MAGNETOHYDRODYNAMICS Vol. 40 (2004), No. 4, pp. 421–430 STRUCTURAL STUDIES OF FERROFLUIDS BY SMALL-ANGLE NEUTRON SCATTERING M. Balasoiu 1,2 , M.V. Avdeev 1 , A.I. Kuklin 1 , V.L. Aksenov 1 , D. Bica 3 , L. V´ ek´ as 3 , D. Hasegan 2 , Gy. T¨ or¨ ok 4 , L. Rosta 4 , V. Garamus 5 , J.Kohlbrecher 6 1 Ural State University, 620083, Ekaterinburg, Russia 2 Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Russia 3 Institute of Space Sciences, Bucharest, Romania 4 Center of Fundamental and Advanced Technical Research, Timisoara Branch of RAS, Romania 5 Research Institute for Solid State Physics and Optics, Budapest, Hungary 6 GKSS Research Centre, Geesthacht, Germany 7 Paul Scherrer Institute, Villigen, Switzerland Small-angle neutron scattering (SANS) is a powerful technique for studying the mi- crostructure of ferrofluids under different conditions. The paper reviews recent results obtained from the analysis of SANS data for some of ferrofluids based on non-polar and polar carriers. Specific features revealed by SANS and their comparison for different types of ferrofluids are discussed. Problems to be solved are emphasized. Introduction. The knowledge of the ferrofluid microstructure plays a key role in understanding their mechanism of stabilization and behaviour under vari- ous conditions. Last decades have showed that the method of small-angle neutron scattering (SANS) can be effectively used in structural analysis of such complex systems as ferrofluids. SANS rests upon the interaction of thermal neutrons (en- ergy ∼10 meV) with atoms of the medium under investigation (e.g., [1]). The neutron scattering is mainly a result of the interaction of neutrons with nuclei of atoms (nuclear scattering). For magnetic materials an additional contribution comes from the interaction of magnetic moments of neutrons and magnetic atoms (magnetic scattering). Neutrons are characterized by a large penetration depth that makes possible to study unmodified bulk samples. With account for specific features of neutron scattering, the following tasks for SANS can be set in the frame of structural studies of ferrofluids. First, this concerns the definition of the nuclear structure in bulk with respect to the parameters of the particle size distribution; thickness and composition of the surfactant shell; micelle formation in ferrofluids; interparticle interaction and particle aggregation. A powerful application in this case is a contrast variation technique employing the fact that if hydrogen (H) in the studied system is substituted (fully or partially) with its isotope deuterium (D), the scattering length density changes significantly for the component, where the substitution takes place. Second, the magnetic scattering of neutrons allows one to get information about the magnetic structure of ferrofluids, namely, the magnetic size of particles and aggregates, as well as magnetic correlation between particles. A special role of polarized neutrons in studying the magnetic structure of ferrofluids should be stressed. In this case, a neutron beam is arranged in such 421