Studies of Fcc Heisenberg Antiferromagnets by Monte Carlo Simulation on Large Spin Arrays

,We report Monte Carlo studies of fcc Heisenberg antiferromagnets carried out on arrays with 108,000 spins. A lattice w&h only JNN # 0 was found to exhibit a Type I AI? order despite the disordered nature of its ground state. Contrary to previous reports, our data indicate in this case a first order phase transition. So far, all reported Monte Carlo (MC) studies of fcc Heisenberg antiferromagnets 11-31 have been done on relatively small samples (up to 4 000 spins). In order to examine the role of finite size effects in such calculations, we have carried out a series of simulations on decidedly larger arrays, with 108,000 fcc sites (i.e., 30 X 30 X 30 cubic cells). In this paper, we report some results obtained for fully occupied lattices. The calculations have been done for classical vector spins, assuming periodic boundary conditions, and using the conventional MC simulation technique [4]. However, for time economy reasons, instead of the commonly used Metropolis algorithm we have applied the heat bath algorithm developed by Walker and Walsted which offers a decidedly faster convergence [5]. The calculations have been run on a CYBER 205 supercomputer using a partially vectorized program, and on two VAXes (8 820 and 780). The fcc antiferromagnetic (AF) lattice exhibits a variety of magnetic structures depending on the relative magnitudes of the NN and NNN exchange interactions. We have investigeted two such cases, (i) a lattice with