Inverse Magnetoresistance Loops in Nearly Percolating Co–Ag Nanostructures and Two-Phase Model

Abstract

We observed unusual inverse magnetoresistance (MR) loops in the transverse configuration for nearly percolating Co–Ag granular thin films deposited by a coevaporation technique onto a room-temperature glass substrate. When the substrate was heated above about 60°C during evaporation, the inverse MR loops disappeared and converted into normal loops. On the other hand, when the sample was annealed below about 120°C after deposition onto a room-temperature substrate, the inverse MR loops remained, whereas when annealed from about 120 to 300°C, the inverse MR loops were converted into “normal+inverse”-type loops and finally into totally normal loops when annealed above about 350°C. The unusual MR loops may have originated from magnetic nanostructures in nearly percolating Co–Ag granular films. A two-phase model incorporating uniaxial anisotropy, exchange interaction and spin-dependent scattering is presented, which is able to qualitatively explain some of the observed results.