  We develop a model for the effect of a magnetic field on quasiparticle
interference in an iron-based superconductor. Recently, scanning tunneling
experiments have been performed on Fe(Se,Te) to determine the relative sign of
the superconducting gap from the magnetic-field dependence of quasiparticle
scattering amplitudes. Using a simple two-band BCS model, we study three
different cases of scattering in a spin-split spectrum. The dominant effect of
a magnetic field in iron-based superconductors is caused by the Pauli limiting
of conduction electrons. Thereby time reversal odd scattering is induced which
enhances the sign-preserving and depresses the sign-reversing peaks in the
quasiparticle interference patterns.
