Stabilization of chiral Bloch skyrmions due to interfacial DMI and asymmetric vertical Bloch line evolution.

We show that Bloch chirality preference in magnetic domain walls and skyrmions can originate from the interplay between an interfacial Dzyaloshinskii–Moriya interaction (iDMI) and a perpendicular magnetic field. This is starkly evident in Pt/Ni/Co thin films with a DMI lower than the critical strength required to form purely Néel walls and skyrmions. Imaging of skyrmion arrays via Lorentz transmission electron microscopy reveals an unmistakable preference for one Bloch chirality which switches. Analytical and micromagnetic modelling show that while conventional iDMI does not break energetic symmetry of the two Bloch chiralities, the energy barrier to switch between two Bloch chiralties becomes asymmetric; an out-of-plane field can thus stabilize a single Bloch chirality both at steady state and upon removing the field. We also demonstrate that the growth of favorable 'worm' domains—precursors to the observed skyrmions—is mediated by vertical Bloch line (VBL) evolution pursuant to the signs of the DMI vector and the applied field. As our modelling and imaging reveal, iDMI breaks the degeneracy in the nucleation energy, free energy, and velocity of VBLs, while the applied field dictates that their asymmetric nucleation and movement favor the same Bloch chirality that is preferred in their absence. Beyond elucidating previously unexplained correlations between preferential Bloch chirality and iDMI, this study offers promising pathways to explore memory devices based on Bloch chirality switching.