CONTINUOUS VERSUS FIRST ORDER TRANSITION IN DILUTE ISING ANTIFERROMAGNETS IN A FIELD

A systematic Monte Carlo investigation of dilute Ising antiferromagnets in a uniform field as function of both dilution and ratio α of nnn to nn interactions on a cubic lattice is presented. A tricritical point is found in the range of dilution 0 ≤ p ≤ 0.3 for a ratio α = -0.5. Increasing p makes the first order region to shrink. On the other hand increasing α enlarges it. We present a new probabilistic mean field treament which yields the correct qualitative features of the phase diagram.