  Magnetic materials with highly anisotropic chemical bonding can be exfoliated
to realize ultrathin sheets or interfaces with highly controllable optical or
spintronics responses, while also promising novel cross-correlation phenomena
between electric polarization and the magnetic texture. The vast majority of
these van-der-Waals magnets are collinear ferro-, ferri-, or antiferromagnets,
with a particular scarcity of lattice-incommensurate helimagnets of defined
left- or right-handed rotation sense, or helicity. Here we use polarized
neutron scattering to reveal cycloidal, or conical, magnetic structures in
DyTe$_3$, with coupled commensurate and incommensurate order parameters, where
covalently bonded double-slabs of dysprosium square nets are separated by
highly metallic tellurium layers. Based on this ground state and its evolution
in a magnetic field as probed by small-angle neutron scattering (SANS), we
establish a one-dimensional spin model with off-diagonal on-site terms,
spatially modulated by the unconventional charge order in DyTe$_3$. The
CDW-driven term couples to antiferromagnetism, or to the net magnetization in
applied magnetic field, and creates a complex magnetic phase diagram indicative
of competing interactions in an easily cleavable helimagnet. Our work paves the
way for twistronics research, where helimagnetic layers can be combined to form
complex spin textures on-demand, using the vast family of rare earth
chalcogenides and beyond.
