Toward Apparent Negative Permittivity Measurement in a Magnetic Nanofluid with Electrically Induced Clusters

It is known that the permittivity of a magnetic nanofluid is controllable by external magnetic fields. The resulting effect of magneto-dielectric anisotropy originates in the magnetic nanoparticle assembly formation induced by the applied magnetic field. The particle assembly in magnetic nanofluids can be induced even by a dc electric field. Then, analogous to the magneto-dielectric effect, a question arises concerning the effect of the dc bias electric field on the nanofluid permittivity in a wide frequency range. We present dielectric spectra (1 mHz-1 MHz) measured on a transformer oil-based magnetic nanofluid at various temperatures. Electrode polarization and an interfacial relaxation process are analyzed. We show that the applied dc bias voltage results in permittivity sign switching at low frequencies. The critical frequency at which the sign is reversed depends on the nanofluid temperature and the dc bias voltage level. The nanoparticle assembly and conductive percolative paths are considered as key mechanisms leading to the transition from capacitive to inductive reactance. The measurement of apparent negative permittivity in a dc electric field can be used as a simple method for the detection of electric field-induced particle assembly and percolation. The permittivity sign control by means of a dc bias voltage may open an alternative avenue for research and applications of magnetic nanofluids.