Development of the Theory of Helical Spin Ordering

Theory of helical spin ordering in a lattice having one magnetic atom per unit cell and its extension to the case of several atoms per unit cell have brought about deeper understanding of magnetic structures than before. These theories are reformulated concisely in the present paper and their applications to ${\mathrm{Cr}}_2{\mathrm{O}}_3‐{\mathrm{Fe}}_2{\mathrm{O}}_3$ solid solutions, $\mathrm{MnP}$ , ${\mathrm{Mn}}_3\mathrm{Sn}$ (spiraling triangular spins), and spinels are discussed. The role of uniaxial anisotropy energy in spin ordering and a particular spin order of an integral period of seven layers in thulium, as well as the pairing of spins in holmium due to sixfold anisotropy energy, are discussed. Transition between a fan structure and the parallel alignment in magnetic field and its example in $\mathrm{MnP}$ are mentioned. Finally, the existence of a spin‐wave mode of zero frequency in a general modified‐helical spin arrangement is pointed out.