E. Olive et P. Molho, THERMODYNAMIC STUDY OF A LATTICE OF COMPASS NEEDLES IN DIPOLAR INTERACTION, Physical review. B, Condensed matter, 58(14), 1998, pp. 9238-9247
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].