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nanoparticles. Two types of measurement can be distinguished: when the relaxa-
tion is fast compared to the timescale, the measurement corresponds to the average
value of the magnetization which tends to zero in zero-applied field. On the
contrary, the static value of the magnetization observed when the relaxation is
slow. Consequently, the decrease of temperature and/or the application of an
external magnetic field are two experimental conditions which tend to overcome
the barrier energy to cancel thus the relaxation phenomena, i.e. to a blocked
magnetic structure. In addition, it is of relevant interest to estimate the blocking
temperature values, T
B
, which are defined as the temperature at which the
superparamagnetic relaxation time is equal to the timescale of the experimental
technique, allowing thus the thermal variation of the relaxation time (log
10
s
m
versus 1/T
B
) to be established (see [
15
] and references therein). Three different
regimes can be observed as a function of the interacting nature of the assembly of
nanoparticles: (1) in presence of negligible or very weak interactions, the linear
behaviour indicates that the properties match the Néel-Brown model; (2) from
weak to medium interactions, a weak curvature is observed in agreement with the
Néel-Brown model which remains rather valid by taking account of an additional
anisotropy temperature dependent contribution, while (3) a critical decrease of the
relaxation time in the case of strong interactions suggests a homogeneous
dynamical process, i.e. a collective freezing of particle moments, similar to that of
the spin freezing in spin glasses which exhibit a phase transition. It is important to
note that one can find in the literature pure superparamagnetic, superparamagnetic
modified by interactions and collective (glass collective state) regimes.
4.4 Magnetic Nanoparticles
During the last twenty years, the chemical and physical properties of nanoparticles
have been widely investigated as well as their potential applications in different
topics but one has to emphasize that the elaboration and the manipulation of mon-
odispersed nanoparticles has opened relevant and promising applications in mate-
rials science. It is clear that the assemblies of nanoparticles without any control of the
size and morphology dispersion and their aggregation due to attractive van der Waals
forces prevent from a fundamental knowledge of their physical properties. Indeed, a
systematic characterization of monodisperse and monomorphological nanoparticles
is required to establish and to control not only the properties of an individual
nanoparticle, but also the collective behaviour of an assembly of nanoparticles,
favouring thus their structural and magnetic modelling. Consequently, a first crucial
point lies on the refinement of the synthesis methods of individual nanoparticles for
the last two decades: they are currently based on conventional chemical processes but
also on new chemical and physical routes. Then, the ability of nanoparticles to self
assemble onto a substrate has been established, in order to elaborate highly ordered
monolayers and 3D ordered arrays. Indeed, hexagonal, cubic or random packing of
