Abstract

We observed spin-dependent tunneling in Co90Fe10/AlOx/Co90Fe10/AlOx/Co90Fe10 ferromagnetic double tunnel junctions. A middle Co90Fe10 layer, which is inserted between two AlOx layers, is discontinuous due to the difference in the surface energy of these layers. The average diameter of Co90Fe10 particles was estimated to be 2.0-4.5 nm from cross-sectional transmission electron microscopy (TEM) images. At low temperature (< 50 K), a Coulomb gap is observed in current-voltage (I-V) curves, and the tunnel magnetoresistance (TMR) ratio within the gap enhances significantly with decreasing temperature, which indicates that the dominant electron transport is inelastic co-tunneling within the Coulomb gap. These observations in a ferromagnetic tunnel junction with 2-dimensional magnetic nano-particle layer insertion accord well with theoretical predictions by Takahashi and Maekawa [Phys. Rev. Lett. 80, 1758 (1998)].

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