  Antiferromagnetic and ferro/ferrimagnetic orders are typically exclusive in
nature, thus, their co-existence in atomic-scale proximity is expected only in
heterostructures. Breaking this paradigm and broadening the range of
unconventional magnetic states, we report here on an atomic-scale hybrid spin
state, which is stabilized in three-dimensional crystals of the polar
antiferromagnet Co$_2$Mo$_3$O$_8$ by magnetic fields applied perpendicular to
the \emph{Co} honeycomb layers and possesses a spontaneous in-plane
ferromagnetic moment. Our microscopic spin model, capturing the observed field
dependence of the longitudinal and transverse magnetization as well as the
magnetoelectric/elastic properties, reveals that this novel spin state is
composed of an alternating stacking of antiferromagnetic and ferrimagnetic
honeycomb layers. The strong intra-layer and the weak inter-layer exchange
couplings together with competing anisotropies at octahedral and tetrahedral
\emph{Co} sites are identified as the key ingredients to stabilize
antiferromagnetic and ferrimagnetic layers in such a close proximity. We show
that the proper balance of magnetic interactions can extend the stability range
of this hybrid phase down to zero magnetic field. The possibility to realize a
layer-by-layer stacking of such distinct spin orders via suitable combinations
of microscopic interactions opens a new dimension towards the nanoscale
engineering of magnetic states.
