28-08-2012, 10:29 AM
FERRIC DEVICES AND APPLICATIONS
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Abstract
Ferri-magnets having low RF loss are used in passive microwave components such as isolators, circulators, attenuator, phase shifters, and miniature antennas operating in a wide range of frequencies (1–100 GHz) and as magnetic recording media owing to their novel physical properties. Frequency tuning of these components has so far been obtained by external magnetic fields provided by a permanent magnet or bypassing current through coils. However, for high frequency operation the permanent part of magnetic bias should be as high as possible, which requires large permanent magnets resulting in relatively large size and high cost microwave passive components. A promising approach to circumvent this problem is to use hexaferrites, such as BaFe12O19 and SrFe12O19, which have high effective internal magnetic anisotropy that also contributes to the permanent bias.
INTODUCTION
A FERRITE is a device that is composed of material that causes it to have useful magnetic properties and, at the same time, high resistance to current flow. The primary material used in the construction of ferrites is normally a compound of iron oxide with impurities of other oxides added. The compound of iron oxide retains the properties of the ferromagnetic atoms, and the impurities of the other oxides increase the resistance to current flow. This combination of properties is not found in conventional magnetic materials. Iron, for example, has good magnetic properties but a relatively low resistance to current flow. The low resistance causes eddy currents and significant power losses at high frequencies Ferrites, on the other hand, have sufficient resistance to be classified as semiconductors.
The compounds used in the composition of ferrites can be compared to the more familiar compounds used in transistors. As in the construction of transistors, a wide range of magnetic and electrical properties can be produced by the proper choice of atoms in the right proportions.
Ferrites have long been used at conventional frequencies in computers, television, and magnetic recording systems. The use of ferrites at microwave frequencies is a relatively new development and has had considerable influence on the design of microwave systems. In the past, the microwave equipment was made to conform to the frequency of the system and the design possibilities were limited. The unique properties of ferrites provide a variable reactance by which microwave energy can be manipulated to conform to the microwave system. At present, ferrites are used as LOAD ISOLATORS, PHASE SHIFTERS, VARIABLE ATTENUATORS, MODULATORS, and SWITCHES in microwave system.
Intrinsic properties of ferrites
Commonly used ferrites are primarily classified into three types: spinels, hexagonal ferrites and garnets, according to their primary crystal lattice. Generally, ferrimagnetism arises from the antiparallel alignment of the magnetic moments on transition metal ions, present on different magnetic sublattices. The origin of the antiparallel coupling can be explained by super-exchange of valence electrons between the filled p-orbitals of O2- and unfilled d-orbitals of the transition metal cations. In ferrites, the oppositely directed magnetic moments do not exactly cancel, thus a net magnetic moment results.
Spinels
Structure and compositions
Spinels have the general formula MeO•Fe2O3, where Me represents a divalent transition ion or a combination of ions having an average valence of two. In spinel ferrites, the relatively large oxygen anions form a cubic close packing with ½ of the octahedral and ⅛ of the tetrahedral interstitial sites occupied by metal ions. Figure 2.1 shows a cubic unit cell containing eight formula units, in which tetrahedral and octahedral sites are labeled as A and B respectively.
Structures and compositions
The group of ferrites possessing hexagonal crystal structures is referred to hexagonal ferrites. Four types of hexagonal ferrites are distinguished and indicated as M, W, Y and Z as shown in the composition diagram in figure 2.2. They correspond to (BaO+MeO)/Fe2O3 ratios of 1:6, 3:8, 4:6 and 5:12 respectively. Me can be a transition cation or a combination of cations as it would occur in spinels. The crystal and magnetic structure of the different types of hexagonal ferrites are remarkably complex, as shown for the most important M-type BaFe12O19 in figure 2.3. The elementary unit cell contains 10 oxygen layers, sequentially constructed from 4 blocks, S (spinel), R (hexagonal), S* and R*. S* and R* have equivalent atomic arrangements as S and R, but are rotated 180° about the c axis with respect to S and R. An S or S* block consists of two O2- layers; while an R or R* block contains three O2- layers, with one oxygen site in the middle layer substituted by a Ba2+ ion.
FERRITE ATTENUATORS
A ferrite attenuator can be constructed that will attenuate a particular microwave frequency and allow all others to pass unaffected. This can be done by placing a ferrite in the center of a waveguide, as shown in figure 3.1. The ferrite must be positioned so that the magnetic fields caused by its electrons are perpendicular to the energy in the waveguide. A steady external field causes the electrons to wobble at the same frequency as the energy that is to be attenuated. Since the wobble frequency is the same as the energy frequency, the energy in the waveguide always adds to the wobble of the electrons. The spin axis of the electron changes direction during the wobble motion and energy is used. The force causing the increase in wobble is the energy in the waveguide. Thus, the energy in the waveguide is attenuated by the ferrite and is given off as heat. Energy in the waveguide that is a different frequency from the wobble frequency of the ferrite is largely unaffected because it does not increase the amount of electron wobble. The resonant frequency of electron wobble can be varied over a limited range by changing the strength of the applied magnetic field.
FERRITE ISOLATORS. –
An isolator is a ferrite device that can be constructed so that it allows microwave energy to pass in one direction but blocks energy in the other direction in a waveguide. This isolator is constructed by placing a piece of ferrite off-center in a waveguide, as shown in figure 3.2. A magnetic field is applied by the magnet and adjusted to make the electron wobble frequency of the ferrite equal to the frequency of the energy traveling down the waveguide. Energy traveling down the waveguide from left to right will set up a rotating magnetic field that rotates through the ferrite material in the same direction as the natural wobble of the electrons. The aiding magnetic field increases the wobble of the ferrite electrons so much that almost all of the energy in the waveguide is absorbed and dissipated as heat. The magnetic fields caused by energy traveling from right to left rotate in the opposite direction through the ferrite and have very little effect on the amount of electron wobble. In this case the fields attempt to push the electrons in the direction opposite the natural wobble and no large movements occur. Since no overall energy exchange takes place, energy traveling from right to left is affected very little.