Numerical studies of the isotropic Heisenberg model for random ferromagnets and spin glasses

Results of Monte Carlo studies of thermodynamical quantities in a generalisation of the Edwards-Anderson model are presented. The model contains a fraction x of antiferromagnetic bonds and (1-x) of ferromagnetic ones. The magnitudes of the exchange constants are given by a gaussian distribution and the Heisenberg spins are coupled by nearest-neighbour interactions. The samples with x=0 and x=0.2 are ferromagnetic, below a critical temperature. The samples with x=0.4 and x=0.5 are spin glasses. In these two cases the specific heat shows a broad maximum whereas the susceptibility seems to follow a mixture of cusp-like and 1/T-behaviours. T=0 studies of the magnetisation as a function of concentration suggest that the spin-glass-ferromagnet transition occurs at x=0.31+or-0.04 and is continuous.