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/Fe3O4 core-shell NCs by electrophoretic deposition (EPD), followed by sintering at 400A degrees 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) a parts per thousand 348 K, which is much larger than the N,el-Brown approximation of T (B) (SP) a parts per thousand 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.