Understanding magnetic spin structures in core-shell Fe₃O₄/Mn_xFe_3-xO₄ nanoparticle polycrystalline assemblies

Magnetic nanoparticles are critical components for many applications, but it is challenging to understand and tailor the governing intra- and inter-particle magnetic interactions. Here, we report the results of polarization analyzed small-angle neutron scattering (PASANS) experiments designed to explicitly probe such correlations in closed-packed polycrystals of core-shell nanoparticles. From other structural measurements, the particles have an average Fe₃O₄ core diameter of 6.4 nm with a 0.5 nm thick Mn_xFe_3-xO₄ shell, with x ~ 1.0. The PASANS data reveal that a significant portion of the nanoparticle magnetic moments are not aligned in a large applied magnetic field, as has been seen in related materials. More strikingly, this system also displays a prominent inter-particle magnetic Bragg peak with components both parallel and perpendicular to the field. The angular dependence of the PASANS data shows further anomalies, indicating that the degree of magnetic correlation varies in extent with temperature and field. The data are fit against a model of close packed, stacked layers where the layer size and thickness can be adjusted to reflect the competition between dipolar and Zeeman energies.