Abstract
A cobalt concentration dependent transition from usual magnetoresistance (MR) to complex MR behavior is reported in the co-sputtered Co-Ag thin films containing cobalt nanoparticles. At 20 K, the MR behavior of 32.6% film consists of three contributions superparamagnetic (SPM), ferromagnetic (FM), and perpendicular magnetic anisotropy (PMA), which are, respectively, operative at high, intermediate, and low fields. Magnetic force microscopy (MFM) provides the supporting evidence for the existence of a magnetic microstructure with magnetization perpendicular to the film plane for a threshold of 32.6% cobalt concentration. MFM images show the presence of regions with perpendicular magnetization having sizes much larger than the particle size. In addition, the magnetization measurements also substantiate the presence of PMA in films above 25.2% cobalt. The observed complex magnetotransport behavior and associated PMA are interpreted in terms of inter-particle (FM-SPM-FM) interactions in which local magnetic fields of the nearby FM particles align the SPM particles out of film plane.
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