  We develop an analytical framework for nonreciprocal radiative heat transfer
in two-body planar systems. Based on our formalism, we identify effects that
are uniquely nonreciprocal in near-field heat transfer in planar systems. We
further introduce a general thermodynamic constraint that is applicable for
both reciprocal and nonreciprocal planar systems, in agreement with the second
law of thermodynamics. We numerically demonstrate our findings in an example
system consisting of magneto-optical materials. Our formalism applies to both
near- and far-field regimes, opening opportunities for exploiting
nonreciprocity in two-body radiative heat transfer systems.
