We propose a string dual to the SU(Nc) N=4 SYM coupled to Nf massless
fundamental flavors in an external magnetic field. The flavors are introduced
by homogeneously smeared Nf D7-branes and the external magnetic field via a
non-trivial Kalb-Ramond B-field. Our solution is perturbative in a parameter
that counts the number of internal flavor loops. In the limit of vanishing
B-field the background reduces to the supersymmetric one obtained in
hep-th/0612118. We introduce an additional probe D7--brane and in the
supersymmetric limit of vanishing B-field perform a holographic renormalization
of its "on-shell" action. We consider also non-supersymmetric probes with fixed
worldvolume gauge field corresponding to a magnetic field coupled only to the
fundamental fields of the probe brane. We study the influence of the
backreacted flavors on the effect of dynamical mass generation. Qualitatively
the physical picture remains unchanged. In the next step we consider the case
when the magnetic field couples to both the backreacted and the probe
fundamental degrees of freedom. At sufficiently strong magnetic field the meson
spectrum signals an instability of the probe D7-brane, which we interpret as
reflecting an instability of the supergravity background.Comment: 33 pages, 5 figures, v2: minor corrections and references adde
