posted on 2020-01-30, 20:45authored byElena H. Sánchez, Marianna Vasilakaki, Su Seong Lee, Peter S. Normile, Giuseppe Muscas, Massimiliano Murgia, Mikael S. Andersson, Gurvinder Singh, Roland Mathieu, Per Nordblad, Pier Carlo Ricci, Davide Peddis, Kalliopi N. Trohidou, Josep Nogués, José A. De Toro
Applications
based on aggregates of magnetic nanoparticles are
becoming increasingly widespread, ranging from hyperthermia to magnetic
recording. However, although some uses require collective behavior,
others need a more individual-like response, the conditions leading
to either of these behaviors are still poorly understood. Here, we
use nanoscale-uniform binary random dense mixtures with different
proportions of oxide magnetic nanoparticles with low/high anisotropy
as a valuable tool to explore the crossover from individual to collective
behavior. Two different anisotropy scenarios have been studied in
two series of binary compacts: M1, comprising maghemite (γ-Fe2O3) nanoparticles of different sizes (9.0 nm/11.5
nm) with barely a factor of 2 between their anisotropy energies, and
M2, mixing equally sized pure maghemite (low-anisotropy) and Co-doped
maghemite (high-anisotropy) nanoparticles with a large difference
in anisotropy energy (ratio > 8). Interestingly, while the M1 series
exhibits collective behavior typical of strongly coupled dipolar systems,
the M2 series presents a more complex scenario where different magnetic
properties resemble either “individual-like” or “collective”,
crucially emphasizing that the collective character must be ascribed
to specific properties and not to the system as a whole. The strong
differences between the two series offer new insight (systematically
ratified by simulations) into the subtle interplay between dipolar
interactions, local anisotropy and sample heterogeneity to determine
the behavior of dense assemblies of magnetic nanoparticles.