  The low-temperature series for the free energy density, pressure,
magnetization and susceptibility of cubic ideal ferromagnets in weak external
magnetic fields are discussed within the effective Lagrangian framework up to
three loops. The structure of the simple, body-centered, and face-centered
cubic lattice is taken into account explicitly. The expansion involves integer
and half-integer powers of the temperature. The corresponding coefficients
depend on the magnetic field and on low-energy effective constants that can be
expressed in terms of microscopic quantities. Our formulas may also serve as
efficiency or consistency check for other techniques like Green's function
methods, where spurious terms in the low-temperature expansion have appeared.
We explore the sign and magnitude of the spin-wave interaction in the pressure,
magnetization and susceptibility, and emphasize that our effective field theory
approach is fully systematic and rigorous.
