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Figure 7.22. Magnetization curve tor a ferrofluid made of magnetite, Fe304, nanoparticies showing the soft (nonhysteretic) magnetic behavior. An oersted corresponds to 10~4 T [Adapted from D. K Kim, J. Magn. Magn. Mater. 225, 30 (2001).]

Analogous properties are observed in liquid crystals, which consist of long molecules having large electric dipole moments, which can be oriented by the application of an electric field in the fluid phase. Electric-field-modulated birefringence or double refraction of liquid crystals is widely used in optical devices, such as liquid crystal displays in digital watches, and screens of portable computers. This suggests a potential application of ferrofluids employing magnetic field induced bifringence. To observe the behavior, the ferrofluid is sealed in a glass cell having a thickness of several micrometers. When a DC magnetic field is applied parallel to m

Figure 7.23. Picture taken through an optical microscope of chains of magnetic nanoparticies formed in a film of a ferrofluid when the DC magnetic field is parallel to the plane of the film. [With permission from H. E. Homig et al., J. Phys. Chem. Solids 62, 1749 (2001 ).J

the surface and the film is examined by an optical microscope, it is found that some of the magnetic particles in the fluid agglomerate to form needle like chains parallel to the direction of the magnetic field. Figure 7.23 depicts the chains viewed through an optical microscope. As the magnetic field increases more particles join die chains, and the chains become broader and longer. The separation between the chains also decreases. Figures 7.24a and 7.24b show plots of die chain separation and the chain width as a function of the DC magnetic field. When the field is applied perpendicular to the face of the film the ends of the chains arrange themselves in the pattern shown

40 60 80 100 MAGNETIC FIELD (Oe) (a)
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