  Motivated by recent experiments on the itinerant helimagnet MnSi, we study
general properties of helical spin crystals - magnetic structures obtained by
superposing distinct spin spirals. An effective Landau description of helical
spin crystals is introduced and simple rules for stabilizing various spin
crystal structures over single spirals are established. Curious properties of
the magnetic structures so obtained, such as symmetry stabilized topological
textures and missing Bragg reflections are pointed out. The response of helical
spin crystals to crystalline anisotropy, magnetic field and non-magnetic
disorder are studied, with special reference to the bcc1 spin structure, a
promising starting point for discussing the 'partial order' phases seen at high
pressure in MnSi. Similar approaches may be applied to other crystallization
problems such as Larkin-Ovchinnikov-Fulde-Ferrel states in spin-imbalanced
superconductors.
