Impacts of Steady-State Domain Wall Configuration on Domain Wall Stiffness and Directional Domain Propagation in the Creep Regime

Abstract

Typically, stabilizing small, robust skyrmions requires a strong Dzyaloshinskii-Moriya Interaction (DMI). Accurately measuring the strength of DMI in prospective skyrmion host materials is therefore crucial to realizing proposed skyrmion-based devices for racetrack memory and neuromorphic computing.[1–2] In the creep regime, the velocity of a domain wall is governed by an Arrhenius law and is inversely proportional to its elastic energy.[3] Early work on the impact of DMI on domain walls showed that the resting energy of a wall is modulated by the DMI's effective field and that the presence of an in-plane field results in asymmetric domain growth due to the (mis)alignment of the two fields.[4] Subsequent studies have proposed creep models that reflect the intricacies of the relevant velocity and elastic energy scales.[5–6] As such, the sign and strength of the Rashba DMI found in thin film systems has been deduced from the asymmetric creep expansion of domains when an in-plane field is present; wall segments with core magnetizations parallel to the in-plane field have reduced energies, resulting in enhanced velocities—and vice-versa.[5–6]However, Brock et al., recently reported experimental results that deviate remarkedly from the above-mentioned creep models. Observations made in [Co(0.7 nm)/Ni(0.5 nm)/Pt(0.7 nm)]N>3 films include highly asymmetric dendritic domain growth perpendicular to the in-plane field for low fields and sharp variations of the growth direction with the strength of the in-plane field.[7–8] While one study focused on the apparent preference of Bloch walls of a specific handedness and attributed most of the phenomena to interlayer DMI [8], further investigation of the findings in [7] revealed that the trends that clearly defy the symmetries expected from an intrinsically favored wall chirality with Hz.[7]In this work, we address these peculiarities by considering the impact of the driving field on the domain wall magnetization, dispersive stiffness, and creep velocity. We show that even in the creep regime, the core magnetization of a domain wall, as per the Landau-Lifshitz-Gilbert equation, can be driven to a steady-state equilibrium that deviates significantly from the static case. We find that for material parameters comparable to those reported in [7], a bubble domain in the presence of an in-plane field can exhibit sharp discontinuities in its steady-state wall magnetization profile. Similar to the findings reported in [10], such a point represents a sudden reorientation of the magnetizations along the domain wall due to the nuanced competition between the effective DMI field, the domain wall anisotropy field, the in-plane field, the driving field and Gilbert damping. Consequently, the domain wall energy and stiffness landscapes can be equally complex, highly sensitive to the in-plane field and significantly different from the static case.Incorporating a steady-state treatment of domain wall profiles into the dispersive stiffness framework developed in [5], we compute the steady-state stiffness and creep velocity profiles of a an expanding domain for the scenarios reported in [7]. As seen in Figure 2., our modelling of the N=3 film shows remarkable agreement with the observed dendritic growth directionalities as a function of the strength of the in-plane field: first propagating uniformly for very weak fields, then at 90° from the field direction, then gradually reducing to ≈20°, and then abruptly reversing direction to ≈200°. Moreover, the model reproduces the curious observation that that the propagation direction is constant for both driving field directions; switching the domain polarization switches the directions of both the DMI and applied field torques, leaving the Bloch chirality at steady state unchanged. Comparing the magnetization profiles to the propagation directions and stiffness values reveals that segments with the lowest elastic energy (and thus the highest velocity) occur close to reorientation positions in the magnetic profile. The sudden emergence of such points is also responsible for the drastic changes in the growth direction.In light of these insights, we discuss the impact of steady-state profiles on DMI values obtained via bubble expansion experiments. Finally, we consider how our steady-state stiffness model could be used in tandem with these growth experiments to more easily determine the value of Gilbert damping in thin films. **