Glass like behavior of magnetization in frozen ferrofluids

The magnetization behavior of frozen ferrofluids shows a typical glassy beh avior, e.g., a maximum in the zero field cooled magnetization at T-m; irrev ersibility and longtime relaxation of residual magnetization, but is distin ct from those of canonical spin glasses. They exhibit many similarities wit h concentrated and re-entrant spin glasses and some with random magnetic an isotropy systems, for which the inhomogeneity of magnetic structure is a ch aracteristic feature. It is suggested that a phase with weak irreversibilit y, ranging from (T) over tilde(f) < 2T(m), is determined by magnetic cluste rs. A crossover to a phase with strong irreversibility at the fictitious fr eezing temperature (T) over tilde(f) is deduced from the change of T-m(H)-c haracteristic in conjunction with the onset of magnetic hysteresis. The var iance of dipole interaction [E(dd)2](1/2) for randomly ordered frozen momen ts were calculated for the investigated ferrofluids. The result T-m approxi mate to 2[E-dd(2)](1/2) supports the hypothesis of the existence of cluster s with non-randomly ordered moments around T-m. From the field dependence o f the relaxation behavior and from the ratio (T) over tilde(f) [E-dd(2)](1/ 2) it was concluded that the ferrofluid with an extended net-like aggregate structure enters in the state with strong irreversibility at significantly higher rescaled temperatures, rather than those with compact, isolated par ticle clusters. (C) 1999 Elsevier Science B.V. All rights reserved.