The role of superlattice phases and interparticle distance in the magnetic behaviour of SPION thin films

Superparamagnetic iron oxide nanoparticles (SPIONs) with tailored surface modifications were employed to fabricate ordered thin films through a drop-casting technique. By systematically varying the ligand chain length using stearic acid, decanoic acid, and hexanoic acid, we precisely controlled the interparticle distances within the films. Comprehensive investigations utilizing superconducting quantum interference device (SQUID) magnetometry elucidated the films' superparamagnetic behaviour at room temperature, as well as notable exchange interactions at lower temperatures. Notably, these exchange characteristics exhibit a correlation with the blocking temperatures of the thin films. We postulate that these characteristics can be explained by different superlattice phases formed in the thin films, as indicated in previous studies, highlighting the profound influence of self-assembly and particle packing on the magnetic properties. To validate our hypothesis regarding the internal structure, we conducted grazing-incidence small-angle X-ray scattering (GISAXS) and scanning transmission electron microscopy (STEM) measurements, enabling us to assess the quality of internal ordering without compromising the integrity of the films. With this study we demonstrated how the use of simple building blocks, guided by the intrinsic driving force of self-assembly, can lead to remarkable magnetic properties in the resulting films.