  We develop a self-consistently renormalized spin-wave theory, within a
mean-field approximation, for the two-dimensional Heisenberg ferromagnet with
perpendicular easy-axis anisotropy on the honeycomb lattice, as well as its
few-layer and bulk extensions. In this method, the magnetization dependence on
temperature is found as the solution of the self-consistency equation.
Furthermore, we account for the physical difference of surface and bulk layers
by treating the layers as separate sublattices. Thus, the method can be readily
generalized to study various magnetic phenomena in a broad range of systems,
including those comprising magnetically inequivalent sublattices. Using our
theory, we calculate the temperature-dependent magnetization for two
chromium-based layered van der Waals insulating magnets, Cr$_2$Ge$_2$Te$_6$ and
CrI$_3$, employing various sets of Heisenberg exchange and single-ion
anisotropy values reported for these materials in the existing literature. As
expected, we observe a strong dimensionality effect where the ordering
temperature is reduced and its sensitivity on the anisotropy is enhanced with
the decrease of dimensionality.
