Introduction to Nanotechnology

186 NANOSTRUCTURED FERROMAGNETISM
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TEMPERATURE (K)
Figure 7.21. Temperature dependence of resistivity (normalized resistance) in sintered samples
of La-Ca-hh-4 in zero magnetic field. (With permission from F. J. Owens and C. P. Poole, Jr.
Electromagnetic Absorption in the Copper Oxide Superconductors, Kluwer/Plenum, 1999, p. 90.)
7.7. FERROFLUIDS
Ferrofluids, also called mugnetoJEuids, are colloids consisting typically of 10-nm
magnetic particles coated with a surfactant to prevent aggregation, and suspended in
a liquid such as transformer oil or kerosene. The nanoparticles are single-domain
magnets, and in zero magnetic field, at any instant of time, the magnetization vector
of each particle is randomly oriented so the liquid has a zero net magnetization.
When a DC magnetic field is applied, the magnetizations of the individual
nanoparticles all align with the direction of the field, and the fluid acquires a net
magnetization. Typically ferrofluids employ nanoparticles of magnetite, Fe304.
Figure 7.22 shows the magnetization curve for a ferrofluid made of 6-nm Fe304
particles exhibiting almost immeasurable hysteresis. Ferrofluids are soft magnetic
materials that are superparamagnetic. Interestingly, suspensions of magnetic parti-
cles in fluids have been used since the 1940s in magnetic clutches, but the particles
were larger, having micrometer dimensions. Application of a DC magnetic field to
this fluid causes the fluid to congeal into a solid mass, and in the magnetic state the
material is not a liquid. A prerequisite for a ferrofluid is that the magnetic particles
have nanometer sizes. Ferrofluids have a number of interesting properties, such as
magnetic-field-dependent anisotropic optical properties.