Abstract
Relations between periods of spatially modulated first-kind antiferromagnetic structure and orthorhombic lattice distortion in an FCC lattice are discussed on the basis of Landau's phenomenological theory. The period of the modulation of the magnetisation is shown to be either the same as or twice that of the lattice distortion. On the basis of the results, the magnetic structure below the Neel point in gamma -Fe precipitated in a Cu matrix, which was recently shown to exhibit periodic lattice distortion, is discussed.