The authors' recent studies of giant magnetoresistance (GMR) in insulating granular films and planar tunneling junctions are reviewed. First, GMR and related properties of sputter-deposited Co-Al-O granular films are described. Electron microscopy observations revealed that Co-Al-O films possess well-defined metal-nonmetal granular structures with Co granules of 2-3 nm in diameter. MR of 10.6% was observed for Co36Al22O42 film at room temperature, which is the largest value of GMR in insulating granular films, and the magnitude of MR is discussed in comparison with those for other granular systems. Anomalous temperature and bias-voltage dependence of MR was found in Co-Al-O granular films, and can be explained by a theory of spin-dependent higher-order tunneling. Improvement of low-field MR response of granular-in-gap (GIG) structures consisting of a Co-Y-O granular film and soft magnetic FeNi films is also shown. Next, GMR and current-voltage characteristics of planar tunneling junctions prepared by an ion beam sputtering technique is shown. MR of 4% was observed for a Fe/Al-O/NiFe/FeMn junction at 77 K. Inserting a thin Co layer between the insulating barrier and the NiFe layer improved the MR up to 18%.
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