  When the magnetosphere of a magnetar is perturbed by crustal deformation, an
electric field $E_\parallel$ parallel to the magnetic field line would appear
via Alvf\'en waves in the charge starvation region. The electron-positron pair
bunches will be generated via two-stream instability in the magnetosphere, and
these pairs will undergo charge separation in the $E_\parallel$ and in the
meantime emit coherent curvature radiation. Following the approach of Yang &
Zhang (2018), we find that the superposed curvature radiation becomes narrower
due to charge separation, with the width of spectrum depending on the
separation between the electron and positron clumps. This mechanism can
interpret the narrow spectra of FRBs, in particular, the spectrum of Galactic
FRB 200428 recently detected in association with a hard X-ray burst from the
Galactic magnetar SGR J1935+2154.
