Microstructure and giant magnetoresistance in granular and multilayer magnetic thin films

Abstract

The microstructure and magnetic properties of phase-separated (FeAg, CoAg) granular and multilayer Permalloy (Py)/Au magnetic thin films exhibiting giant magnetoresistance (GMR) havebeen investigated. Surprisingly, two Fe-Ag films of similar composition grown under identicalconditions and having substantially different microstructures yet display similar GMR. The microstructure of these films is characterized by Fe-rich or Co-rich regions respectively, 350–700 nm in extent and surrounded by a Ag-rich matrix. Within the Ag-rich regions, the Fe concentration varies from 20 to 25 atomic % 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 Ag matrix. The Co and Fe particles are ~15 to 25Å in diameter. It is these small particles which account for the GMR 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. On the other hand, high resolution electron microscopy (HREM) analysis of as-grown permalloy (Py)/Au multilayers in (111) orientation revealed that the Au and Py layers have sharp interfaces with defects such as twin 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 nm² has been observed. On mild annealing, the twin density decreases and the multilayer interfaces become rough. The magnetoresistance of the as-grown Py/Au multilayer decreased from 11% to 8% on annealing at 250 °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.