Ferro fluids, also called magnetofluids, 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 particles 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 die 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.

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