  Recently reported measurements of specific heat on the compound Mn-formate
di-Urea (Mn-f-2U) by Takeda et al. [Phys. Rev. B 63, 024425 (2001)] are
considered. As a model to describe the overall thermodynamic behavior of such
compound, the easy-axis two-dimensional Heisenberg antiferromagnet is proposed
and studied by means of the 'pure quantum self-consistent harmonic
approximation' (PQSCHA). In particular it is shown that, when the temperature
decreases, the compound exhibits a crossover from 2D-Heisenberg to 2D-Ising
behavior, followed by a 2D-Ising-like phase transition, whose location allows
to get a reliable estimate of the easy-axis anisotropy driving the transition
itself. Below the critical temperature T_N=3.77 K, the specific heat is well
described by the two-dimensional easy-axis model down to a temperature T*=1.47
K where a T^3-law sets in, possibly marking a low-temperature crossover of
magnetic fluctuations from two to three dimensions.
