Biomedical Engineering Reference
In-Depth Information
samples treated at 900 °C, as examples [59]. The XRD patterns showed the presence
of the main refl ections attributable to the cubic cobalt ferrite phase, superimposed
to an amorphous silica halo [45]. No other phase has been detected, thus confi rm-
ing the stabilizing effect on the CoFe
2
O
4
phase of the silica matrix, with respect
to thermal treatments. TEM investigations allow a highlighting of the effect of the
matrix on nanoparticle morphology and particle size distribution. HR- TEM images
of the N50_900 and N15_900 samples showed the irregular morphology of the
particles for samples with the highest ferrite concentration, and the spherical
morphology of the nanocrystals for the diluted samples. The particle size distribu-
tion, obtained via TEM images, is well fi tted with a log-normal function (Figure
12.8a). The increase in silica content leads to a reduction of
<
D
>
, and to a sharper
particle size distribution.
The magnetic properties have been studied by both static magnetization mea-
surements and Mössbauer spectroscopy. Measurements of magnetization as a
function of temperature, carried out using the TRM procedure, showed in all
samples that
M
TRM
decreased with increasing temperature, and for N15_T900 and
N30_T900 vanished above 108 K and 180 K, respectively. In contrast, N50_T900
showed a nonzero value for the whole temperature range, indicating that the
largest particles were blocked, even at room temperature. For an assembly of
noninteracting particles, the derivative of
M
TRM
with respect to temperature pro-
vides an estimate of the anisotropy energy barrier distribution (Figure 12.8b). The
decrease in ferrite contents leads to a more regular distribution of the anisotropy
(a)
(b)
12
28
0.032
26
11
0.030
0.028
24
10
0.026
22
9
0.024
20
0.022
8
N50_T900
N30_T900
N15_T900
0.020
18
N50_T900
N30_T900
N15_T900
7
0.018
16
0.016
6
14
0.014
5
0.012
12
4
0.010
10
0.008
3
8
0.006
2
6
0.004
0.002
1
4
0.000
0
2
0
50
100
150
200
250
300
0
10
20
30
40
50
60
<D>
(nm)
E
a
/k
B
(K)
Δ
Figure 12.8
(a) Log - normal fi ts of particles size distribution
for the samples N50_T900; N30_T900; N5_T900;
(b) Distribution of magnetic anisotropy extracted from
thermoremanent magnetization measurements.
