  Iron-based superconductors can exhibit different magnetic ground states and
are in a critical magnetic region where frustrated magnetic interactions
strongly compete with each other. Here we investigate the longitudinal modes of
spin fluctuations in an unified effective magnetic model for iron-based
superconductors. We focus on the collinear antiferromagnetic phase (CAF) and
calculate the behavior of the longitudinal modes when different phase
boundaries are approached. The results can help to determine the nature of the
magnetic fluctuations in iron-based superconductors.
