GIANG MAGNETORESISTANCE AND MICROSTRUCTURAL CHARACTERISTICS OF EPITAXIAL FE-AG AND CO-AG GRANULAR THIN-FILMS

The microstructure and magnetic properties of phase-separated Fe-Ag an d Co-Ag granular alloy films, grown epitaxially on (001) NaCl substrat es and exhibiting giant magnetoresistance (GMR), have been investigate d. Surprisingly, two Fe-Ag films of similar composition grown under id entical conditions are found to have substantially different microstru ctures yet display similar GMR. The microstructure of the films is cha racterized by Fe-rich or Co-rich regions, respectively, 350-700 nm in extent, surrounded by a Ag-rich matrix. Within the Ag-rich regions, th e Fe concentration varies from 20 to 25 at % and the Co concentration is congruent-to 16 at %. Within these regions essentially pure fcc Co particles and bcc Fe particles are in parallel and rotated epitaxial a lignment respectively with the fcc silver matrix. The Co and Fe partic les are congruent-to 15-25 angstrom in diameter. It is these small par ticles which most likely account for the giant magnetoresistance exhib ited by these alloys. This suggests that a size distribution of magnet ic particles, sharply peaked at the optimum size with limited bulk seg regation, might give rise to larger GMR values.