  We extend the well known phenomenon of magnetoresistance (extra resistivity
of materials in transverse magnetic field) to a new and unexplored regime where
in addition to a transverse magnetic field, a transverse AC field of resonant
frequency is also applied. In a magnetic field, electron spin levels are Zeeman
split. In a resonant AC field, we uncover a new channel of momentum relaxation
in which electrons in upper Zeeman level can deexcite to lower Zeeman level by
generating spin fluctuation excitation in the lattice (similar to what happens
in Electron Spin Resonance (ESR) spectroscopy). An additional resistivity due
to this novel mechanism is predicted in which momentum randomization of Zeeman
split electrons happen via bosonic excitations (spin fluctuations). An order of
magnitude of this additional resistivity is calculated. The whole work is based
upon an extension of Einstein's derivation of equilibrium Planckian formula to
near equilibrium systems.
