Introduction to Nanotechnology

176 NANOSTRUCTURED FERROMAGNETISM
on the surface, which means that they can have very different magnetic properties
than larger grain particles. It has been shown that treating the surfaces of
nanoparticles of z-Fe that are 600 nm long and 100 nm wide with various chemicals
can produce variations in the coercive field by as much as 50%, underlining the
importance of the surface of nanomagnetic particles in determining the magnetic
properties of the grain. Thus the dynamical behavior of very small magnetic particles
is somewhat more complicated than predicted by the SW model, and remains a
subject of continuing research.
7.4. NANOPORE CONTAINMENT OF MAGNETIC PARTICLES
Another area of ongoing research in nanomagnetism involves developing magnetic
materials by filling porous substances with nanosized magnetic particles. In fact,
there are actually naturally occurring materials having molecular cavities filled with
nanosized magnetic particles. Ferritin is a biological molecule, 25% iron by weight,
which consists of a symmetric protein shell in the shape of a hollow sphere having
an inner diameter of 7.5 nm and an outer diameter of 12.5 nm. The molecule plays
the role in biological systems as a means of storing Fe3+ for an organism. One
quarter of the iron in the human body is in ferritin, and 70% is in hemoglobin. The
ferritin cavity is normally filled with a crystal of iron oxide 5Fe20, . 9H20. The iron
oxide can be incorporated into the cavity from solution, where the number of iron
atoms per protein is controlled from a few to a few thousand per protein molecule.
The magnetic properties of the molecule depend on the number and kind of particles
in the cavity, and the system can be engineered to be ferromagnetic or antiferro-
magnetic. The blocking temperature TB is the temperature below which thermally
assisted hopping, between different magnetic orientations, becomes frozen out.
Figure 7.9 shows that the blocking temperature decreases with a decrease in the
number of iron atoms in the cavity. Femtin also displays magnetic quantum
tunneling at very low temperatures. In zero magnetic field and at the very low
temperature of 0.2 K the magnetization tunnels coherently back and forth between
two minima. This effect makes its appearance as a resonance line in frequency-
dependent magnetic susceptibility data. Figure 7.1 0 shows the results of a measure-
ment of the resonant frequency of this susceptibility versus the number of iron atoms
per molecule. We see that the frequency decreases from 3 x lo8 Hz for 800 atoms to
IO6 Hz for 4600 atoms. The resonance disappears when a DC magnetic field is
applied, and the symmetry of the double walled potential is broken.
Zeolites are crystalline silicates with intrinsic pores of well-defined shape, Fig. 6.24
(of Chapter 6) gives a schematic of a zeolite structure. These materials can be used as a
matrix for the confinement of magnetic nanoparticles. Measurements of the tempera-
ture dependence of the susceptibility of iron particles incorporated into the pores
exhibited paramagnetic behavior, with the magnetic susceptibility x obeying the Curie
law, x = C/T, where C is a constant, but there was no evidence of ferromagnetism.