Anomalous magnetic aftereffect of a frozen magnetic fluid

A very stably dispersed magnetic fluid (MF) was zero-field-cooled from room temperature to 5 K followed by the application of a magnetic field of 2.86 MA/m for 300 s. After the field was removed (t=0), its residual magnetization M was measured as a function of time t for 80 000 s. After measurement, the MF sample was heated to room temperature, and the experiment was repeated after zero-field cooling to 5 K and again applying and removing the 2.86 MA/m field. We performed the same experiment several times, and obtained a different M versus t curve each time. With each cycle, the average M (at t=0) increased and the M versus t curve converged to a universal curve. In the early cycles´ experiments, M surprisingly increased with t during the later stages of the experiment. From other different experiments, it was concluded that the isolated surfactant molecules in the MF solvent played an important role. We propose a model wherein the magnetic colloids form closed magnetic flux circuits by forming collective micelle structures with temperature decrease. It is suggested that after application and removal of the field at 5 K, the micelles in the frozen MF break spontaneously, leading to an increase in the residual magnetization.