  Two-dimensional materials such as graphene or hexagonal boron nitride are
indispensable in industry. The recently discovered 2D ferromagnetic materials
also promise to be vital for applications. In this work, we develop a
phenomenological description of non-centrosymmetric 2D ferromagnets with
trigonal prismatic crystal structure. We chose to study this special symmetry
group since these materials do break inversion symmetry and therefore, in
principle, allow for chiral spin structures such as magnetic helices and
skyrmions. However, unlike all non-centrosymmetric magnets known so far, we
show that the symmetry of magnetic trigonal prismatic monolayers neither allow
for an internal relativistic Dzyaloshinskii-Moriya interaction (DMI) nor a
reactive spin-orbit torque. We demonstrate that the DMI only becomes important
at the boundaries, where it modifies the boundary conditions of the
magnetization and leads to a helical equilibrium state with a helical
wavevector that is inherently linked to the internal spin orientation.
Furthermore, we find that the helical wavevector can be electrically
manipulated via dissipative spin-torque mechanisms. Our results reveal that 2D
magnets offer a large potential for unexplored magnetic effects.
