Birefringence of magnetite-based magnetic fluids: the effect of the surface-coating layer

Summary form only given. Fe/sub 3/O/sub 4/ nanoparticles obtained by chemical precipitation were treated with two molecular species (dodecanoic acid and a C/sub 12/-C/sub 14/ polyalcohol) to produce three magnetite-coated magnetic fluid (MF) samples. Diluted MF samples (8/spl times/10/sup 15/ particle/cm/sup 3/) were investigated at room temperature using static magnetic birefringence measurements (SMB). The mean particle diameter (8.9 /spl plusmn/ 0.2 nm) and the diameter dispersity (0.31 /spl plusmn/ 0.01) obtained from the fitting of the SMB data were in excellent agreement with the data obtained from transmission electron microscopy (9.4 /spl plusmn/ 0.1 nm and 0.32 /spl plusmn/ 0.01). Such agreement could only be achieved by considering the field dependence of the magnetic permeability and dimer formation. The dimer magnetic permeability depends upon the nanoparticle coating, thus suggesting particle-particle interaction effects . The dimer magnetic permeability coating layer thickness dependence is examined.