  The interplay of spin angular momentum and thermal radiation is a frontier
area of interest to nanophotonics as well as topological physics. Here, we show
that a thick planar slab of a nonreciprocal material, despite being at thermal
equilibrium with its environment, can exhibit nonzero photon spin angular
momentum and nonzero radiative heat flux in its vicinity. We identify them as
the persistent thermal photon spin (PTPS) and the persistent planar heat
current (PPHC) respectively. With a practical example system, we reveal that
the fundamental origin of these phenomena is connected to spin-momentum locking
of thermally excited evanescent waves. We also discover spin magnetic moment of
surface polaritons in nonreciprocal photonics that further clarifies these
features. We then propose a novel thermal photonic imaging experiment based on
Brownian motion that allows one to witness these surprising features by
directly looking at them using a lab microscope. We further demonstrate the
universal behavior of these near-field thermal radiation phenomena through a
comprehensive analysis of gyroelectric, gyromagnetic and magneto-electric
nonreciprocal materials. Together, these results expose a surprisingly little
explored research area of thermal spin photonics with prospects for new avenues
related to non-Hermitian topological photonics and radiative heat transport.
