Uniform, organically functionalized, 8.5-nm-diameter nanocrystals of Fe 3 O 4 were synthesized and assembled into close-packed monolayer as well as multilayer arrays using the Langmuir-Blodgett technique. The ac and dc magnetic susceptibility measurements on arrays in comparison with isolated particles indicate strong dipolar interactions and a spin-glass-like slowing down of relaxation times. Differences in magnetic characteristics between monolayer to multilayer arrays were observed, such as larger remanent magnetization and a higher blocking temperature in the two-dimensional system.