The magnetic dipole transition strength B(M1) of 48Ca is dominated by
a single resonant state at an excitation energy of 10.23 MeV. Experiments
disagree about B(M1) and this impacts our understanding of spin flips in
nuclei. We performed ab initio computations based on chiral effective field
theory and found that B(M1:0+→1+) lies in the range from 7.0 to
10.2μN2. This is consistent with a (γ,n) experiment but larger
than results from (e,e′) and (p,p′) scattering. Two-body currents
yield no quenching of the B(M1) strength and continuum effects reduce it by
about 10%. For a validation of our approach, we computed magnetic moments in
47,49Ca and performed benchmark calculations in light nuclei