CRITICAL-BEHAVIOR OF DILUTED HEISENBERG FERROMAGNETS WITH COMPETING INTERACTIONS

The pure and the site-diluted classical Heisenberg model on the face centered cubic (fee) lattice with ferromagnetic exchange J(nn) between nearest neighbors and antiferromagnetic exchange J(nnn) = - J(nn)/2 between next nearest neighbors is studied by Monte Carlo simulation. Data are generated by the heat bath algorithm for lattice sines L = 4, 8, 12, 16, 20 and 24, using histogram reweighting techniques and sampling up to several hundred configurations of the random site disorder. From a finite size scaling analysis both the critical temperature and the critical exponents are estimated. For the purl system, the data are in very good agreement with the critical exponent estimates 1/v approximate to 1.42, beta/v approximate to 0.51 obtained from other methods (as a check of the accuracy of our approach, we also study the nearest neighbor model - where J(nnn) = 0 - and again obtain very good agreement with the known behavior). However, for the diluted systems evidence for a new universality class is found. While for concentration c = 0.875 of occupied sites strong crossover phenomena preclude us from giving exponent estimates, for c = 0.75 we find 1/v approximate to 1.2 and beta/y approximate to 0.45. Possible reasons why the Harris criterion may not apply for this system are discussed. The application of this study to experiments on EuxSr1-xS is briefly mentioned.