Low-energy properties of two-dimensionalmagnetic nanostructures: interparticle interactions anddisorder effects

Abstract

The low-energy properties of two-dimensional ensembles ofdipole-coupled magnetic nanoparticles are studied as a function of structural disorder and particle coverage. Already smalldeviations from a square particle arrangement lift thedegeneracies of the microvortex (MV) magnetic configuration andresult in a strongly inhomogeneous magnetic order of the particle ensemble. The energy distribution of metastable statesis determined. For a low degree of disorder a stronglyasymmetric shape with a pronounced peak of the ground-stateenergy results. In contrast, for a strong disorder aGaussian-like distribution is obtained. The average dipoleenergy Ēdip decreases with increasingstructural disorder. Above a coverage-dependent degree ofdisorder Ēdip resembles the average dipole energy of a random particle set-up, for which a simplescaling behaviour is derived. The role of vacancies has beenstudied for a square particle array by determining the angulardistribution of the preferred MV angle as a function ofthe vacancy concentration. Preferred angles along the axial aswell as along the diagonal directions of the square array areobtained. A corresponding investigation for disturbed squarearrays yields preferred MV angles only along the axialdirections. The effect of dipole-quadrupole correctionsresulting from the finite size of the particles is quantified.