Fig. 9 reveals the magnetization as function of the magnetic
field up to 5 T at 5 and 300 K for A25, a sample with smaller mean
particle size of 6.5 nm. As in the case of the C25, the typical
behaviour of a magnetic state with spontaneous magnetization
in which the magnetic moment increases when the temperature
decreases and takes a zero value in the coercivity field at 300 K
was also observed. Thus the A25 behaves as a superparamagnetic.
At 5 K, the onset of a magnetic hysteresis loop with a
coercive field of 275 Oe is observed. This loop is slightly higher
than the one obtained for the C25 magnetite (248 Oe), see Fig. 9
upper inset. These low coercivities values indicate that synthesized
magnetite samples as nanoparticles are soft magnets at
these low temperatures. On the other hand, the bigger Fe3O4
nanoparticles corresponding to the C25 with a mean size of
12.5 nm have a saturation moment of 3.2mB which is larger than
A25 sample’s saturation moment of 2.1mB following the
characteristic behaviour for this kind of nanosized particles
[4]. The saturation magnetization of the magnetite was smaller
than the bulk value (4.1mB) and decreased with the reduction of
particle size. It can be explained as in MnFe2O4 due to a
disordered distribution of the magnetic spin moments, which
provokes the formation of a magnetically dead layer on the
surface of particles [43,44].