  We investigate the low-temperature behavior of ferromagnets with a
spontaneously broken symmetry O(3) $\to$ O(2). The analysis is performed within
the perspective of nonrelativistic effective Lagrangians, where the dynamics of
the system is formulated in terms of Goldstone bosons. Unlike in a
Lorentz-invariant framework (chiral perturbation theory), where loop graphs are
suppressed by two powers of momentum, loops involving ferromagnetic spin waves
are suppressed by three momentum powers. The leading coefficients of the
low-temperature expansion for the partition function are calculated up to order
$p^{10}$. In agreement with Dyson's pioneering microscopic analysis of the
cubic ferromagnet, we find that, in the spontaneous magnetization, the
magnon-magnon interaction starts manifesting itself only at order $T^4$. The
striking difference with respect to the low-temperature properties of the O(3)
antiferromagnet is discussed from a unified point of view, relying on the
effective Lagrangian technique.
