ISSN 1027-4510, Journal of Surface Investigation. X-ray, Synchrotron and Neutron Techniques, 2015, Vol. 9, No. 2, pp. 320–325. © Pleiades Publishing, Ltd., 2015. Original Russian Text © I.V. Gapon, V.I. Petrenko, M.V. Avdeev, L.A. Bulavin, Yu.N. Khaydukov, O. Soltwedel, V. Zavisova, I. Antal, P. Kopcansky, 2015, published in Poverkhnost’. Rentgenovskie, Sinkhrotronnye i Neitronnye Issledovaniya, 2015, No. 4, pp. 8–13. 320 INTRODUCTION Magnetic fluids (MFs) are unique systems combin- ing magnetic properties and fluidity. They consist of magnetic nanoparticles (with a characteristic size of 10 nm) located in a liquid medium. To prevent aggre- gation, the particle surface is coated by a surfactant layer. Study of MFs is of great practical and fundamen- tal importance [1–4]. Despite a large number of papers devoted to the bulk structure of magnetic flu- ids, little attention was paid to their microstructure near the interface with other media. Currently much interest is shown specifically in objects at the liquid– solid interface [5–8]. In this paper, we report the results of studying the interface between silicon and a polar magnetic fluid based on nanomagnetite particles dispersed in water and stabilized by a double sodium-oleate layer. Previ- ously, the analysis of specular neutron reflection at this interface [8] revealed the ordering of magnetic nano- particles when deposited onto a silicon surface in the case of high-concentration (magnetite volume frac- tion up to 10%) systems. In addition, strong diffuse scattering (due to small-angle scattering from nanopar- ticles) was observed. When selecting specular-reflection curves, this scattering was taken into account using sup- plementary small-angle neutron scattering (SANS) data for bulk samples. This approach implies identical structures of the liquid phase in the bulk and at the interface. At the same time, recent SANS experiments for the MF under consideration [9] revealed the pres- ence of stable aggregates, the structure of which was sensitive to the preparation conditions and the pres- ence of additional components. Individual MF parti- cles and their aggregates may exhibit different adsorp- tion properties. To answer the question to what extent the MF structure at the interface repeats the bulk structure, one must obtain a specular reflection curve with allowance for the diffuse background without using SANS data. In this study we analyze two ways to take into account the background when processing experimental neutron-reflectometry data. An inter- face with a low-concentration MF is considered. Based on this consideration, it is concluded that the MF structures at the interface and in the bulk differ. EXPERIMENTAL A detailed description of neutron reflectometry as applied to different interfaces can be found, for exam- ple, in [10]. Note that a specularly reflected neutron beam contains information about the distribution of the scattering length density ρ along the normal to the Consideration of Diffuse Scattering in the Analysis of Specular Neutron Reflection at the Magnetic Fluid–Silicon Interface I. V. Gapon a, b , V. I. Petrenko a, b , M. V. Avdeev a , L. A. Bulavin b , Yu. N. Khaydukov c , O. Soltwedel c , V. Zavisova d , I. Antal d , and P. Kopcansky d a Frank Neutron Physics Laboratory, Joint Institute for Nuclear Research, ul. Zholio-Kyuri 6, Dubna, Moscow oblast, 141980 Russia e-mail: gapon@jinr.ru b Shevchenko National University, Kyiv, 3022 Ukraine c Max Plank Institute for Solid State Research, Garching, Stuttgart, 85747 Germany d Institute of Experimental Physics, Slovak Academy of Sciences, Košice, 04001 Slovakia Received April 22, 2014 Abstract—Neutron reflectometry data on the interface between a magnetic fluid (magnetite/sodium ole- ate/D 2 O) and silicon are considered with allowance for a background of diffuse scattering from magnetic- fluid nanoparticles adsorbed on the silicon surface. Two ways to subtract the background in selecting the spec- ular reflection curve are analyzed. Based on a comparison of neutron reflectometry and small-angle neutron scattering data, it is concluded that individual nanoparticles rather than their aggregates (which are also present in the liquid phase) are preferably adsorbed on the silicon surface. In addition, free sodium-oleate molecules are adsorbed from the magnetic-fluid bulk. Keywords: neutron reflectometry, adsorption of magnetic particles, magnetic fluids, diffuse scattering DOI: 10.1134/S1027451015010073