  The thermal properties of antiferromagnetic films -- in particular, the
square-lattice antiferromagnet -- subjected to an external magnetic field
pointing into the direction of the staggered magnetization are explored. The
effective field theory analysis of the free energy density is carried out to
two-loop order. While the emphasis is on finite temperature, we also discuss
the behavior of the magnetization and staggered magnetization at zero
temperature. Our results imply that the staggered magnetization increases in
presence of the magnetic field -- reminiscent of magnetic catalysis. Most
remarkably, if staggered and magnetic field strength are kept fixed, the
magnetization initially grows when temperature increases.
