  We present a computational search for spin spiral ground states in
two-dimensional transition metal halides that are experimentally known as van
der Waals bonded bulk materials. Such spin spirals break the rotational
symmetry of the lattice and lead to polar ground states where the axis of
polarization is strongly coupled to the magnetic order (type II multiferroics).
We apply the generalized Bloch theorem in conjunction with non-collinear
density functional theory calculations to find the spiralling vector that
minimizes the energy and then include spin-orbit coupling to calculate the
preferred orientation of the spin plane with respect to the spiral vector. We
find a wide variety of magnetic orders ranging from ferromagnetic, stripy
anti-ferromagnetic, 120$^\circ$ non-collinear structures and incommensurate
spin spirals. The latter two introduce polar axes and are found in the majority
of materials considered here. The spontaneous polarization is calculated for
the incommensurate spin spirals by performing full supercell relaxation
including spinorbit coupling and the induced polarization is shown to be
strongly dependent on the orientation of the spiral planes. We also test the
effect of Hubbard corrections on the results and find that for most materials
LDA+U results agree qualitatively with LDA. An exception is the Mn halides,
which are found to exhibit incommensurate spin spiral ground states if Hubbard
corrections are included whereas bare LDA yields a 120$^\circ$ non-collinear
ground state.
