THERMODYNAMIC STUDY OF A LATTICE OF COMPASS NEEDLES IN DIPOLAR INTERACTION

We present the study of an original system, which is an experimental d ipolar model allowing a precise understanding of dipolar effects. This system is a square lattice of 22x22 magnetized needles (compass needl es). Its thermodynamic properties are studied by considering the actio n of a random external field supposed to mimic the thermal fluctuation s. In parallel, a Monte Carlo numerical simulation of the experimental system is performed. Both studies show the existence of a phase trans ition between an ordered phase and a disordered one. We introduce the notion of ''line length'' which is the relevant quantity to describe t he obtained configurations and to give a precise description of the ob served transition. This phase transition is characterized by the emerg ence of a preferential direction, called the ''director,'' and by an i ncrease of the average line length. The order parameter expressing the symmetry breaking is defined. Our results show an example of a succes sful mapping of a dynamic dissipative system excited by a random field onto equilibrium statistical mechanics. [S0163-1829(98)05638-0].