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A negative magnetoresistance was measured between 15 and 300 K under a maximum field H=70 kOe on two granular systems obtained by compacting Fe nanoparticles surrounded by an oxide shell ∼2 nm thick. The effect depended on the Fe core average size D that was of 8 and 18 nm in the two samples, as by x-ray diffraction. The maximum relative resistance change, about 5%, was observed at 50 K in the sample with smaller D. The results have been interpreted considering intraparticle and interparticle magnetic correlations and microscopic mechanisms similar to those responsible for the magnetoresistance in other granular systems.

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Magnetization and magnetoresistance were measured at room temperature and above on Au 80 Fe 20 platelets and ribbons obtained by solid-state quenching and melt spinning. The as-quenched samples contain a solid solution of Fe in Au and exhibit a paramagnetic ͑Curie-Weiss͒ behavior in the considered temperature range; magnetic data indicate very short-ranged magnetic correlation among adjacent spins, enhanced by local composition fluctuations. The solid solution is very stable. Only a very limited fraction ͑never exceeding 1%͒ of nanometer-sized, bcc Fe particles appears after long-time isothermal anneals at suitable temperatures. A negative magnetoresistance was observed at room temperature in all examined samples. The observed effect is anhysteretic, isotropic, and quadratically dependent on magnetic field H and magnetization M. The signal scales with M rather than with H, indicating that it depends on the field-induced magnetic order of the Fe moments, as it does for conventional giant magnetoresistance in granular magnetic systems. This effect derives from spin-dependent scattering of conduction electrons from single Fe spins or very small Fe clusters. The scattering centers are almost uncorrelated at a distance of the order of the electronic mean free path ͑of the order of 1.5 nm, or a few atomic spacings, at RT͒.

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