The amount by which the plane of polarization of the waves will be rotated depends on the length and thickness of the ferrite material, and on the strength of the dc magnetic field. This has the effect of rotating the plane of polarization of the waves propagating through the ferrite and is similar to the behavior of light, which Michael Faraday discovered in 1845. Because of the precession, a magnetic component at right angles to the other two is produced, as shown in Figure 10-39b. Furthermore, it is identical to the behavior that an ordinary gyroscope would exhibit under these conditions. This is due to the gyroscope forces involved and occurs at a rate that depends on the strength of the dc magnetic field. When the RF magnetic field due to the propagating electromagnetic waves is also applied, it is perpendicular to the axial dc magnetic field, so that the electrons precess about their original spin axis. Each electron behaves very much like a gyroscope. The so-called unpaired spin of electrons in a ferrite causes individual electrons to have angular momentum and a magnetic moment along the axis of spin. In other materials spin is said to take place also, but each pair of electrons has individual members spinning in opposite directions, so that there is an overall cancellation of spin momentum. Electrons spin because this is a magnetic material. With only the axial dc magnetic field present, the spin axes of the spinning electrons align themselves along the lines of magnetic force, just as a magnetized needle aligns itself with the earth’s magnetic field. The situation may be somewhat simplified if weak and strong interactions are considered separately. If an axial magnetic field from a permanent magnet is applied as well, a complex interaction takes place in the ferrite. When electromagnetic waves travel through a ferrite, they produce an RF magnetic field in the material, at right angles to the direction of propagation if the mode of propagation is correctly chosen. Because the ferrites have strong magnetic properties, external magnetic fields can be applied to them with several interesting results, including the Faraday rotation mentioned in connection with wave propagation. (Gamets are vitreous mineral substances of various colors and composition, several of them being quite valuable as gems.) Since all these materials are insulators, electromagnetic waves can propagate in them. Among the more common ferrites are manganese ferrite (MnFe 2O 3), zinc ferrite (ZnFe 2O 3) and associated ferromagnetic oxides such as yttrium-iron-garnet, or YIG for short. A ferrite is a nonmetallic material (though often an iron oxide compound) which is an insulator, but with magnetic properties similar to those of ferrous metals.
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