  Collinear and non-collinear spin structures of wurtzite phase CoO often
appearing in nano-sized samples are investigated using first-principles density
functional theory calculations. We examined the total energy of several
different spin configurations, electronic structure and the effective magnetic
coupling strengths. It is shown that the AF3-type antiferromagnetic ordering is
energetically most stable among possible collinear configurations. Further, we
found that a novel spiral spin order can be stabilized by including the
relativistic spin-orbit coupling and the non-collinearity of spin direction.
Our result suggests that a non-collinear spin ground state can be observed in
the transition-metal-oxide nanostructures which adds an interesting new aspect
to the nano-magnetism study.
