  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.
