  MnSi is an itinerant magnet which at low temperatures develops a helical spin
density wave. Under pressure it undergoes a transition into an unusual
partially ordered state whose nature is debated. Here we propose that the
helical spin crystal (the magnetic analog of a solid) is a useful starting
point to understand partial order in MnSi. We consider different helical spin
crystals and determine conditions under which they may be energetically
favored. The most promising candidate has bcc structure and is reminiscent of
the blue phase of liquid-crystals in that it has line-nodes of magnetization
protected by symmetry. We introduce a Landau theory to study the properties of
these states, in particular the effect of crystal anisotropy, magnetic field
and disorder. These results compare favorably with existing data on MnSi from
neutron scattering and magnetic field studies. Future experiments to test this
scenario are also proposed.
