When colloidal nanoparticles made of soft magnetic materials have strong interparticle interactions, new magnetic phases may be formed, such as super-ferromagnets or super-spin glasses, with different local temperature and field-dependent magnetization dynamics. Magneto-transport experiments on nanoscale tunnel junctions formed in magnetic nanoparticle arrays, defined either by scanning probe tips or patterned nanoscale electrodes, can be used as probes of the magnetization dynamics.

The electronic properties of arrays and isolated magnetite nanocrystals were studied using tunneling spectroscopy. Macroscopic tunnel junctions were used to study stacked arrays of the nanocrystals. The temperature dependent resistance measurements showed an abrupt increase of the resistance around 100 K, attributed to the Verwey metal-insulator transition, while the current-voltage characteristics exhibit a sharp transition from an insulator gap to a peak in the density of states near the Fermi energy. This conductance peak was sensitive to in-plane magnetic field showing large magnetoresistance. The tunneling spectra obtained on isolated particles using a Scanning Tunneling Microscope exhibit a gap-like structure below the transition temperature that gradually disappeared with increasing temperature, ending with a small peak structure around zero bias.