Retarded response of the optical transmittance through a magnetic fluid film under switching-on/off external magnetic fields

In our research, we develop the setup to observe the time-dependent response of the optical transmission through a magnetic fluid film under switching-on or -off the external magnetic field. The optical transmittance is reduced when an external magnetic field is applied perpendicularly to a magnetic fluid film. It was further found that the transmittance starts to decrease in tens of milliseconds after switching-on the external field (referred as the charging process). This retarded response of the transmittance with respect to the external of the field also occurs when the field is switched off (referred as the discharging process), with the retarding time ranging from several to tens of milliseconds, depending on field strength. In addition, the exponential decay or increase in the transmitted intensity was observed for the charging or discharging process, respectively.