Critical behavior of a bcc Ising antiferromagnet in a magnetic field

Field-induced phase transitions in the bcc Ising antiferromagnet with nearest-neighbor interactions have been studied using a Monte Carlo method. The properties of $N\times N\times N$ lattices with periodic boundary conditions have been determined for $N\le 16$ and the size effects analyzed using finite-size scaling theory. We find no field dependence of the critical exponents and only a slow variation of critical amplitudes in small fields in agreement with the "smoothness postulate." At low temperatures the critical-field curve shows a weak, but clear maximum which is about 2% greater than the zero-temperature value $H_c$. Crossover to noncritical exponential behavior due to thermally induced spin flips is observed as we move away from the low-temperature phase boundary.