Disorder Dissipation in Superparamagnetic Nanoparticle Chains Assembled in a Rotating Magnetic Field

Chaining of superparamagnetic particles is well understood in two limits: Finite-temperature assembly of magnetic nanoparticles in a static magnetic field, and far-from equilibrium assembly of larger magnetic beads in a rotating field. Here, we study chaining of magnetic nanoparticles in a rotating field, achieving a combined regime where self-assembly occurs both away from thermal equilibrium and at finite temperatures. Theories applicable to both limits qualitatively describe the observed chain length distribution, except that chains are significantly shorter than in either limit. Most notably, if the chains are incubated in the rotating field for tens of minutes, the disorder initially present in the chains gradually dissipates. The disorder dissipation can be sped up by increasing the solution ionic strength or the particle concentration, both of which increase the velocity of chain assembly. We therefore conclude that the chain order is not only improved through thermal fluctuations acting on individual chains, but also by way of the assembly, during which chains continually grow and fall apart, providing an influx of energy sufficient to liberate chains from kinetic traps so they can evolve toward a higher degree of order.