Phase diagrams of site-frustrated Heisenberg models on simple cubic, bcc, and fcc lattices

Abstract

We have determined the phase diagrams of the site-frustrated Heisenberg model in three dimensions over the entire concentration regime of competing ferromagnetic F and antiferromagnetic A sites for the three basic lattice types: simple cubic sc , body-centered cubic bcc and face-centered cubic fcc using Monte Carlo methods. Long-range ferromagnetic FM or antiferromagnetic AF order is established at a finite temperature, second-order phase transition whenever F or A sites percolate. Since site percolation thresholds in three dimensions are less than 1 2 , FM and AF order coexist over a wide composition regime. The only apparent effect of frustration is to cause the AF and FM order to occur perpendicular to one another. Strong evidence is found to suggest that all of the transitions remain in the Heisenberg universality class and that there exists a tetracritical point at x= 1 2 where TC=TN.