The effect of magnetic nanoparticle assembly formation at a planar interface between a transformer oil-based ferrofluid and single-crystal silicon under non-homogeneous magnetic fields was studied by specular neutron reflectometry. The ferrofluid probed represents a stable suspension of dispersed iron oxide nanoparticles (characteristic size about 10 nm) coated with oleic acid in a low-polarity liquid carrier (transformer oil). It was shown that the reflectivity curves are sensitive to the application of a non-homogeneous external magnetic field. In particular, at a low magnetic flux density (< 35 mT) they are well described in terms of the simplest model of a strict boundary between two semi-infinite homogeneous media. Nevertheless, slight concentrating of the ferrofluid bulk at the interface was detected. At a high magnetic flux density (35-75 mT), the analysis gives a three-layered structure of in-depth profiles of the scattering length density, thus showing the formation of two effective adsorption layers with different content of magnetic nanoparticles in them.