A Critical Comparison of Ferrites with Other Magnetic Materials ® a Division of Spang and Company
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® Division of Spang & Company A Critical Comparison of Ferrites with Other Magnetic Materials ® A Division of Spang and Company Basic Differences- Material Magnetic Losses The magnetic losses in an A.C. Composition Considerations- application can be represented by and Structure Magnetic and the familiar Legg equation: The difference in properties and Mechanical Rm = µfL(ef + aBm +c) performance of ferrites as compared Properties with most other magnetic materials where: Rm = total core loss in is due to the fact that the ferrites are Saturation Magnetization ohms oxide materials rather than metals. As mentioned previously, the e = eddy current Ferromagnetism is derived from the highest saturation values are found coefficient unpaired electron spins in only a few in the metals and alloys. Thus, if high a = hysteresis metal atoms, these being iron, flux densities are required in high coefficient cobalt, nickel, manganese, and power applications, the bulk metals, c = residual loss some rare earth elements. It is not iron, silicon-iron and cobalt-iron are coefficient surprising that the highest magnetic unexcelled. Since the flux in max- µ = magnetic moments and therefore the highest wells Ø = BA, where B = flux density permeability saturation magnetizations are to be in gausses and A = cross-sectional f = frequency in found in the metals themselves or in area in cm², obtaining high total flux hertz alloys of these metals. The oxides, in materials such as ferrites or per- L = inductance in on the other hand, suffer from a dilu- malloy powder cores can be accom- henries tion effect of the large oxygen ions in plished only by increasing the Bm = maximum flux the crystal lattice. In addition, the net cross-sectional area. Powdered iron density in moment resulting from ferromag- has a fairly high saturation value, but gausses netic alignment of the atomic spins exhibits low permeabilities. is reduced because a different, less Eddy current losses will increase efficient type of exchange mecha- Curie Temperatures quite rapidly with frequency. In bulk nism is operative. The oxygen ions All magnetic materials lose their metals, these high frequency losses do serve a useful purpose, however, ferromagnetism at the Curie temper- can be reduced by reducing the since they insulate the metal ions ature. One overriding consideration thickness of the material perpendic- and, therefore, greatly increase the for a magnetic material is that the ular to the flux flow. This is accom- resistivity. This property makes the Curie point of the material be well plished by using thin gauge tapes or ferrite especially useful at higher fre- above the proposed operating tem- laminations or by powdering and quencies. The purpose of this paper peratures. Table 1 lists the Curie insulating the particles. In ferrites, is to list the various considerations Temperatures of the various the same result is obtained by which enter into the choice of a ma- materials. The Curie point depends increasing the resistivity by many terial for a specific application and to only on composition and not on orders of magnitude. Thus, at the contrast pertinent ferrite properties geometry. Even though some of the highest operating frequencies where with those of bulk metal or powdered magnetic materials shown can be further gauge or particle reduction is metal materials. used at higher operating tempera- impractical, ferrites are the only tures than others, very often the tem- available materials. perature limitations of the accessory items (wire insulation, potting or damping compound) can be more limiting; in this case, no practical advantage may be gained by the higher curie point materials. 1 The hysteresis losses are propor- Permeability this phase is emphasized by the fact tional to the flux density and can be Permeability is a function of com- that the market value for computer depicted as the area inside the hys- position and processing. The highest magnetics is now greater in dollars teresis loop. High hysteresis losses initial permeabilities (those meas- to that of the power materials market. are accompanied by the presence of ured at very low flux levels) are found It is interesting to note that disk unwanted harmonics. The nickel- in the nickel-iron alloys, particularly media and bubble materials are fer- iron (permalloy) alloys have low hys- in supermalloy where the value is rite type oxides. teresis losses and a great asset to about 100,000. Powdered iron cores the permalloy powder core is that have low permeabilities (10-100) Brittleness these low losses are maintained with while permalloy powder cores are One drawback to the ferrite core the accompanying reduction in eddy somewhat higher (15-550). Ferrites as compared with metal cores is its current losses. can be made over a wide range of brittleness. Being ceramic in nature, The residual losses are not too permeabilities. The linear filter type care must be exercised in the han- well understood and perhaps repre- permeabilities vary from 100-2000, dling of these cores. Powder cores sent an expression of our ignorance while those used in power applica- are also somewhat brittle and similar of the system. They apparently are tions range from 3000-1 5,000. As the precautions are required. Although tied in partially to absorption of operating frequency increases, fer- metal tape cores are not brittle, energy from the system by gyromag- rites with lower permeabilities are (except amorphous metal cores), netic resonance. used because these have distinctly they nevertheless are sensitive to A listing of the various losses in the lower losses in these regions. The strain and mechanical shock, espe- materials under consideration is permeabilities for a variety of cially in the high permeability given in Table 1. materials are listed in Table 1. materials. Consequently, tape wound cores are often embedded in Figure of Merit a damping compound which pre- A useful figure of merit for linear vents the transfer of strain or shock core materials is the µQ product. to the cores. Values of this factor are tabulated in Table 1 . At frequencies of 100 KHz Hardness and above, the value for ferrites is Ferrites are very hard materials as considerably above all other compared with the other materials materials. under consideration. This property is especially useful in applications in Squareness which wear is a factor. Conse- The squareness ratio is defined as quently, ferrite material is being used the ratio of Br to Bm and is especially extensively in magnetic recorder important in memory and switching head applications. core applications. Magnesium- manganese ferrites can be produced with extremely high squareness ratios. While some metal tape and bobbin cores possess similar high ratios, their higher cost and difficulty in miniaturization made the ferrites the material of choice in large scale memory applications in early com- puter models. Thin magnetic film memories, which may be considered bulk metals, have become increas- ingly important, and along with semi- conductor, disk, tape and bubble memories, have replaced the old core memories. The importance of 2 MAGNETICS • BUTLER, PA ® A Division of Spang and Company Geometry Tunability Magnetic Shielding Considerations An exact inductance is required in If magnetic components are rela- certain L-C circuits. If the shape of tively close in a circuit, the fields Formability the inductor is toroidal, the induc- produced by one component may The three types of materials- bulk tance can be trimmed only by the affect the performance of other metal, powdered metal and ferrite - addition or removal of turns, a time cores. One solution is to increase the are produced by widely varying tech- consuming and costly procedure. If space between components. This niques and consequently the availa- a ferrite pot core is used, the tuning increases the overall size of the sys- ble geometries also vary. can be accomplished by means of a tem. Another is to use a magnetic Bulk metals - These are screw-type trimmer core which shield which increases weight and produced mostly by standard changes the effective air gap of the size. A ferrite pot core is inherently metallurgical process involving core. Threaded rods of powdered self-shielding by nature of the melting followed by hot and cold iron or ferrite materials are used enclosed magnetic circuit. rolling. The sheet material extensively as tuning elements in produced is either slit and wound slug tuned inductors. into tape or bobbin cores or punched into laminations. Pho- Winding Considerations toetching, a new method of form- Winding turns on a toroid involves ing small complex parts, avoids specialized equipment and the proc- costly tooling, and produces ess involves winding each core stress-free parts. separately in a relatively time con- suming operation. The bobbins used Powdered Iron and permalloy - in ferrite pot cores can be wound These materials are always die- many at a time on a rather simple pressed into toroids or slugs, machine. This ease of winding con- molypermalloy usually in toroids stitutes an important advantage for and powdered iron into slugs. ferrite pot cores. Ferrites - Because ferrites are produced by a ceramic technique, they can be made in a large num- ber of shapes. Unlike the bulk metals, they can be molded directly, and unlike the powdered permalloy, they can be machined and ground to close tolerances after firing. Various forming processes for ferrites include die pressing, extrusion, hydrostatic pressing and hot pressing. The available shapes include toroids, E-l cores, U-l cores, pot cores, rods, tubes, beads and blocks. 3 Inductance Figure 1 — Ferrite Toroids Stability Initial Permeability (µo) Vs. Temperature Considerations Temperature Stability In telecommunications circuitry (tuned L-C filters), the maintenance of a near-constant inductance as a function of temperature and time is most critical. One method of achiev- ing this stability is by the insertion of an air gap.