Magnetic dipole strength functions are deduced from averages of a large number of $M1$ transition strengths calculated within the shell model for the nuclides ${}^{90}\mathrm{Zr}$, ${}^{94}\mathrm{Mo}$, ${}^{95}\mathrm{Mo}$, and ${}^{96}\mathrm{Mo}$. An enhancement of $M1$ strength toward low transition energy is found for all nuclides considered. Large $M1$ strengths appear for transitions between close-lying states with configurations including proton as well as neutron high-$j$ orbits that recouple their spins and add up their magnetic moments coherently. The $M1$ strength function deduced from the calculated $M1$ transition strengths is compatible with the low-energy enhancement found in (${}^3\mathrm{He}$, ${}^3\mathrm{He}^{\prime }$) and ($d$, $p$) experiments. The Letter presents an explanation of the experimental findings.

• Received 13 August 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.232504