Dispersion of magnetic susceptibility and the microstructure of magnetic fluid

The microstructure of magnetic fluid produced on the basis of kerosene with oleic acid as a stabilizer is studied experimentally. An analytical procedure based on the known dependence of the time of Brownian relaxation of the magnetic moment of the colloidal particle on its size and the expansion of a low-frequency spectrum of dynamic susceptibility into the series of Debye functions is used. Magnetic susceptibility is measured at frequencies from 10 Hz to 100 kHz and temperatures from 225 to 360 K for colloidal solutions with the volume fraction of magnetite from 0.08 to 0.17. The clusters with uncompensated magnetic moments and sizes varying from 50 to 70 nm that are three-or fourfold larger than the mean diameter of a single colloidal particle are found. It is revealed that characteristic sizes of clusters are virtually independent of temperature and concentration of colloidal particles. The contribution of clusters to the equilibrium susceptibility of magnetic fluid grows exponentially with decreasing temperature, being manyfold larger at low temperatures than that of single particles. The obtained temperature dependence of equilibrium susceptibility is compared with that predicted from current theoretical models.