  In recent years, there has been increasing interest in the understanding and
application of nanoparticle assemblies driven by external fields. Although
these systems can exhibit marked transitions in behavior compared to
non-interacting counterparts, it has often proven challenging to connect their
dynamics with underlying physical mechanisms or even to verifiably establish
their structure under realistic experimental conditions. We have studied
colloidal iron oxide nanoparticles that assemble into ordered, few-particle
linear chains under the influence of oscillating and pulsed magnetic fields.
Cryo-EM has been used to flash freeze and image the structures formed by
oscillatory drive fields, and magnetic relaxometry has been used to extract the
multiple time constants associated with magnetic switching of the short chains.
Armed with the physical structure from cryo-EM and the field-dependent
switching times from magnetic measurements, we have conducted extensive
micromagnetic simulations, revealing probable mechanisms for each time constant
regime spanning $10^{9}$ in time and how switching develops from individual
particles to entire chains. These types of magnetic nanomaterials have great
potential for biomedical technologies, particularly magnetic particle imaging
and hyperthermia, and rigorous elucidation of their physics will hasten their
optimization.
