  We study the phase diagram of a quasi-two dimensional magnetic system ${\rm
Rb_2MnF_4}$ with Monte Carlo simulations of a classical Heisenberg spin
Hamiltonian which includes the dipolar interactions between ${\rm Mn}^{2+}$
spins. Our simulations reveal an Ising-like antiferromagnetic phase at low
magnetic fields and an XY phase at high magnetic fields. The boundary between
Ising and XY phases is analyzed with a recently proposed finite size scaling
technique and found to be consistent with a bicritical point at T=0. We discuss
the computational techniques used to handle the weak dipolar interaction and
the difference between our phase diagram and the experimental results.
