  Reflection and transmission of electrons scattered by a rectangular potential
step in the presence of an external magnetic field parallel to the electron
beam is described with the use of the Dirac equation. It is shown that in
addition to the known effects present in the so called Klein paradox, the
presence of magnetic field gives rise to electron components with reversed spin
in the reflected and transmitted beams. The spin-flip scattering processes are
caused by the spin-orbit interaction activated by electric field of the
potential step and transverse momentum components of electron motion induced by
the magnetic field. The contemporary understanding of the Klein paradox,
consisting in the finite transmission even when the potential height tends to
infinity, is generalized to the presence of magnetic field and spin-reversed
electron beams. The spin-reversed beams are shown to occur also for electrons
moving above the step. It is proposed that, accounting for the anomalous value
of the electron spin $g$-factor related to radiation corrections, the
reflection and transmission scattering from the potential step can be used as
an electron spin filter.
