  We reveal the assembly of monodisperse magnetite nanoparticles of sizes 5 nm,
15 nm and 25 nm from dilute water-based ferrofluids onto an amorphous magnetic
template with out-of-plane anisotropy. From neutron reflectometry experiments
we extract density profiles and show that the particles self-assemble into
layers at the magnetic surface. The layers are extremely stable against
cleaning and rinsing of the substrate. The density of the layers is determined
by and increases with the remanent magnetic moment of the particles.
