  Superparamagnetic colloids present interesting assembly dynamics and
propulsion in time-varying magnetic fields due to their magnetic relaxation.
However, little is known about the mechanisms governing this magnetic
relaxation, which is commonly attributed to the interactions and polydispersity
of the ferromagnetic nanoparticles distributed within the colloid. We measure
this relaxation from the effective potential between colloids subjected to
rotating magnetic fields. Remarkably, our results indicate the presence of
magnetic relaxation times much longer than what has been reported, which
furthers our understanding of the magnetization of colloids in complex magnetic
fields.
