Thermodynamics of finite magnetic two-level systems
  We use Monte Carlo simulations to investigate the thermodynamical behaviour of aggregates consisting of few superparamagnetic particles in a colloidal suspension. The potential energy surface of this classical two-level system with a stable and a metastable `ring' and `chain' configuration is tunable by an external magnetic field and temperature. We determine the complex `phase diagram' of this intriguing system and analyze thermodynamically the nature of the transition between the ring and the chain `phase'. 
Peter Borrmann (1), Heinrich Stamerjohanns (1), Eberhard Hilf (1),   Philippe Jund (2), Seon Gon Kim (2), David Tomanek (2) ...