Investigation of the field dependence of magnetic fluids exhibiting aggregation

Measurement of the frequency dependent, complex susceptibility, chi(omega) = chi'(omega) - ichi"(omega), over the frequency range 100 Hz to 1 MHz, of a colloidal suspension of cobalt ferrite particles, by means of the toroidal technique, indicates the presence of aggregation in the samples: the aggregation being identified by the appearance of a low-frequency loss-peak in the chi"(omega) component of the susceptibility. By means of the application of a polarising field, H (A/m), the field dependence of the complex susceptibility of the aggregates, chi(omega, H), the frequency dependent loss tangent, tandelta and mean power loss, T (J/m(3)), are investigated for polarising fields over the approximate range 0-13.6 kA/m. It is shown that the plots of both chi(omega), H and tandelta may be fitted in terms of the Cole-Cole parameter, alpha, and it is demonstrated how the fits can be used to determine data outside the measurement frequency range. A novel method for determining the tandelta data is also presented. It is also shown how, with increase in polarising field, the magnetic losses of the fluid are reduced. (C) 2004 Elsevier B.V. All rights reserved.