Besides the restrictions imposed on the possible symmetry changes occurring in magnetic second-order phase transitions by the Landau theory, an additional restriction exists due to the specific form of the density function involved in these transitions. This restriction is presented as a "selection rule" for irreducible representations of the space group of the high-symmetry phase inducing the phase transitions. A general mathematical form of this selection rule is derived differing from that obtained by Kovalev. The role of crystal structure type in this rule is investigated, and results concerning some particular structure types are obtained. Prediction of magnetic symmetry of ferromagnetic Eu compounds is made.

• Received 25 November 1974

DOI:https://doi.org/10.1103/PhysRevB.12.1071