  Using the parallel tempering algorithm and GPU accelerated techniques, we
have performed large-scale Monte Carlo simulations of the Ising model on a
square lattice with antiferromagnetic (repulsive) nearest-neighbor(NN) and
next-nearest-neighbor(NNN) interactions of the same strength and subject to a
uniform magnetic field. Both transitions from the (2x1) and row-shifted (2x2)
ordered phases to the paramagnetic phase are continuous. From our data
analysis, reentrance behavior of the (2x1) critical line and a bicritical point
which separates the two ordered phases at T=0 are confirmed. Based on the
critical exponents we obtained along the phase boundary, Suzuki's weak
universality seems to hold.
