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Low-energy enhancement of magnetic dipole radiation

Magnetic dipole strength functions have been deduced from averages of a large number of $M1$ transition strengths calculated within the shell model for the nuclides $^{90}$Zr, $^{94}$Mo, $^{95}$Mo, and $^{96}$Mo. An enhancement of $M1$ strength toward low transition energy has been 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 re-couple 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 ($^3$He,$^3$He') and $(d,p)$ experiments. The present work presents for the first time an explanation of the experimental findings.