Time solved susceptibility spectra of magnetic fluids

Measurements of the frequency (f) and polarizing field (H) dependent complex magnetic susceptibility, χ(f, H)=χ′(f, H)–iχ″(f, H) of a magnetic fluid over the frequency range 1kHz to 2MHz, in various polarizing fields from zero to approximately 7kA/m are presented. The sample was a commercial magnetic fluid with magnetite particles in isopar-M. Usually, Brownian relaxation time measurements are involved in the investigation of particle agglomeration in magnetic fluids. In some situations the presence of miscellaneous particles may induce erroneous conclusions. The paper shows that over time measurements can solve the problem in giving confirmation of particle agglomeration within magnetic fluids. Measurements performed in a constant polarizing field of, H=7kA/m, have shown that after 30min both components of the complex magnetic susceptibility increased and the relaxation peak shifted towards higher frequencies. The phenomenon is correlated to the particle aggregation process and demonstrates that the relaxation peaks are due to the rotation of small aggregates. •Magnetic susceptibility, χ=χ′–iχ″, in various polarizing fields, H, was measured.•The sample was a magnetic fluid with magnetite particles dispersed Isopar-M.•In constant H, over time measurements, χ(t) and χ(t+Δt) were also performed.•Over the range 1kHz to 2MHz, for H=7kA/m and Δt=30min, χ has increased.•The result is due to the time evolution of particle agglomerations in the sample.