Phenomenological consideration of spin-wave spectrum in noncollinear antiferromagnet Mn3NiN

A spin-wave spectrum in noncollinear cubic antiferromagnet Mn3NiN of perovskite structure type is investigated theoretically using the phenomenological Lagrangian method. The activation of one branch of a spin-wave spectrum is shown to vanish at the points of spin-rotational phase transitions examined experimentally in this crystal. The activation of two other branches does not vanish at the points of phase transitions. The splitting of these branches is proportional to the magnitude of the weak ferromagnet moment detected experimentally in the Mn3NiN crystal.