Spin-dependent tunneling between ferromagnetic metals in a new type of tunnel junction (abstract)

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We report on tunneling experiments in tunnel junctions with ferromagnetic electrodes (cobalt or permalloy) separated by a relatively thick (15 nm) Al₂O₃ layer in which small cobalt clusters are embedded. Electron transport is, thus, by successive tunnelings from electrode to cluster and from cluster to cluster. This structure for multichannel and multistep tunneling is less liable to metallic bridging by pinholes than junctions with an unique thin layer of insulator, and also makes easier the control of the total tunnel resistance. Layers of Al₂O₃ and discontinuous ultrathin layers of cobalt ("cluster layers") are alternately deposited by sputtering through masks. The structure is characterized by transmission electron microscopy (Co clusters of 3 nm typical diameter, with about 2 nm of Al₂O₃ between successive cluster layers and between clusters and electrode). The total tunnel resistance can reach 25 k at 4.2 K. Its variation with magnetic field reflects the magnetization reversal of the electrodes and cobalt clusters, with a maximum fractional change of about 21% at room temperature. ©1997 American Institute of Physics.