Analysis of magnetic structures of iron nitrides by Landau's theory of second-order phase transitions

The magnetic structures of iron nitrides are studied by Landau's theory of second-order phase transitions. It is found that the magnetization direction of ɛ-Fe3N must be parallel to the c-axis of the hexagonal structure, which may conclude the debates on the easy axes of ɛ-Fe3N in this field. The easy axes of α″-Fe16N2 are turned out to be along [001], or [100], or [110] direction, with the former two cases already found by the experiments. The magnetization along [111] is forbidden, which solves rigorously the puzzle why the easy axes of the α″-Fe16N2 on InGaAs and Fe are different from each other. Finally, the magnetic structure of γ′-Fe4N is also determined, and the result shows that there are three possibilities for the magnetization: parallel to the axial, or the body diagonal, or the face diagonal direction of the cubic structure, among which the first case has been found in experiments.