Observation of isotropic giant magnetoresistance in paramagneticAu80Fe20

Magnetization and magnetoresistance were measured at room temperature and above on ${\mathrm{Au}}_{80}{\mathrm{Fe}}_{20}$ 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-Weiss) 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 RT).