Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors*

We report on magnetotransport investigations of Co/Ni/Co ferromagnetic single electron transistors, fabricated using a high-precision alignment procedure invoked during e-beam writing. As a result of reduced size, the devices exhibit single-electron transistor characteristics at 4.2K. Magnetotransport measurements carried out at 1.8K reveal TMR traces with negative coercive fields, which we interpret in terms of a switching mechanism driven by the shape anisotropy of the central wire-like Ni island. A large TMR of about 18{\%} is observed within a small source-drain bias regime. The TMR decreases rapidly with increasing bias, which we primarily attribute to the excitation of magnons in the central island.