  We investigate the thermodynamic behavior of antiferromagnetic films in
external magnetic fields oriented perpendicular to the staggered magnetization.
Within the systematic effective Lagrangian framework we first calculate the
two-point function and the dispersion relation for the two types of magnons up
to one-loop order. This allows us to split the two-loop free energy density
into a piece that originates from noninteracting dressed magnons, and a second
piece that corresponds to the genuine magnon-magnon interaction. We then
discuss the low-temperature series for various thermodynamic quantities,
including the parallel staggered and uniform perpendicular susceptibilities,
and analyze the role of the spin-wave interaction at finite temperature.
