Giant magnetoresistance and microstructural characteristics of epitaxial Fe-Ag and Co-Ag granular thin films

Abstract

The microstructure and magnetic properties of phase-separated Fe-Ag and Co-Ag granular alloy films, grown epitaxially on (001) NaCl substrates and exhibiting giant magnetoresistance (GMR), have been investigated. Surprisingly, two Fe-Ag films of similar composition grown under identical conditions are found to have substantially different microstructures yet display similar GMR. The microstructure of the films is characterized by Fe-rich or Co-rich regions, respectively, 350-700 nm in extent, surrounded by a Ag-rich matrix. Within the Ag-rich regions, the Fe concentration varies from 20 to 25 at % and the Co concentration is ≅16 at %. Within these regions essentially pure fcc Co particles and bcc Fe particles are in parallel and rotated epitaxial alignment respectively with the fcc silver matrix. The Co and Fe particles are ≅15-25 Å in diameter. It is these small particles which most likely account for the giant magnetoresistance exhibited by these alloys. This suggests that a size distribution of magnetic particles, sharply peaked at the optimum size with limited bulk segregation, might give rise to larger GMR values.