Composition dependence of particle size distribution and giant magnetoresistance in Co-Al-O granular films

We have investigated the particle size distributions in Co-Al-O insulating granular films with various Co concentrations (x(Co)) by analyzing the superparamagnetic behavior of magnetization. The comparison between the particle size distributions estimated from the magnetization analysis and the TEM observation has made it possible to distinguish between the topological size and the magnetic size of particles. It is suggested that some Co particles couple ferromagnetically to form larger magnetic clusters as x(Co) increases. The magnetoresistance (MR) ratio decreases slightly with x(Co) for x(Co) > 36 at%, which is different from the result for metallic granular systems where the MR ratio decreases drastically with the coarsening of magnetic clusters. (C) 2000 Elsevier Science B.V. All rights reserved.