Field Dependence of the Spin Relaxation Within a Film of Iron Oxide Nanocrystals Formed via Electrophoretic Deposition

The thermal relaxation of macrospins in a strongly interacting thin film of spinel-phase iron oxide nanocrystals (NCs) is probed by vibrating sample magnetometry (VSM). Thin films are fabricated by depositing FeO/Fe 3 O 4 core–shell NCs by electrophoretic deposition (EPD), followed by sintering at 400°C. Sintering transforms the core–shell structure to a uniform spinel phase, which effectively increases the magnetic moment per NC. Atomic force microscopy (AFM) confirms a large packing density and a reduced inter-particle separation in comparison with colloidal assemblies. At an applied field of 25 Oe, the superparamagnetic blocking temperature is T B SP ≈ 348 K, which is much larger than the Néel-Brown approximation of T B SP ≈ 210 K. The enhanced value of T B SP is attributed to strong dipole–dipole interactions and local exchange coupling between NCs. The field dependence of the blocking temperature, T B SP (H), is characterized by a monotonically decreasing function, which is in agreement with recent theoretical models of interacting macrospins.