this type of material compared to competing conventional (Co,Ni)Fe 2 O 4 ferrites are
shown graphically in Figures 8.39 and 8.40.
The data in Figure 8.39 show that, although the susceptibility of the new exchangecoupled material is less than that of a commercial cobalt/nickel ferrite, in contrast to
the ferrite the maximum frequency of application goes beyond 10
7 Hz, in this case at
least up to 10
8 Hz. (The frequency of application for this ferrite is limited by Bloch
wall resonance, as indicated in Figure 8.39, by an increase of susceptibility. During
-1
-0.5
0
0.5
1
-150
-100
-50
0
50
100
150
magnetic
moment
[Am
2 kg
-1 ]
no coating
3 nm SiO 2
6 nm SiO 2
9 nm SiO 2
μ 0 H [T]
Figure 8.38 Magnetization curves of soft magnetic FeNi coated with SiO 2 of different thickness.
As expected, the mass-related magnetization is reduced with increasing thickness of the
nonmagnetic coating [24].
Figure 8.39 Real part of the complex
susceptibility of soft magnetic composite
consisting of FeNi kernels coated with SiO 2 of
different thickness compared to a commercially
available (Co,Ni)Fe 2 O 4 ferrite. The susceptibility
of the commercially available material was
higher; however, due to Bloch wall resonances
the application was limited to frequencies
significantly below 10
7 Hz [24].
202j 8 Magnetic Properties of Nanoparticles