Frequency dependence of dielectric anisotropy in ferrofluids

We have conducted broadband, temperature-dependent dielectric spectroscopy on ferrofluids of various concentrations. We study the change of the permittivity on application of a homogeneous magnetic field (∼0.5 T, parallel or orthogonal). The results reveal that the field-induced magnetodielectric effect is markedly frequency dependent and should be attributed to two different mechanisms: The structural anisotropy affects not only the interfacial polarisation of the nanoparticles, but also the strength of the hopping process observed in the samples. Both these effects influence the low-frequency permittivity values. High-frequency measurements should be used to derive information on the microstructure by means of effective media analysis