Giant negative magnetoresistance in granular ferromagnetic systems (invited)

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Abstract
Giant negative magnetoresistance (GMR) has been observed in a number of granular ferromagnetic systems [Co–Ag, Co–Cu, Fe–Cu, Fe–Ag, and (Fe–Ni)–Ag] with effect sizes as much as 85% at 5 K and 25% at 300 K. It is shown that the GMR is isotropic and is due to magnetic scattering of the conduction electrons by the nonaligned magnetic entities. The essential contribution to the resistivity is ρm[1−F(M/Ms)], where F(M/Ms) measures the spin disorder from ferromagnetic alignment and ρm is the magnetic resistivity that defines the size of the GMR. The magnitude of GMR is affected by the size and density of the magnetic entities which can be controlled by varying the composition and the process conditions. When the composition is varied, the maximum GMR is realized in systems with magnetic constituents of about 25%.