Raman spectroscopy on surfacted ferrofluids in a magnetic field

We investigated the effect of an external magnetic field up to 0.25 T on the Raman spectra of surfacted ferrofluids with various magnetic-particle concentrations. With increasing magnetic field the Raman spectra, which display the characteristic broad bands associated with vibrations of surfactant and water molecules, show a pronounced decrease in intensity in the range up to 50 mT. We interpret this behavior as due to an increase of the local particle concentration in the magnetic fluid. At larger fields a magnetic excitation with an intensity growing in proportion to the magnetization of the ferrofluid becomes apparent at around 4400 cm(-1). We consider both effects as evidence for the formation of a solid structure by the magnetic nanoparticles in the presence of a magnetic field.