  The existence of magnetic dipolar nanoparticle chains at zero field has been
predicted theoretically for decades, but these structures are rarely observed
experimentally. A prerequisite is a permanent magnetic moment on the particles
forming the chain. Here we report on the observation of magnetic dipolar chains
of spherical iron oxide nanoparticles with a diameter of
\SI{12.8}{\nano\meter}. The nanoparticles are embedded in an ultrathin polymer
film. Due to the high viscosity of the polymer matrix, the dominating
aggregation mechanism is driven by dipolar interactions. Smaller iron oxide
nanoparticles (\SI{9.4}{\nano\meter}) show no permanent magnetic moment and do
not form chains but compact aggregates. Mixed monolayers of different iron
oxide nanoparticles and polymer at the air-water interface are characterized by
Langmuir isotherms and in-situ X-ray reflectometry (XRR). The combination of
the particles with a polymer leads to a stable polymer nanocomposite film at
the air-water interface. XRR experiments show that nanoparticles are immersed
in a thin polymer matrix of \SI{3}{\nano\meter}. Using atomic force microscopy
(AFM) on Langmuir-Blodgett films, we measure the lateral distribution of
particles in the film. An analysis of single structures within transferred
films results in fractal dimensions that are in excellent agreement with 2D
simulations.
