Effects of static electric field and temperature on the dynamic dielectric responses of mixed oil-based and bilayer-stabilised magnetic fluids

Magnetic fluids based on non-polar liquids constitute attractive materials exhibiting magnetic field-sensitive dielectric relaxation processes. In this study, we focus on the dielectric response of three magnetic fluids with different bilayer stabilisation. The first stabilising layer is a fatty acid, while the second layer is polymeric. The dielectric spectra are studied on thin layers of magnetic fluids in the frequency range from 0.1 Hz to 200 kHz. The presence of the bilayer on the magnetic particle surfaces gives rise to two distinctive relaxation processes observable in permittivity and dissipation factor spectra. We show that the relaxation maxima are significantly sensitive to the acting direct current bias electric voltage (0-3 V). It is found that the bias electric field shifts the relaxation maxima towards higher frequencies and greater permittivity and dissipation factor values. The shift is similar to the effect of temperature, which is also documented in this study. The application of the Havriliak-Negami fitting functions on the studied dielectric spectra is employed in the analysis. The free charge and the resulting conductivity contribution are also taken into account. The direct current-sensitive dielectric response of magnetic fluids may find applications in multifunctional sensors that detect both electric and magnetic fields.