DYNAMICS AND FINITE-SIZE EFFECTS IN 2-DIMENSIONAL LATTICES OF PLANAR DIPOLES

A two-dimensional (2D) system of classical dipoles free to rotate in a plane and fixed on an N x N square lattice (N less-than-or-equal-to 32) has been studied as a function of temperature (for thermal energies less than the typical magnetic energy). Spin dynamics simulations have been performed using periodic boundary conditions and the spin-wave excitation spectra measured. A long-lived mode related to a continuous degeneracy of the ground state of this system has been observed. A calculation to study both librational and rotational motion in this mode has been made. In the librational regime the measured frequencies are in good quantitative agreement with the calculations. For systems below a critical size the rotational regime of this mode is observed. The critical size predicted by the theory is in fair agreement with the observations.