Violation of the Brownian relaxation law for cobalt ferrite particles in water

Abstract

In the temperature range 0–90°C, the dynamic susceptibility of three samples of a magnetic fluid based on cobalt ferrite particles stabilized in water with a double layer of a surfactant was measured. The first layer of the stabilizer was formed of auric acid molecules. The second layer consisted of a mixture of lauric acid and sodium dodecyl sulfate. The liquid samples had a small (about 10%) volume fraction of particles in order to reduce the influence of interparticle interactions and eliminate non-Newtonian rheological properties. The main parameter in which the samples differed was viscosity. It was changed by adding a small amount of polyvinyl alcohol. For all samples, in the studied temperature range the frequency dependence of the dynamic susceptibility had a quasi-Debye form with a pronounced maximum for the imaginary part. The value of the magnetization relaxation time was determined from the frequency of the maximum on the imaginary part of the dynamic susceptibility. As a result, the dependences of the magnetization relaxation time on temperature were obtained. To analyze the results obtained, viscosities of the samples were measured. All samples exhibited the properties of Newtonian fluids – the value of dynamic viscosity did not depend on the shear rate. The obtained results on the dynamic susceptibility and viscosity were compared by plotting relaxation times as a function of the ratio of dynamic viscosity to absolute temperature. The resulting dependences do not fit on one universal straight line, as it is supposed to be according to the theory. The actual graph looks like a family of three different curves that diverge as viscosity increases. In order to assess the influence of Néel relaxation on the resulting susceptibility, the dynamic susceptibility of a frozen sample of the magnetic fluid was measured. The resulting susceptibility value was about 3% of the initial susceptibility of the liquid sample. Thus, despite the insignificant contribution of the Néel mechanism of magnetization relaxation to the dynamic susceptibility, it is impossible to explain the obtained measurement results on the basis of the Brownian relaxation mechanism alone.