  We investigate the contributions of finite-temperature magnetic fluctuations
to the thermodynamic properties of bcc Fe as a function of pressure. First, we
apply a tight-binding total-energy model parameterized to first-principles
linearized augmented planewave computations to examine various ferromagnetic,
anti-ferromagnetic, and noncollinear spin spiral states at zero temperature.
The tight-binding data are fit to a generalized Heisenberg Hamiltonian to
describe the magnetic energy functional based on local moments. We then use
Monte Carlo simulations to compute the magnetic susceptibility, the Curie
temperature, heat capacity, and magnetic free energy. Including the
finite-temperature magnetism improves the agreement with experiment for the
calculated thermal expansion coefficients.
