MICROSTRUCTURE AND GIANT MAGNETORESISTANCE IN GRANULAR AND MULTILAYERMAGNETIC THIN-FILMS

The microstructure and magnetic properties of phase-separated (FeAg, C oAg) granular and multilayer Permalloy (Py)/Au magnetic thin films exh ibiting giant magnetoresistance (GMR) havebeen investigated. Surprisin gly, two Fe-Ag films of similar composition grown under identicalcondi tions and having substantially different microstructures yet display s imilar GMR. The microstructure of these films is characterized by Fe-r ich or Go-rich regions respectively, 350-700 nm in extent and surround ed by a Ag-rich matrix. Within the Ag-rich regions, the Fe concentrati on varies from 20 to 25 atomic % and the Co concentration is similar t o 16 at%. Within these regions essentially pure fee Co particles and b cc Fe particles are in parallel and rotated epitaxial alignment respec tively with the fee Ag matrix. The Co and Fe particles are similar to 15 to 25 Angstrom in diameter. It is these small particles which accou nt for the GMR exhibited by these alloys. This suggests that a size di stribution of magnetic particles, sharply peaked at the optimum size w ith limited bulk segregation might give rise to larger GMR values. On the other hand, high resolution electron microscopy (HREM) analysis of as-grown permalloy(Py)/Au multilayers in (111) orientation revealed t hat the Au and Py layers have sharp interfaces with defects such as tw in boundaries and misfit dislocations. The density of twin boundaries decreases from the Pt-seed layer to the top surface of the multilayer. An average of 1 twin boundary/195 nm(2) has been observed. On mild an nealing, the twin density decreases and the multilayer interfaces beco me rough. The magnetoresistance of the as-grown Py/Au multilayer decre ased from 11% to 8% on annealing at 250 degrees C for 40 minutes. From the measured roughness and total thickness of Au layers it was found that Au preferentially intermixes with Py, which reduces the magnitude of GMR.