Suppression of blocking behavior in a macroscopic fcc crystal of nanoparticles

We have used a protein crystallization technique to fabricate three-dimensional (3D) ordered arrays of magnetoferritin nanoparticles up to a few hundreds of micrometers in size in the form of free standing perfectly faceted crystals. We compare the ac and dc susceptibilities of the nanoparticles in an uncrystallized system (amorphous assembly) and in a macroscopic 3D fcc crystal of nanoparticles. Completely different magnetism is measured due to the different interparticle interactions in the two systems. For example, at temperatures above 50 K the uncrystallized nanoparticle system is superparamagnetic while the 3D crystal displays no superparamagnetism up to 400 K.