  Applications based on aggregates of magnetic nanoparticles are becoming
increasingly widespread, ranging from hyperthermia to magnetic recording.
However, although some uses require a collective behavior, other 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
($\gamma$-Fe$_2$O$_3$) 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.
