Magnetic susceptibility of concentrated ferrocolloids

The initial susceptibility of ferrocolloids (magnetic fluids) is studied within wide ranges of temperature and concentration. Experimental results are compared with the predictions of current theoretical models. A modified formula is proposed for an equilibrium ferrocolloid susceptibility, which allowed us to obtain quantitative agreement with the experimental data. It is shown that magnetodipole interparticle interactions lead to a manyfold increase in the susceptibility; however, no evidence of the spontaneous transition of ferrocolloids to the magnetoordered state are disclosed. For ferrocolloids with a dispersion medium having a low melting point, the loss of fluidity and transition to the solidified state at temperatures above the melting point are revealed. The independence of the solidification temperature on the magnetic phase concentration and strong temperature dependence on the type of stabilizer make it possible to suggest that the transition is associated with molecular interactions. The solidification of a magnetic fluid is accompanied by the appearance of a peak on the temperature dependence of susceptibility in all cases when large particles are present in a colloidal solution, and the relaxation of particle magnetic moment is due to thermal motion of the particles.