Spin filter effect in iron oxide nanocrystal arrays

Integrating nanocrystals (NCs) into magnetic tunnel structures is of considerable interest due to expectation of novel properties from their spin selective transport and single electron features. Superstructures by cplloidal NCs having translational and orientational order and interesting collective magnetic properties can be prepared by solution casting through sensitive interparticle and particle-substrate interactions. In this work, we discuss the study on magnetic field induced assembly of mono-dispersed iron oxide NCs to obtain spin filter effect across (he superlattice array, when sandwiched between gold electrodes. The deposition of mixed phase Fe3O4@gamma-Fe2O3 NCs on SiO2/Au surface proceeds through slow solvent evaporation and are studied for controlled interparticle spacing. For specific NC concentration, the ordering depends on the substrate chemistry and the ligands passivating NC surface, which affects the concentration of cluster nuclei formed. In presence of a magnetic field, the tunnel structure exhibits enhanced positive tunnel magnetoresistance at low temperatures, which could be related to their ferromagnetism and the attempts by electrons to percolate NC superlattice with preserved spin. A sign reversal for magnetoresistance is exhibited by the vertical tunnel junctions on raising the temperature.