Magnetic domain formation in monolayer nanoparticle films

Self-assembled magnetic nanoparticle films offer promise as data storage media, but an understanding of the interactions is missing. Modified Langmuir-Blodgett methods were used to prepare monolayer films of 7 and 11 nm diameter Fe$_3$O$_4$ nanoparticles with large structural domains. Small-angle neutron scattering (SANS) shows a peak at a wavevector $Q$ corresponding to the particle size and spacing, and scattering at intermediate $Q$ indicating possible long-range correlations. We extend to lower $Q$ with off-specular neutron reflectivity, achieving high intensity by sacrificing resolution along one in-plane direction $y$ while retaining high resolution in the other in-plane direction $x$ and the normal direction $z$. We measure in saturation and zero field to extract magnetic scattering. In high fields, the specular scattering ($Q_x=0$) is increased, consistent with aligned moments. Preliminary results show weak magnetic scattering for nonzero $Q_x$ . Since the maximal $Q_x$ roughly corresponds to the lowest $Q$ in SANS, the combination of these techniques allows us to quantify field-dependent magnetic domain size.