  We consider a phenomenological holographic model, inspired by the D3/D7
system with a 2+1 dimensional intersection, at finite chemical potential and
magnetic field. At large 't Hooft coupling the system is unstable and needs
regularization; the UV cutoff can be decoupled by considering a certain double
scaling limit. At finite chemical potential the model exhibits a phase
transition between states with filling fractions plus and minus one--half as
the magnetic field is varied. By varying the parameters of the model, this
phase transition can be made to happen at arbitrary values of the magnetic
field.
