  In the background of a charged AdS black hole, we consider a Dirac particle
endowed with an arbitrary magnetic dipole moment. For non-zero charge and
dipole coupling of the bulk fermion, we find that the dual boundary theory can
be plagued with superluminal modes. Requiring consistency of the dual CFT
amounts to constraining the strength of the dipole coupling by an upper bound.
We briefly discuss the implications of our results for the physics of
holographic non-Fermi liquids.
