Re-orientation Transition in Molecular Thin Films: Potts Model with Dipolar Interaction

Abstract

We study the low-temperature behavior and the phase transition of a thin film by Monte Carlo simulation. The thin film has a simple cubic lattice structure where each site is occupied by a Potts parameter which indicates the molecular orientation of the site. We take only three molecular orientations in this paper which correspond to the 3-state Potts model. The Hamiltonian of the system includes: (i) the exchange interaction $J_{ij}$ between nearest-neighbor sites $i$ and $j$ (ii) the long-range dipolar interaction of amplitude $D$ truncated at a cutoff distance $r_c$ (iii) a single-ion perpendicular anisotropy of amplitude $A$. We allow $J_{ij}=J_s$ between surface spins, and $J_{ij}=J$ otherwise. We show that the ground state depends on the the ratio $D/A$ and $r_c$. For a single layer, for a given $A$, there is a critical value $D_c$ below (above) which the ground-state (GS) configuration of molecular axes is perpendicular (parallel) to the film surface. When the temperature $T$ is increased, a re-orientation transition occurs near $D_c$: the low-$T$ in-plane ordering undergoes a transition to the perpendicular ordering at a finite $T$, below the transition to the paramagnetic phase. The same phenomenon is observed in the case of a film with a thickness. We show that the surface phase transition can occur below or above the bulk transition depending on the ratio $J_s/J$. Surface and bulk order parameters as well as other physical quantities are shown and discussed.

7 pages, 11 figures, submitted for publication

References in corpus (4)

• The Spin Reorientation Transition and Phase Diagram of Ultrathin Ferromagnetic Films
• Monte Carlo Study of Magnetic Resistivity in Semiconducting MnTe
• Reexamination of the long-range Potts model: a multicanonical approach
• Frustration Effects in Antiferromagnetic FCC Heisenberg Films

Cited by in corpus (3)

• Phase transition in frustrated thin films -- physics at phase boundaries
• Vortex Structure in Magnetic Nanodots: Dipolar Interaction, Mobile Spin Model, Phase Transition and Melting
• Theory and Simulation of Magnetic Materials: Physics at Phase Frontiers