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October 4, 2021

Magnetic nanoparticles can be assembled in stable plane clusters by magnetic domain wall traps

Background & motivation: The stable assembly of fluctuating nanoparticle clusters on a surface represents an important technological problem. Magnetic nanoparticles are of widespread interest for their special physical properties. Despite techniques allowing manipulation and control of clusters at the colloidal micrometer lengthscale and above, magnetic nanoparticles remain a challenge for a number of reasons. For example, reliable trapping via optical tweezers becomes difficult because the doping with iron oxide makes the particles light adsorbing and therefore too thermally active. Alternatively, to overcome fluctuations with an external magnetic field, large field gradients are required. Such techniques are typically combined with the use of lithographic patterns composed of fixed microstructures restricting external control. Here we demonstrate a technique to stably confine in two dimensions clusters of interacting nanoparticles via size-tunable, virtual magnetic traps free of any geometric constrictions.

Figure: Triangular lattice of ferromagnetic domais or magnetic bubbles (upper left: schematic; bottom left: experiment). Inset: six particles trapped above a single bubble and their probability distribution. Generic model for particles subjected to a field-tuned harmonic confinement, repulsive interactions and thermal fluctuations (upper right) predicts collective states of three types (bottom right).

Findings: Experimentally, we use cylindrical Bloch walls arranged to form a triangular lattice of ferromagnetic domains within a ferrite garnet film. To rationalize the experiment, we develop a theory suggesting that: i) At each domain (magnetic bubble), the magnetic stray field generates an effective harmonic potential with a field tunable stiffness. ii) Pairwise interactions between the particles are of dipolar nature, leading to central, strictly repulsive forces. Thus, for clusters of magnetic nanoparticles, the stationary collective states arise from the competition between repulsion, confinement and the tendency to fill the central potential well. Using a numerical simulation model as a quantitative map between the experiments and theory we explore the field-induced crystallization process for larger clusters and unveil the existence of three different dynamical regimes. More generally, our method provides a model platform for investigations of the collective phenomena emerging when strongly confined nanoparticle clusters are forced to move in an idealized, harmonic-like potential.

P. Tierno, T.H. Johansen, A.V. Straube,
Thermally active nanoparticle clusters enslaved by engineered domain wall traps,
Nature Commun. 12, 5813 (2021) (open access)