Tricritical points in a nearest-neighbor FCC antiferromagnet in an external field

We use a microcanonical simulation algorithm to study phase transitions in a nearest-neighbor antiferromagnetic FCC Ising model in a magnetic field. From the temperature dependence of the internal energy we obtain the phase diagram in the temperature-field space, which is in good agreement with previous Monte Carlo calculations. The new results refer to the nature of the phase transitions, where we find indications of second-order transitions for magnetic fields in the ranges 3.0J < H < 3.5J and 4.3J < H < 4.7J.