Peculiar 'from-edge-to-interior' spin freezing in a magnetic dipolar cube

By molecular dynamics simulation, we have investigated classical Heisenberg spins, which are arrayed on a finite simple cubic lattice and interact with each other only by the dipole-dipole interaction, and have found its peculiar from-edge-to-interior freezing process. As the temperature is decreased, spins on each edge predominantly start to freeze in a ferromagnetic alignment parallel to the edge around the corresponding bulk transition temperature, then from each edge grow domains with short-ranged orders similar to the corresponding bulk orders, and finally the system is expected to end up with a unique multidomain ground state at the lowest temperature. We infer that these freezing characteristics are attributed to the anisotropic and long-range nature of the dipole-dipole interaction combined with a finite-size effect.