Investigation of possible hysteresis effects arising from frequency- and field-dependent complex susceptibility measurements of magnetic fluids

Abstract Measurement of frequency and field dependence of the complex magnetic susceptibility, χ ( ω , H )= χ ′( ω , H )−i χ ″( ω , H ), of water- and hydrocarbon-based magnetic fluids over the frequency range 100 Hz–1 MHz are presented. The magnetic polarising field, H , is initially increased from 0 to 13.6 kA/m in 15 steps of approximately 900 A/m and then decreases in steps from 13.6 to 0 kA/m. A comparison is then made of the real and imaginary components of χ ( ω ) at each value of the polarising field to see if there is any significant difference between the susceptibility for increasing and decreasing polarising fields and, thus, to determine whether there is any hysteresis. Upon careful analysis of the results it is concluded that no significant hysteresis arises from the cyclic variation of H . This conclusion is supported by the results obtained in the case of the water-based fluid where, for the larger field values, a double loss-peak occurs in χ ″( ω ), indicating the presence of two different aggregate sizes in the sample. On decreasing H this double peak reduces to a single peak, indicating that the particles have reverted to their original state.