Annealing Effects on Tunnel Magnetoresistance in Polyimide-Co Granular Films

Recently, we reported on the synthesis of a high-quality polyimide-Co (PI-Co) granular thin film using vapor deposition polymerization. This thin film had a tunnel magnetoresistance (TMR) ratio of 2.6% at room temperature. Because thermal annealing is considered effective for manipulating the grain size in granular systems composed of metallic ferromagnetic grains samples were annealed at temperatures between 473 K and 673 K in high vacuum. All samples were superparamagnetic at room temperature, and the average diameter of the Co particles, as deduced from Langevin analysis, increased significantly in the sample annealed at 623 K. The temperature dependence of the electrical resistance indicated tunneling conduction between Co particles in all samples. The TMR ratio increased to 4.8% at an annealing temperature of 473 K but decreased significantly after annealing above 623 K. This change was attributed to PI decomposition, as determined from the IR spectra.