SHORT-TIME DYNAMICS OF CORRELATED MAGNETIC-MOMENTS IN SUPERPARAMAGNETIC CU-CO MELT-SPUN ALLOYS EXHIBITING GIANT MAGNETORESISTANCE

Evidence for correlation among superparamagnetic particles in melt-spu n Cu100-xCox systems (x = 5-20) exhibiting a giant magnetoresistance i s obtained by plotting this quantity as a function of reduced magnetiz ation. Two ranges, R-theta(H-e) and R-theta(H-e), have been recently i ntroduced to describe the extent of correlation among angles of tilt ( theta) and of twist (phi) of superparamagnetic moments precessing arou nd a local field axis. The angle of tilt appears to be spatially corre lated over a distance larger by a factor of 3 than the angle of twist. This difference is explained by analyzing the short-time dynamics of magnetic moments in superparamagnetic granular systems with long-range interactions (of dipolar and the RKKY-like type). The typical time co nstants characterizing the process of scattering of conduction electro ns by adjacent magnetic moments (electronic time of flight, relaxation times for theta and phi) are discussed in detail. An explicit express ion for R-phi(H) is obtained by considering the competition between a magnetic interaction favoring parallel (or antiparallel) alignment or adjacent moments, and thermal disturbances resulting in a continuous l oss of the phase coherence. (C) 1997 American Institute of Physics.