Raman study of magnetic field effects on surfacted and ionic ferrofluids

We report Raman spectroscopic measurements on surfacted (SFF) and ionic ferrofluids in magnetic fields up to 0.25 T. For the SFF Raman spectra display the characteristic broad bands associated with vibrations of surfactant and water molecules. With increasing magnetic field the overall Raman intensity decreases abruptly up to about 50 mT, leveling out above this field. In contrast, at high magnetic fields a broad peak which is attributed to a magnetic excitation becomes apparent in the spectra at frequencies in the range of 4000 cm(-1). Our results can be explained in terms of a local increase in magnetic-particle concentration and the formation of ordered structures in the presence of a magnetic field. The Raman spectra of the electrostatically stabilized ferrofluids, which are dominated by the H2O stretching vibrations, exhibit a different magnetic-field behavior of their intensity probably indicating the incipient formation of large nanograin clusters. (C) 2003 Elsevier B.V. All rights reserved.