitation, in line with previous reports on PMA systems. A weak oscillatory component, independent of fluence and attributed to coherent surface phonons, is resolved in low-period samples where ultrafast demagnetization is moderate. However, in samples with strong demagnetization, this structural oscillation is masked by the dominant magnetic response. Thus, $I_+$ provides a baseline for separating structural and collective magnetic texture responses but does not resolve internal wall-specific modes.

Wall-Specific Dynamics and Oscillatory Modes

The dichroic signal ($I_-$), measured via helicity difference, displays a robust, strongly fluence-dependent oscillatory response, absent in the helicity-summed signal, and is thus attributed to internal degrees of freedom of chiral Néel walls. The oscillatory component has a frequency that decreases monotonically with increasing pump fluence, indicative of a softening effect. Quantitative analysis, based on the Walker-wall model, associates this oscillation with the precessional motion of the internal wall angle $\phi$ and possibly the transient evolution of wall width. The fluence dependence reflects transient changes in effective perpendicular anisotropy ($K_\text{eff}$) and saturation magnetization ($M_s$), which renormalize the restoring field for wall distortions.

Strong numerical findings include:

• Oscillation frequencies decrease linearly (within the measured fluence window) as fluence is raised from 1.5 to 4.5 mJ/cm$^2$.
• The larger stack (sample I, Ir-based, $n=15$) shows both a higher initial frequency and greater fluence sensitivity compared to the W-based multilayer (sample II, $_{2,3}$0).
• The amplitude of oscillatory wall motion decays within a few hundred picoseconds, revealing significant damping and indicating strong coupling to the environment.

These observations are consistent with theoretical scaling $_{2,3}$1, where transient thermal effects following optical excitation reduce $_{2,3}$2 and $_{2,3}$3, thereby lowering the characteristic frequency of the wall mode.

Theoretical and Experimental Implications

This work validates dichroic, time-resolved XRMS as a selective probe of internal domain-wall modes, as opposed to collective magnetization or domain pattern recovery. It establishes that the dynamics of internal wall degrees of freedom (as encoded in $_{2,3}$4 and $_{2,3}$5) can be quantitatively tracked in reciprocal space on sub-nanosecond timescales. The data support models in which transient heating leads to a fast and reversible modification of wall structure, relevant for ultrafast control of chiral spin textures.

The dependence of mode softening on multilayer stack composition and domain period underscores the role of materials engineering in setting not only static wall structure but also dynamical response functions in technologically relevant timescales. The finding that wall modes can be selectively excited and interrogated, and that their frequencies are tunable by optical fluence, opens possibilities for dynamic control of wall-mediated phenomena in ultrathin ferromagnets and devices exploiting chiral dynamics.

Future Directions

Key open questions include the microscopic origin of damping for these modes, the degree of coupling between internal wall excitations and lattice or electronic baths, and the comparative roles of i-DMI, wall stiffness, and pinning. Extension to thinner stacks, multilayers exhibiting more pronounced DMI, or topological spin textures like skyrmions would be a logical progression. Experimentally, the combination of time-resolved reciprocal-space probes with direct real-space imaging (e.g., Lorentz TEM, time-domain MFM) could provide further insight into the spatiotemporal evolution of chiral wall excitations.

On the theory side, incorporation of these experimental results into micromagnetic modeling frameworks, including stochastic thermal effects and full stack-dependent anisotropy, is warranted. The implications for magnetic logic and memory—where domain-wall velocity, tunability, and robustness are critical—are substantial as the ability to both characterize and control internal wall dynamics is central to next-generation device engineering.

Conclusion

The paper provides unambiguous evidence that time-resolved, dichroic XRMS discerns ultrafast internal dynamics of chiral domain walls in perpendicularly magnetized multilayers. It isolates a wall-specific precessional mode whose characteristic frequency is both material- and fluence-dependent, offering direct insight into wall stiffness renormalization under optical excitation. The findings lay the foundation for refined modeling and dynamic control of chiral spin textures with implications for ultrafast spintronics and domain-wall-based devices.