Relaxation processes and stochastic dynamics ...

Two-dimensional (2D) ensembles of interacting magnetic nanoparticles are currently the subject of an intense fundamental and technological research activity. The magnetic particles in these systems are subject to both local couplings and long-range interactions which interplay can be experimentally tuned by varying the particle size and surface coverage. The magnetic response can then be correlated with the size distribution and geometrical arrangement of the nanoparticles. A problem of particular interest is the slow magnetic dynamics like magnetic ageing, rejuvenation, or magnetic viscosity. These non- equilibrium effects are characteristic of a spin-spin glass behavior of nanostructures where giant magnetic moments interact via competing exchange and dipole couplings.

The magnetic relaxation processes in disordered two-dimensional ensembles of dipole-coupled magnetic nanoparticles are investigated from a local perspective. The energy landscape of the system is explored by determining saddle points, adjacent local minima, energy barriers, and the associated minimum energy paths (MEPs) as functions of the structural disorder and particle density. The changes in the magnetic order of the nanostructure along the MEPs connecting adjacent minima are analyzed.  In particular, we have determined the extension of the correlated region where the directions of the particle magnetic moments vary significantly. This shows that with increasing degree of disorder the magnetic correlation length decreases, i.e., the elementary relaxation processes become more localized.

Quantitative results for the distribution of the energy barriers, and their relation to the changes in the magnetic configurations and the MEP lengths provide new insights on the long-time magnetic relaxation dynamics of these nanostructures.