Introduction to Nanotechnology

192 NANOSTRUCTURED FERROMAGNETISM
depends on the strength of the applied DC magnetic field. In effect, we have a
tunable dihction grating, which can be adjusted to a spccific wavelength by
changing the strength of the DC magnetic field.
Ferrofluids have a number of present commercial uses. They are employed as
contaminant exclusion seals on hard drives ofpersonal computers, and vacuum seals
for high-speed high vacuum motorized spindles. In this latter application the
ferrofluid is used to seal the gap between the rotating shaft and the pole piece
support structure, as illustrated in Fig. 1.29. The seal consists of a few drops of
ferrofluid in the gap between thc shaft and a cylindrical permanent magnet that
forms a collar around it. The fluid forms an impermeable O-ring around the shaft,
while allowing rotation of the shaft without significant friction. Seals of this kind
have been utilized in a variety of applications. Ferrofluids are used in audiospeakers
in the voice gap of the driver to dampen moving masscs. Nature even employs
ferrofliiids. For example, it is belicved that ferrofluids play a role in the directional
sensing of trout fish. Cells near the nose of the trout arc thought to contain a
ferrofluid of magnetite nanoparticles. When the trout changes its orientation with
respect to the Earth’s magnetic field the direction of magnetization in the ferrofluid
of the cells changes, and this change is processed by tlic brain to give the fish
orientational information.
MAGNETICALLY
PERMEABLE POLE PIECE
CYLINDRICAL
PERMANENT MAGNET
Figure 7.29. Illustration of the use 01 a ferrofluid as a vacuum seal on a rotating magnetically
permeable shalt mounted on the pole pieces of a permanent magnet.