  Ballistic transport in graphene p-n junctions in the presence of magnetic
field exhibits two distinct regimes: At low fields, transport is partially
suppressed by the field. When the field exceeds a certain critical value, the
junction is pinched off by the Landau level formation. Transmission and
conductance are found in the entire range of fields using Lorentz boost and
mapping to the Landau-Zener problem. We show that perfect transmission occurs
at a field-dependent collimation angle, indicating that the chiral dynamics of
massless Dirac fermions persists at a finite magnetic field. A current switch,
utilizing field-tunable collimation angle, is proposed. With a generalization
of the developed approach we study transmission through p-n junctions in
graphene bilayer.
