Three-dimensional (3D) superstructures of individual magnetic nanocrystals that form artificial solids provide a new way to study collective behaviour. These innovative magnetic metamaterials are the basis of a new paradigm in magnetism where a nanoparticle can be considered to be the “atom” of a crystalline solid. Clear evidence of this is provided by comparing the magnetism of a disordered and 3D FCC ordered array, both made with the same nanoparticle “atom”.

Recently [1] we have demonstrated a successful route to make 3D crystals of nanoparticles of magnetite. We use cage like proteins to carry nanoparticles and to crystallize them into periodic 3D array. We studied [2] the structure of these bio-metamaterials in comparison with non –crystalline counterpart and have shown that the interparticle (centre-to-centre) separation is different in both systems by only 0.5 nm. Thus it is a viable to suggest that the only structural difference between two studied systems is special ordering of nanoparticles in the FCC array in comparison to absence of long range order in the non-crystalline closed packed assembly.

The results reported here show a dramatic change in coercivity and remanence when short range order/disorder is replaced by long range FCC-order and there is only a small change in interparticle separation. The magnetization measurements of the disordered nanoparticle system reveal typical nanomagnetism. Dynamical freezing of the single domain magnetization when cooling from temperatures where the nanoparticles are superparamagnetic through to the nanoparticle magnetization being blocked (around 50 K) is observed. When the nanoparticles are superparamagnetic, no coercivity is observed (as expected). By contrast, the magnetism of the 3D periodic array of nanoparticles exhibits no dynamical freezing and coercivity and remnant magnetization are measured at all temperatures from 2 to 400 K, manifesting dipolar ferromagnetism  of FCC system of magnetic nanoparticles.

[1] O. Kasyutich et.al. J. of Physics D: Applied Physics, 41, 134022 (2008)

[2] O. Kasyutich et.al. J. Appl. Phys. 105 (2009)

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