  In quantum field theory, charged particles with spin $\geq 1$ may become
tachyonic in the present of magnetic fluxes above some critical field,
signaling an instability of the vacuum. The phenomenon is generic, in
particular, similar instabilities are known to exist in open and closed string
theory, where a spinning string state can become tachyonic above a critical
field. In compactifications involving RR fluxes $F_{p+2}$, the quantum states
which could become tachyonic by the same Nielsen-Olesen mechanism are Dp
branes. By constructing an appropriate background with RR magnetic flux that
takes into account back-reaction, we identify the possible tachyonic Dp brane
states and compute the formula for the energy spectrum in a sector. More
generally, we argue that in any background RR magnetic flux, there are high
spin Dp quantum states which become very light at critical fields.
