  We consider the consequence of nonreciprocity in near-field heat transfer by
studying systems consisting of magneto-optical nanoparticles.We demonstrate
that, in thermal equilibrium, a nonreciprocal many-body system in heat transfer
can support a persistent directional heat current, without violating the second
law of thermodynamics. Such a persistent directional heat current cannot occur
in reciprocal systems, and can only arise in many-body systems in heat
transfer. The use of nonreciprocity therefore points to a new regime of
near-field heat transfer for the control of heat flow in the nanoscale.
