  Recent experiments showed that Co undergoes a phase transition from
ferromagnetic hcp phase to non-magnetic fcc one around 100 GPa. Since the
transition is of first order, a certain region of co-existence of the two
phases is present. By means of \textit{ab initio} calculations, we found that
the hcp phase itself undergoes a series of electronic topological transitions
(ETTs), which affects both elastic and magnetic properties of the material.
Most importantly, we propose that the sequence of ETTs lead to the
stabilisation of a non-collinear spin arrangement in highly compressed hcp Co.
Details of this non-collinear magnetic state and the interatomic exchange
parameters that are connected to it, are presented here.
