Short-range magnetic order in two-dimensional cobalt-ferrite nanoparticle assemblies

Abstract

Magnetic order in two-dimensional islands of spherical $21\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ cobalt-ferrite $(\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4})$ nanoparticles is studied by magnetic force microscopy and spectroscopy. Images obtained at a temperature of $105\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ clearly reveal the presence of repulsive and attractive areas on top of the islands. Monte Carlo simulations on hexagonal arrangements of nanoparticles have been carried out and are consistent with the experimental findings. The simulations show that the magnetization patterns are determined by a competition between interactions among the nanoparticle magnetic dipole moments and alignment of the individual moments along the easy axes of magnetization. At an elevated temperature of $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, it will be shown that dipole-dipole interactions lead to short-range ferromagnetic order, which fluctuates rapidly in time due to thermal excitations. Upon reduction of the temperature to $105\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the dipole moments are forced toward the easy axes of magnetization, thereby increasing the amount of disorder.