Low-energy $M1$ strength functions of ${}^{60,64,68}\mathrm{Fe}$ are determined on the basis of large-scale shell-model calculations with the goal to study their development from the bottom to the middle of the neutron shell. We find that the zero-energy spike, which characterizes nuclei near closed shells, develops toward the middle of the shell into a bimodal structure composed of a weaker zero-energy spike and a scissorslike resonance around 3 MeV, where the summed strengths of the two structures change within only 8% around a value of $9.8{\mu }_N^2$. The summed strength of the scissors region exceeds the total $\gamma$ absorption strength from the ground state by a factor of about three, which explains the discrepancy between total strengths of the scissors resonance derived from ($\gamma$, ${\gamma }^{\prime }$) experiments and from experiments using light-ion induced reactions.

• Received 16 January 2017

DOI:https://doi.org/10.1103/PhysRevLett.118.092502