Experimental determination of the $6s^2 ^1S_0 -> 5d6s ^3 D_1$   magnetic-dipole transition amplitude in atomic ytterbium
  We report on a measurement of the highly forbidden $6s^2 ^1S_0 \to 5d6s ^3 D_1$ magnetic-dipole transition in atomic ytterbium using the Stark-interference technique. This amplitude is important in interpreting a future parity nonconservation experiment that exploits the same transition. We find $| < 5d6s ^3 D_1 | M1 | 6s^2 ^1S_0 > | ~ = ~ 1.33(6)_{Stat}(20)_{\beta} \times 10^{-4} \mu_0$, where the larger uncertainty comes from the previously measured vector transition polarizability $\beta$. The $M1$ amplitude is small and should not limit the precision of the parity nonconservation experiment. 
J. E. Stalnaker, D. Budker, D. P. DeMille, S. J. Freedman, and V. V.   Yashchuk