Glass like behavior of magnetization in frozen ferrofluids

The magnetization behavior of frozen ferrofluids shows a typical glassy behavior, e.g., a maximum in the zero field cooled magnetization at T-m; irreversibility and longtime relaxation of residual magnetization, but is distinct from those of canonical spin glasses. They exhibit many similarities with concentrated and re-entrant spin glasses and some with random magnetic anisotropy systems, for which the inhomogeneity of magnetic structure is a characteristic feature. It is suggested that a phase with weak irreversibility, ranging from (T) over tilde(f) < 2T(m), is determined by magnetic clusters. A crossover to a phase with strong irreversibility at the fictitious freezing temperature (T) over tilde(f) is deduced from the change of T-m(H)-characteristic in conjunction with the onset of magnetic hysteresis. The variance of dipole interaction [E(dd)2](1/2) for randomly ordered frozen moments were calculated for the investigated ferrofluids. The result T-m approximate to 2[E-dd(2)](1/2) supports the hypothesis of the existence of clusters with non-randomly ordered moments around T-m. From the field dependence of 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 particle clusters. (C) 1999 Elsevier Science B.V. All rights reserved.