Spin-dependent tunneling in discontinuous metal/insulator multilayers

We have studied the structural, magnetic and transport properties of (Co/SiO₂) discontinuous multilayers. These multilayers consist of layers of Co particles embedded in an insulating SiO₂ matrix. The current-in-plane (CIP) and current-perpendicular-to-the-plane (CPP) resistivities of the discontinuous multilayers can be tuned independently over orders of magnitude by varying the nominal thicknesses of the metallic and insulating layers. Negative magnetoresistance (MR) due to spin-dependent tunneling has been observed in both CIP and CPP geometries. At room temperature the two magnetoresistive responses are similar, differing only in magnitude. At lower temperatures, the two responses are remarkably different. The CIP-MR saturates readily following the magnetization curve whereas the CPP-MR exhibits hysteresis up to magnetic fields higher than 20 kOe. These differences suggest the nature of the magnetic domain structure in each metallic plane. These systems should permit a combination of ease in preparation with high magnetoresistance sensitivity at low fields.