FLUCTUATION-INDUCED 1ST-ORDER TRANSITION IN A BCC ISING-MODEL WITH COMPETING INTERACTIONS

We study the simple system of (S=12) Ising spins on a bcc lattice with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange. Particular emphasis is placed on the region of competition between ferromagnetic and antiferromagnetic ordering of the second kind [AF(2)]. Renormalization-group ε-expansion methods along with Monte Carlo analysis leads to the suggestion that the mean-field bicritical phase diagram becomes, with the inclusion of fluctuations, a critical-end-point diagram. © 1979 The American Physical Society.