Glass like behavior of magnetization in frozen ferrofluids

Abstract

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 ̃ f <2T m , is determined by magnetic clusters. A crossover to a phase with strong irreversibility at the fictitious freezing temperature T ̃ 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 2 dd 〉 1/2 for randomly ordered frozen moments were calculated for the investigated ferrofluids. The result T m≈2〈 E 2 dd 〉 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 ̃ f /〈E 2 dd〉 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.