Theory of helical spin crystals: phases, textures and properties

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.