  We have studied the anisotropic three-dimensional nearest-neighbor Ising
model with competitive interactions in an uniform longitudinal magnetic field
$H$. The model consists of ferromagnetic interaction $J_{x}(J_{z})$ in the
$x(z)$ direction and antiferromagnetic interaction $J_{y}$ in the $y$
direction. We have compared our calculations within a effective-field theory in
clusters with four spins (EFT-4) in the simple cubic (sc) lattice with
traditional Monte Carlo (MC) simulations. The phase diagrams in the
$h-k_{B}T/J_{x}$ plane ($h=H/J_{x}$) were obtained for the particular case
$\lambda_{1}=J_{y}/J_{x} (\lambda_{2}=J_{z}/J_{x})=1$ (anisotropic sc). Our
results indicate second-order frontiers for all values of $H$ for the
particular case $\lambda_{2}=0$ (square lattice), while in case
$\lambda_{1}=\lambda_{2}=1$, we observe first- and second-order phase
transitions in the low and high temperature limits, respectively, with presence
of a tricritical point. Using EFT-4, a reentrant behavior at low temperature
was observed in contrast with results of MC.
