Effective collective barrier for magnetic relaxation in frozen ferrofluids

Magnetic relaxation and frequency response were measured in frozen magnetic colloids of different concentrations. A crossover from reversible to irreversible behavior is observed for increasing concentration. In the irreversible state (below blocking temperature), magnetic relaxation is time-logarithmic over seven decades of the experimental time window. Effective collective barrier, U(M), was obtained as a function of the irreversible magnetization, M, by utilizing a master curve construction within developed phenomenological model. It was found that U(M) logarithmically diverges. This might provide evidence for self-organized critical behavior during magnetic relaxation in frozen ferrofluids.