2D to 3D crossover of the magnetic properties in ordered arrays of iron oxide nanocrystalsElectronic supplementary information (ESI) available: Details concerning the synthesis of the nanoparticles, the preparation of the arrays and the dispersion, the characterization methods and the Monte Carlo simulations, as well as further results from magnetization measurements. See DOI: 10.1039/c2nr33013j

The magnetic 2D to 3D crossover behavior of well-ordered arrays of monodomain γ-Fe 2 O 3 spherical nanoparticles with different thicknesses has been investigated by magnetometry and Monte Carlo (MC) simulations. Using the structural information of the arrays obtained from grazing incidence small-angle X-ray scattering and scanning electron microscopy together with the experimentally determined values for the saturation magnetization and magnetic anisotropy of the nanoparticles, we show that MC simulations can reproduce the thickness-dependent magnetic behavior. The magnetic dipolar particle interactions induce a ferromagnetic coupling that increases in strength with decreasing thickness of the array. The 2D to 3D transition in the magnetic properties is mainly driven by a change in the orientation of the magnetic vortex states with increasing thickness, becoming more isotropic as the thickness of the array increases. Magnetic anisotropy prevents long-range ferromagnetic order from being established at low temperature and the nanoparticle magnetic moments instead freeze along directions defined by the distribution of easy magnetization directions. Thickness dependent magnetic interactions in well ordered arrays of maghemite nanospheres studied by GISAXS, magnetometry and Monte Carlo simulations.