Model simulations of ground-state and finite-temperature properties of disordered magnetic nanostructures

The properties of two-dimensional ensembles of magnetic nanoparticles that interact by magnetic dipole coupling are investigated. The low-temperature magnetic arrangements, the average binding energy E-dip due to dipolar interactions, and its scaling behavior with respect to the particle density C are calculated for different types of structural disorder and particle-size distributions. Many different metastable magnetic states are obtained, which exhibit strong noncollinearities and are reminiscent of a spin-glass behavior. For a given C, \E-dip\ increases with increasing disorder of the particle positions. For random distributions at low particle densities C less than or equal to 0.2, E-dip is dominated by the contributions of short interparticle distances. Thus, it scales as \E-dip\ proportional to C-alpha with an unusually small exponent alpha = 0.85-1. The straightforward scaling of the dipole interaction, alpha similar or equal to 3/2: is obtained only for C greater than or equal to 0.5 or for nearly periodic ensembles. The finite temperature behavior of these disordered interacting nanomagnets is explored. The specific heat and magnetic susceptibility are calculated by performing Monte Carlo simulations. The onset of long-range magnetic order is discussed. In addition we determine hysteresis loops at finite temperatures and compare the results for different degrees of disorder.