Frustration of coupled helices in weakly perpendicular magnetic thin films*

Anti-symmetric exchange, or the Dzyaloshinskii–Moriya interaction (DMI), is responsible for non-collinear spin textures such as canting, helices and skyrmions. Analogously, some thin films without DMI can have competing magnetic anisotropies which stabilise non-collinear spin textures. Using resonant soft x-ray scattering, we report on the peculiar phenomenon of collective stripe rotation in an FeGd film under an applied field which also accompanies the transition from the stripe phase to the six-fold dipole skyrmion lattice (SkL). By changing the sample tilt, we control the resultant field vector, and in turn the threshold and abruptness of the observed rotation. We performed micromagnetic simulation to reproduce the experimental result and on inspection of the simulated 3D spin structure we identify two strongly coupled helical structures. As the magnetic field is increased in the simulation, the helical domain wall region unwinds before rotation and then reverts to a more wound structure in the rotated state. Our simulations highlight the importance of setting the polarity of the shared center of the coupled helices as well as the frustration effects induced by a combined in and out-of-plane magnetic field.