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The phenomenon of ferromagnetism is associated only with the solid state of matter; like iron, nickel, cobalt and some rare earth metals and their alloys. Thus, up to now, there is no intrinsic homogeneous fluid having ferromagnetic properties; although, theories admit the possibility of ferromagnetism in the liquid state, and suggest that there is no inherent reason why they should not exist [1-3,5]. Ferromagnetism occurs when paramagnetic ions in a solid lock together in such a way that their spins all point (on the average) in the same direction . At a certain temperature this locking breaks down and ferromagnetic materials become paramagnetic. This transition temperature is called the Curie point (TC), which is invariably well below the melting point of the corresponding material [1,8,12].
1.2 MAGNETIC FLUIDS:
Magnetic fluids ( MF ) are stable colloidal suspensions of ultrafine ferro- or ferri- magnetic particles ( D100Ao ), coated with a surfactant like oleic acid, in a suitable liquid carriers such as Isopar-M, kerosene, decalin, etc., [1,2].
The idea of MF was put forward independently and almost simultaneously by several investigators. The first prepared MF was developed by Stephen Papell of the National Aeronautics and Space Administration (NASA) in the early 1960s. After that, Ronald E. Rosensweig and his colleagues succeeded in formulating MFs that were 10 times as strong magnetically as Papell`s original MF .
The special feature of magnetic fluids is given by the fact, that they combine normal liquid behavior with superparamagnetic properties, as will be discussed later. The possibility of magnetic fluid control gave rise to the development of many technical applications in our everyday life [1-3].
Mixtures are classified into three kinds: colloidal suspensions, suspensions, and solutions, and one of the principal features that differentiate one kind of mixture from another is the size of the particles . Colloidal suspensions (also called colloids) are mixtures that are intermediate between solutions and suspensions. Typically the radius of the particle is of the order 100Ao. Since this is much smaller than the size of a single magnetic domain in bulk solids, which is about , the magnetization of the individual particle is saturated, but the direction of the magnetic moment is subject to thermal agitation [1,2,5]. The particles used are commonly Fe3O4, Fe, Co and Ni.
MF is a two-phase system with three components, which combines magnetic properties, carried by the solid magnetic fine particles, with fluidity of the carrier liquid.
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