  In magnetic tunnel junctions based on iron oxide nanoparticles the disorder
and the oxidation state of the surface spin as well as the nanoparticles
functionalization play a crucial role in the magnetotransport properties. In
this work, we report a systematic study of the effects of vacuum annealing on
the structural, magnetic and transport properties of self-assembled ~10 nm
Fe3O4 nanoparticles. The high temperature treatment (from 573 to 873 K)
decomposes the organic coating into amorphous carbon, reducing the electrical
resistivity of the assemblies by 4 orders of magnitude. At the same time, the
3Fe2+/(Fe3++Fe2+) ratio is reduced from 1.11 to 0.13 when the annealing
temperature of the sample increases from 573 to 873 K, indicating an important
surface oxidation. Although the 2 nm physical gap remains unchanged with the
thermal treatment, a monotonous decrease of tunnel barrier width was obtained
from the electron transport measurements when the annealing temperature
increases, indicating an increment in the number of defects and hot-spots in
the gap between the nanoparticles. This is reflected in the reduction of the
spin dependent tunneling, which reduces the interparticle magnetoresistance.
This work shows new insights about influence of the nanoparticle interfacial
composition, as well their the spatial arrangement, on the tunnel transport of
self-assemblies, and evidence the importance of optimizing the nanostructure
fabrication for increasing the tunneling current without degrading the spin
polarized current.
