Towards control of the chirality sign at ultrathin metal films: Bi at 2Ni/Co

Proximity effects can be used to introduce spin–orbit interactions in magnetic metallic layers in contact to a heavy metal (HM). This well known phenomenon has been exploited to induce chiral spin textures at Co ultrathin films, where the left- or right- handedness can be tuned by the HM layer position, based on the broken inversion symmetry of the film and the existence of an additive interface effect. Here we show that structural and chemical features introducing further symmetry reductions can be added to this scenario ultimately enabling control over the definition of a unique winding sense. We focus on 2Ni/Co heterostructures and Bi, a scarcely explored HM metal of large size, to combine a chemically inhomogeneous ferromagnetic stack along the normal to the surface with in-plane asymmetries. Our results are contrasted to 2Co layers combined with Ir.