We report on the fabrication and properties of (cobalt/alumina/iron oxide) tunnel junctions. We observe magnetoresistance (MR) effects reaching 43% at 4.2 K and 13% at room temperature. This large MR is ascribed to the presence of a ${\mathrm{Fe}}_{\text{3−x}}{\mathrm{O}}_{\mathrm{4}}$ (close to half-metallic magnetite) phase identified by electron diffraction. At low temperature, the MR drops sharply when the bias voltage is smaller than 10 mV, which suggests that the magnetoresistance originates from the activation of tunneling channels through spin polarized states below and above the Fermi level in the iron oxide.

We have fabricated ${\mathrm{C}\mathrm{o}/\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3/\mathrm{C}\mathrm{o}$ tunnel junctions in which the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer includes a unique layer of small and disconnected cobalt clusters, with a typical mean diameter ranging from 2.0 to 4.0 nm. We observe spin-dependent tunneling properties with, below about 50 K, typical Coulomb-blockade effects induced by intermediate electron tunneling into cobalt clusters. The tunnel magnetoresistance ratio is approximately the same in the Coulomb-blockade regime (low-temperature range with very high tunnel resistance) and in the room-temperature regime without Coulomb blockade. It also depends weakly on the applied voltage.

We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.2 nm (25–7000 atoms) prepared by sequential sputtering of Co and ${\mathrm{Al}}_2{\mathrm{O}}_3.$ The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared with previous structural characterizations. The distribution of activation energies has been independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant $K_{\mathrm{eff}}.$ We find that $K_{\mathrm{eff}}$ is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal process is shown to proceed classically down to the lowest temperature investigated (1.8 K).