Correlation of microstructural, magnetic, and transport properties of composite metal-insulator films

The goal of this research was to achieve a thorough understanding of a model granular metal-insulator system, and correlate the microstructural, electrical transport, magnetic and magneto-transport properties. Co-SiO ₂ is an ideal system from a structural standpoint since the Co/SiO ₂ interfaces are of high quality, and there is no evidence of intermixing. Below 46 volume percent Co, co-deposited films consist of approximately spherical particles; above that, the particles begin to connect forming elongated chain-like structures. The charge transport is due to a hopping conductivity over Coulomb energy barriers present for such small particles. The films exhibit magnetoresistance (MR) due to spin-dependent tunneling. The MR follows a (M/M_s) 2 dependence and is consistent with a temperature independent polarization of 0.26 for Co. For a film with 41 volume percent Co, which consists of isolated 40Å particles, MR, AC magnetic susceptibility, and Small Angle Neutron Scattering (SANS) are indicative of a magnetic transition from uncorrelated behavior above 150K to an ordered magnetic state below that temperature. The ordered state consists of 40Å particles that are coupled ferromagnetically over a length scale of 550Å, and those regions are aligned antiferromagnetically. The ordering is attributed to strong dipolar interactions. Films with 32 volume percent Co show signs of weaker magnetic interactions, but no long-range order. Discontinuous Co-SiO₂ multilayers were also studied since they are more readily saturated than co-deposited films. MR and M(H) data suggest that both ferromagnetically coupled particles and uncoupled particles are necessary for the sharpest MR response. Preliminary results on CoFe-SiO ₂ films indicate that films below percolation, in which the conductivity is non-metallic and due to hopping, exhibit magnetically soft properties.