
402 Biosensors Based on Nanomaterials and Nanodevices
moment. Superparamagnetism is useful in biomedical applications because as the applied
magnetic eld is removed, magnetization disappears then there will be no agglomeration
of NPs. The superparamagnetic behavior can be characterized in terms of a relaxation
time, τ, which is the time it takes for the system to achieve zero magnetization after the
removal of the external magnetic eld. The relaxation time, τ, for a particle is given by the
Néel expression:
17
tt=
Ê
Á
0
exp
DE
(16.2)
where τ
0
is the characteristic time of the order 10
−10
to 10
−12
s for non-reacting particles and
weakly depends ...