We have studied the role of long-range interactions on the thermodynam
ics of magnetic systems. We have simulated, through the Monte Carlo me
thod, magnetization curves of a two-dimensional classical Ising model
including a long-range dipole-dipole-like interaction, where the range
of interaction is tuned by a parameter ct. Based on the conjectures o
f Tsallis statistics, we show that, for alpha/d less than or equal to
1 (d=2), the appropriate form of the equation of state is given by M/N
=m(T,H*) with T*=T/N* and H*=H/N*. The normalization factor N*[N*=(N(
1-alpha/d)-1)/(1-alpha/d)] emerges from the nonextensivity of thermody
namic variables of energy type. The crossover from nonextensive to ext
ensive behavior at alpha/d=1 occurs smoothly and similarly to other qu
ite different systems, thus suggesting it to be a general result.