  Nuclear level densities for $^{56,57}$Fe have been extracted from the primary
$\gamma$-ray spectra using ($^3$He,$^3$He$^{\prime}\gamma$) and ($^3$He,$\alpha
\gamma$) reactions. Nuclear thermodynamic properties for $^{56}$Fe and
$^{57}$Fe are investigated using the experimental level densities. These
properties include entropy, Helmholtz free energy, caloric curves, chemical
potential, and heat capacity. In particular, the breaking of Cooper pairs and
single-quasiparticle entropy are discussed and shown to be important concepts
for describing nuclear level density. Microscopic model calculations are
performed for level densities of $^{56,57}$Fe. The experimental and calculated
level densities are compared. The average number of broken Cooper pairs and the
parity distribution are extracted as a function of excitation energy for
$^{56,57}$Fe from the model calculations.
