Experimental verification of interactions between randomly distributed fine magnetic particles

We experimentally examined whether or not a magnetic fluid (MF) is really superparamagnetic by comparing the initial magnetic susceptibilities of the mother MFs with those of their highly diluted solutions (more than 1000 times diluted) in which the dipole-dipole interaction between the particles was negligible. We used three mother MFs, SA(1), SB1, and SC1, and their highly diluted solutions, SA(2), SB2, and SC2, respectively. The particles' dispersability was best in SA(1) and poorest in SC1. From the static field experiment, it was found that the mutual interaction between the particles in SB1, and SC1 made clusters of particles with magnetically closed flux circuits even at zero field while no interaction was detected in SA(1). The initial complex magnetic susceptibility, (χ) over tilde, as a function of temperature, T, under an AC field experiment revealed that the complex susceptibility of both the samples SA(1) and SA(2) showed peaks as a function of T. However, their (χ) over tilde vs. T curves were not similar, leading to the conclusion that the sample SA(1) was not super-paramagnetic. Instead, SA(1) was a magnetic spin-glass induced by the weak interaction between the particle spins. The existence of the spin-glass state was also confirmed by the Volgel-Fulcher law dependence of the AC-susceptibility peak temperature, T-p, or the frequency of the AC field. (C) 2003 Elsevier B.V. All rights reserved.