  The performance of nanoscale magnetic devices is often limited by the
presence of thermal fluctuations, while in micro-nanofluidic applications the
same fluctuations may be used to spread reactants or drugs. Here we demonstrate
the controlled motion and the enhancement of diffusion of magnetic
nanoparticles that are manipulated and driven across a series of Bloch walls
within an epitaxially grown ferrite garnet film. We use a rotating magnetic
field to generate a traveling wave potential that unidirectionally transports
the nanoparticles at a frequency tunable speed. Strikingly, we find an
enhancement of diffusion along the propulsion direction and a frequency
dependent diffusion coefficient that can be precisely controlled by varying the
system parameters. To explain the reported phenomena, we develop a theoretical
approach that shows a fair agreement with the experimental data enabling an
exact analytical expression for the enhanced diffusivity above the magnetically
modulated periodic landscape. Our technique to control thermal fluctuations of
driven magnetic nanoparticles represents a versatile and powerful way to
programmably transport magnetic colloidal matter in a fluid, opening the doors
to different fluidic applications based on exploiting magnetic domain wall
ratchets.
