Frequency- and magnetic-field-dependent properties of ordered magnetic nanoparticle arrangements

We present a frequency and magnetic field dependent investigation of ordered arrangements of 20 nm magnetic<br>
nanoparticles (MNPs) consisting of magnetite (Fe3O4) by employing micro Brillouin light scattering<br>
microscopy. We utilized electron beam lithography to prepare hexagonally arranged, circularly shaped MNPassemblies<br>
consisting of a single layer of MNPs using a variant of the Langmuir-Blodgett technique. By<br>
comparing the results with non-structured, layered superlattices of MNPs, further insight into the influence<br>
of size and geometry of the arrangement on the collective properties is obtained. We show that at low static<br>
external field strengths, two signals occur in frequency dependent measurements for both non-structured and<br>
structured assemblies. Enlarging the static external field strength leads to a sharpening of the main signal,<br>
while the satellite signal decreases in its intensity and increases in its linewidth. The occurrence of multiple<br>
signals at low external field strengths is also confirmed by sweeping the static external field and keeping the<br>
excitation frequency constant. Micromagnetic simulations unravel the origin of the different signals and their<br>
dependence on the static external field strength, enabling an interpretation of the observed characteristics in<br>
terms of different local environments of an MNPs forming the MNP assembly.