Magnetic hyperthermia occurring in colloids of magnetic
nanoparticles has been demonstrated to reduce tumour size in
human beings [1]. However, at the present time the mechanism
by which the heat is generated is not fully understood [2]. In
this Fast Track Communication we report a detailed study of
the heating properties of magnetic nanoparticles of different
sizes for hyperthermia applications. In particular we have
modified the colloidal properties for three samples with
different particle sizes so as to control the mechanisms leading
to heat dissipation.
Several heating mechanisms are possible, associated with
susceptibility loss, hysteresis loss and viscous heating, i.e.
stirring. Susceptibility loss occurs in superparamagnetic
particles and has two relaxation times associated with N´eel
relaxation and Brownian rotation of the particles as they are
in a liquid environment [3]. The N´eel (τN) and Brownian (τB)
relaxation times are given by