  Many characterization techniques for magnetic nanoparticles depend on the
usage of external fields. This is not the case in Thermal Noise Magnetometry
(TNM), where thermal fluctuations in the magnetic signal of magnetic
nanoparticle ensembles are measured without any external excitation. This can
provide valuable information about the fundamental dynamical properties of the
particles, due to the purely observative experiments of this relatively new
technique. Until now, TNM signals have been detected only by a superconducting
quantum interference device (SQUID) sensor. We present a tabletop setup using
Optically Pumped Magnetometers (OPMs) in a small magnetic shield, offering a
flexible and accessible alternative and show the agreement between both
measurement systems for two different commercially available nanoparticle
samples. We argue that the OPM setup with high accessibility complements the
SQUID setup with high sensitivity and bandwidth. Furthermore, because of its
excellent sensitivity in the lower frequencies, the OPM tabletop setup is well
suited to monitor aggregation processes where the magnetization dynamics of the
particles tend to slow down, e.g. in biological processes. As a proof of
concept, we show for three different immobilization and clustering processes
the changes in the noise spectrum measured in the tabletop setup: 1) the
aggregation of particles due to the addition of ethanol, 2) the formation of
polymer structures in the sample due to UV exposure, and 3) the cellular uptake
of the particles by THP-1 cells. From our results we conclude that the tabletop
setup offers a flexible and widely adoptable sensor measurement unit to monitor
the immobilization and clustering of magnetic nanoparticles over time for
different applications.
