Theory and simulation of positionally frozen Heisenberg spin systems

The structure, thermodynamics and the ferromagnetic phase transition of a positionally frozen disordered Heisenberg spin system are studied by means of extensive Monte Carlo calculations in combination with finite size scaling techniques, as well as resorting to the Replica Ornstein-Zernike formalism. The system is formed by a collection of Heisenberg spins whose spatial distribution corresponds to a soft sphere fluid with its particle positions frozen at a certain quench temperature. The spin orientations are allowed to equilibrate at a given equilibrium temperature. If the quench and equilibrium temperatures are similar the properties of the positionally frozen system are practically indistinguishable from those of the fully equilibrated Heisenberg spin fluid. On the other hand, one observes that as the quenching temperature of the spatial degrees of freedom increases, so does the Curie temperature of the Heisenberg spins. The theory fails to reproduce the location of the ferromagnetic transition, despite its relative accuracy in the determination of the orientational structure in the supercritical region.Received: 25 July 2003, Published online: 9 September 2003PACS: 75.50.Lk Spin glasses and other random magnets - 64.60.Fr Equilibrium properties near critical points