Deterministic Bloch Chirality in Interfacial DMI Systems: An Astroid for DW Switching

Abstract

The interfacial Dzyaloshinskii–Moriya interaction (DMI) emerges in ferromagnet/heavy metal thin films with broken z-mirror symmetry, stabilizing Néel domain walls and skyrmions of one chirality as dictated by its Rashba-type field. On the other hand, both Bloch chiralities remain energetically degenerate at static equilibrium for mixed domain walls in films with a moderate DMI. Multiple research groups have nevertheless independently reported significant Bloch chirality asymmetries in the Bz-stabilized domain microstructures of different interfacial DMI systems [1, 2].In this work, we demonstrate how the asymmetric wall energy landscape in such systems can produce a deterministic Bloch chirality when domain walls are driven away from static equilibrium. For D < Dc, the two ground state domain walls—having Bloch components of opposite chiralities—exhibit different restoring torques and steady state configurations. As dictated by the driving field’s direction and sign of DMI, one wall, which we term the lax wall, becomes labile at a lower Bz prior to the Walker field. At this lability point, the wall lies at an inflection point in its energy landscape and the domain wall susceptibility (χdw ∝ (σΦΦ)-1) diverges. The lax wall therefore reorients at higher fields to the steady state configuration of its counterpart, which we call the locked wall (Fig. 1).Using the Slonczewski equations and the 1D wall energy model, we analytically derived a DMI–Bz phase diagram of steady state domain wall Bloch chirality (Fig. 2), which includes an astroid analogous to the Stoner–Wohlfarth domain switching model. While these equations predict that locked walls precess beyond the Walker field, it was observed that they can instead persist due to DMI-induced asymmetries in vertical Bloch line (VBL) evolution—extending the Bloch monochirality regimes [3]. Here, we generalize these Bloch chirality preferences to arbitrary VBL behavior in these systems, before discussing how the theory of horizontal Bloch line evolution could similarly mediate Bloch chirality preferences [4].  Steady state Bloch monochirality via DMI-induced asymmetric evolution of lax & locked walls.  DMI–Bz phase diagram of steady state domain wall Bloch chirality