Monte Carlo simulations of moderately frustrated three-dimensional Heisenberg spin systems show evidence for two distinct ordering events. The upper one, at ${\mathit{T}}_{\mathit{c}}$, marks the onset of long-range ferromagnetic order and exhibits fluctuations which scale as the system size. The lower event, at ${\mathit{T}}_{\mathit{x}\mathit{y}}$, marks the freezing of transverse degrees of freedom and does not affect the long-range order in the system. Critical levels of frustration for the onset of ferromagnetism have been determined on several lattices and a phase diagram is presented.

• Received 12 September 1991

DOI:https://doi.org/10.1103/PhysRevB.45.3129