  Bubbles have a rich history as transducers in particle-physics experiments.
In a solid-state analogue, we use bubble domains in nanomagnetic films to
measure magnetic nanoparticles. This technique can determine the magnetic
orientation of a single nanoparticle in a fraction of a second, and generate a
full hysteresis loop in a few seconds, which is much faster than any other
reported technique. We achieve this unprecedented speed by tuning the
nanomagnetic properties of the films, including the Dzyaloshinskii-Moriya
interaction, in the first application of topological protection from the
skyrmion state to a nanoparticle sensor. We demonstrate the technique on
iron/nickel nanorods and iron oxide nanoparticles, which delineate a wide range
of properties and applications. Bubble magnetometry enables the first
measurement with high throughput for statistical analysis of the magnetic
hysteresis of dispersed nanoparticles, and the first direct measurement of a
transition from superparamagnetic behavior as single nanoparticles to
collective behavior in nanoscale agglomerates. These results demonstrate a
breakthrough capability for measuring the heterogeneity and interaction of
magnetic nanoparticles.
