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-spun Cu100-xCox systems (x = 5-20) exhibiting a giant magnetoresistance is obtained by plotting this quantity as a function of reduced magnetization. Two ranges, R-theta(H-e) and R-theta(H-e), have been recently introduced to describe the extent of correlation among angles of tilt (theta) and of twist (phi) of superparamagnetic moments precessing around a local field axis. The angle of tilt appears to be spatially correlated 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 constants characterizing the process of scattering of conduction electrons by adjacent magnetic moments (electronic time of flight, relaxation times for theta and phi) are discussed in detail. An explicit expression 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 loss of the phase coherence. (C) 1997 American Institute of Physics.