  In the paper a theoretical study of critical (Curie) temperature of diluted
ferromagnet based on multilayer graphene (or graphite) with hydrogen adatoms
deposited over carbon atoms belonging to single sublattice is presented. The
calculations are performed within Pair Approximation (PA) for diluted
ferromagnetic system with long-range interactions. The method is able to take
into account the spin-space anisotropy of coupling. The results obtained within
Mean Field Approximation (MFA) are also presented for comparison. The assumed
interaction between hydrogen adatom spins is inversely proportional to their
mutual distance, with the additional exponential attenuation reflecting the
presence of disorder in the system. The results obtained for a wide range of
impurity concentrations and interaction decay length are discussed. The
strongly non-linear behaviour of critical temperature as a function of dilution
is predicted, at variance with MFA predictions. Moreover, MFA tends to
overestimate heavily the critical temperature values compared to PA. An
universal dependence of critical temperature on impurity concentration and
interaction decay length is found for strong dilution regime.
