Ferrites forlinear applicationsI-PropertiesThe properties of magnetic ferrites have been continuouslyimproved during recent years, chiefly because of the increasedunderstanding of the nature of magnetization processes

E. C. Snelling Mulilard Research Laboratories

This two-part tutorial article describes properties and During the 25 years since the introduction of ferrites onapplications of magnetically soft ferrites. These ma- an industrial scale the magnetically soft ferrites haveterials, which are characterized by high permeabil- remained by far the largest class in terms of weight ofity and low losses, are used in very large quantities as material manufactured. It is difficult to estimate thecores for inductors and transformers. This first in- current annual world production of soft ferrites but itstallment provides an elementary introduction to the certainly exceeds 25 000 tonnes or about 600 millionprocesses of magnetization in ferrites. It also includes cores (excluding small extrusions and other very smalla survey of available grades and a summary of the pieces). This quantity consists mainly of inductor andtechnical properties of typical modern ferrites. Part transformer cores for telecommunication, and deflection1I, to appear in February, reviews the main applica- yokes and line-scanning transformer cores for televisiontions, describing how the material properties and per- receivers.formance requirements come together in the design Soft ferrites have a cubic crystal structure analogous toof the device. the mineral spinel.4 The general formula is MeFe2O4,

where Me usually represents one or more of the divalent;---- transition metals Mn, Fe, Co, Ni, Cu, and Zn. Other

divalent metals can also be used and it is possible to re-The physical nature of linear ferrites place some or all of the trivalent iron ions with other

Ferrites are magnetic ceramics usually composed of trivalent metals. It is even possible to replace the oxygenoxides of iron and other metals. Many different composi- (with sulfur or selenium for example), but so far this hastions have been developed to meet the requirements of a not led to useful materials. In practice almost all thewide range of applications. In addition to the magneti- linear ferrite applications are covered by mixed ferritescally soft linear ferrites considered in this article, ferrites in which Me is either Mn + Zn or Ni + Zn. These twohave long been established as materials for microwave basic compositions will be referred to as MnZn and NiZndevices, for data storage in computer memories, and for ferrites respectively. In each case the proportion of zincthe manufacture of high-coercivity permanent magnets. may be adjusted to provide the best match between theHowever, the main target of the original developments'-3 properties and the application requirements.was a magnetically soft ceramic-that is, a material Although single-crystal specimens may bfi~peparedhaving high permeability and low losses combined with for research purposes and have even been introduced onhigh electrical resistivity for use as a core material for a commercial basis for special applications such as low-inductors and transformers. This class of material is wear recording heads,5 virtually all manufactured softreferred to as linear or magnetically soft ferrite. ferrites are polycrystalline. They are prepared by pro-

42 IEEE spectrum JANUARY 1972

now substantially composed of ferrite spinel, is milled,mixed with a small proportion of binder, and granulatedinto a readily flowing powder. This powder is then formedinto the required shape, usually by pressing in a die or,after plasticization, by extrusion. After drying, the piece

B - _/ |parts are sintered at about 1250°C. During this processthe grains grow and the material is densified to a hard,black, polycrystalline ceramic.

Technically useful properties can be achieved only bycareful control of the composition and microstructure,7'8which in turn means control of the powder technologyand ofthe temperature and atmosphere during sintering.The grains, which in a typical ferrite have an average

0) Oxygen (anions) size between about 1 and 20 ,m, are imperfect single crys-

* Cation on tetrahedral site tals of spinel ferrite. The ideal spinel structure consists of*Cation on octahedral site two interlaced face-centered cubic sublattices of metal

ions (cations). In one sublattice the cations are sur-FIGURE 1. Small element of the spinel unit cell showing a rounded by a tetrahedral arrangement of oxygen ionstetrahedral site(A) and adjacent octahedral site(B). (anions) and in the other the arrangement is octahedral.

A small element of the overall lattice, illustrating thesetwo sites, is shown in Fig. 1. It can be seen that all the

FIGURE 2. Idealized arrangement of magnetic domains. cations are separated from each other by anions. Theoxygen plays an important role in the magnetic behavior

\______________________ _> / of these materials.In contrast, let us consider a metal magnetic material

such as iron. Here the ions are arranged in a body-centered cubic formation with no other elements present.

__________________________________________\ Because of their uncompensated electron spins, the ionshave a net magnetic moment and interact directly in sucha way that the magnetic moments of adjacent ions areheld in parallel-aiding alignment. The resultant ordering

________________________ _ <\is complete over regions that may extend over manythousands of interatomic distances. These regions, whichare called domains, thus exhibit spontaneous saturationmagnetization. This fully cooperative phenomenon iscalled ferromagnetism.

Returning to the magnetism of ferrites, the cations inthe spinel lattice still exhibit the appropriate magneticmoments but the net magnetization is much less. This isbecause (1) the cations are distributed in a nonmagneticoxygen matrix and (2) except in rare cases, cation inter-

cesses similar to those used in the manufacture of other actions via the intermediate oxygen ions cause the align-ceramics. The starting composition is formed by mixing ment between cations on the tetrahedral and octahedralthe correct proportions of metal oxides or carbonates, sites respectively to be substantially antiparallel. If theetc., or, as more recently described, by chemical co- sublattices had equal magnetic moments per unit volume,precipitation.6 In the former case milling is required to the net spontaneous magnetization would be zero andobtain sufficiently small grain size. The powder is usually the material would be classed as antiferromagnetic. Incalcined at about 1000°C, and during this process most of magnetic ferrites the sublattice magnetic moments onlythe solid-state reaction takes place. The resulting material, partially cancel, so there is a resultant spontaneous

Snelling- Ferrites for linear applications 43

magnetization; this is referred to as ferrimagnetism. to the surface represents the anisotropy energy when theIn the absence of an applied magnetic field it is ener- magnetization lies along that direction. In a cubic lattice

getically favorable for the spontaneous magnetization this anisotropy is generally small and may be minimizedto break up into domains. The direction of magnetization by careful adjustment of the composition. If the anisot-in adjacent domains differs by 90° or 1800 and the pattern ropy were zero the surface of Fig. 4 would become spheri-is such that the macroscopic magnetization is initially cal.zero; see Fig. 2. The regions between the domains are Other obstructions to wall movements include latticecalled walls. Within the thickness of these walls the imperfections, pores, and grain boundaries, which givedirection of the magnetization changes gradually (on rise to discontinuous and irreversible movements thatan atomic scale) from the direction in one domain to that dissipate energy and cause nonlinearity of the flux-in the next, as shown in Fig. 3. density/field-strength relation. This is the origin of mag-When a magnetic field is applied, the walls move in netic hysteresis. If the applied field is sinusoidal, with an

such a way that the domains having a component of mag- amplitude much smaller than the material coercivity,netization in the direction of the applied field expand at the energy loss caused by hysteresis may be expressed inthe expense of the others and a macroscopic magnetiza- terms of the resulting phase angle between the fluxtion may be observed. As the applied field strength is density and applied field strength. As the amplitude of theincreased, the observed magnetization increases until, latter approaches zero the phase angle attributable toat saturation, the spontaneous magnetization is aligned hysteresis tends to zero. The reason is that wall move-with the field throughout the material. From the fore- ments, as they become smaller, encounter fewer obstruc-going discussion of magnetism in ferrite materials it is tions and the motion becomes reversible or elastic.clear that the available saturation magnetization per However, as the field strength approaches zero someunit volume will be much less than for a metal such as loss mechanisms remain. The phase angle representingiron. This is reflected in the saturation flux density values this loss tends to a residual value referred to as the re-observed; about 0.5 tesla (5 kilogauss) for MnZn ferrite sidual loss angle; see Fig. 5. At low frequencies this lossagainst about 2 teslas for iron. is mainly due to thermal fluctuations of the domain

If the domain walls move readily under the influence of magnetization; at high frequencies it is mainly due to thean applied field the observed magnetization Mwill greatly onset offerrimagnetic resonance. This latter phenomenonexceed the applied field strength H. Such a material will is well known in microwave ferrites and is essential toexhibit a high susceptibility, K (= M/H), and consequently their gyromagnetic properties. It arises from the facta high permeability, A (= B/H, where B is the flux den- that the spinning electron behaves like a magnetic gyro-sity). scope. Thus, when disturbed, it precesses at a frequency

Since one of the desirable characteristics of a soft determined by the strength of the internal magnetic fieldferrite is high permeability, the walls must be able to that holds it in alignment; see Fig. 6. Application of anmove freely. The most obvious impediment to wall external magnetic field, alternating at the precessionmovement would be the resistance to rotation of the frequency, causes a resonance to occur. The result is amagnetization because of constraints within the lattice. dispersion of the observed permeability and a rise inSuch constraints will arise from preferred directions of losses at frequencies in the vicinity ofthe resonance.magnetization, which are referred to as magnetocrys- For a ferrite having no applied polarizing field thetalline anisotropy. Figure 4 shows the anisotropy energy ferrimagnetic resonance frequency is given by1surface for the lattice ofan MnZn ferrite having a positive 23 4 X 10MM.Hanisotropy. The length of a radius vector from the origin fies *

IHz

t- 1

where M,t is the saturation magnetization in A-m-1and jU is the small-signal permeability at low frequency.The inverse relation betweenfj. and pt is of particular

FIGURE 3. Transition of magnetization direction at a significance in the application of soft ferrites.boundary between two domains having opposite mag- This very brief and qualitative description of the mag-netizations. netization processes in ferrites is intended only as an

introduction to a survey of the technical parameters bywhich the properties of linear ferrites are specified.Fuller treatments may be found in the literature; see,for example, Refs. 4,7,9, and 10.

Electrical and magnetic parametersThe main parameters by which the technical properties

of linear ferrites are specified have become standardizedpartly by usage and partly by the work of TechnicalCommittee 51 of the International ElectrotechnicalCommission."' Although a number of these parametersare well known in a general sense it will be useful todefine them here in the context of ferrite properties. Amore extensive treatment may befound in Ref. 12.

Initial penmeability. The initial permeability )At is thesmall-signal value of the permeability. Its value relativeto that ofvacuum is given by

IEEE spectrum JANUARY 1972

<l00>

lo>. FIGURE 4. Anisotropy ener-

gy surface for an MnZn fer-rite having positive aniso-tropy. The (100) directionis the direction of easy mag-netization and the hard di-rections are (111) and (110).

1 B s=1s-i 3Pt= -lim- (2)

A

- (3)A1oH -0 H where pA' and A.' are the real and imaginary components,

where go = magnetic constant = 47r X 10-7, and B and respectively, expressed in terms of series elements (anal-H are in teslas andA* m-1 respectively. ogous to series inductance and resistance).

Because loss mechanisms cause a phase difference Temperature factor. The temperature factor ap isbetween the (sinusoidal) B and H, p, is complex and may defined by the following relation:be expressed in its real (inductive) and imaginary (loss) A2 - A

components: 2(02- 01) (4)

where ul and P2 are the permeabilities measured at tem-peratures 01 and 02 respectively. The permeability is

FIGURE 5. Phase angle a between B and H as a function ofsignal amplitude.

FIGURE 6. Ferrimagnetic resonance. A-Spinning elec-tron having axis aligned with a static magnetic field H.

X ! B-Precession about the resultant field direction when ac small perpendicular field h is applied. C-Precession

decaying to zero as a result of damping. D-Precessionresonance when h is alternating at the precession fre-

.6 | quency.

-ii_ )̂A B -- C D

Amplitude h h h h

Snelling-Ferrites for linear applications 4S

normally taken to be the real part of the initial perme- 1. Survey of some ferrite gradesability. Class*: I 11This factor has the property that when it is multiplied Initial permeability: 800-2500 500-1000

by the effective permeability of a gapped core it gives the Main applications: Inductors Inductors. an-temperature coefficient of inductance of that core. tenna rods

Disaccommodation factor. When a magnetic material issubjected to a transitory disturbance, which may be of ___ __mechanical, magnetic, or thermal origin, the pattern of Approximate frequency range: < 200 kHz 100 kHz-2 MHzdomains is disturbed and at the cessation of the dis- _ __ _turbance the walls do not return to their original posi- Country and manufacturertions. In those regions of the material where the direction (trade names In parentheses):ofspontaneous magnetization has changed there follows a Franceredistribution of some of the cations through the lattice. Cofelec (Ferrinox) TIO, T14, T22 T31. B1OThis results in an increasing anisotropy, so the wall LTT (Fermalite, Fernilite) 2002, 1004, 2005 1005mobility is reduced and the observed permeability falls RTC, La Radiotechnique-Com- 3B, 3B3, 3B5, 3B7. 3D3to a somewhat lower, more stable, value as a function of pelec (Ferroxcube) 3H1time. time. ~~~~~~~~~~~GermanyFor the purpose of specification a standard disturbance Krupp Widia-Fabrik (Hyperox) DI, DIS2, D1S3. D11

is usually adopted. This consists of the application of an D1S4alternating field of sufficient strength to saturate the Neosid Pemetzrieder K.G. F02 FOB

Siemens A. G. (Siferrit) N22, N28, N29, M25, M33material and then the reduction of the amplitude N32smoothly to zero. The initial permeability is measured at Steatit-Magnesia (Keraperm) 417 615two time intervals after the cessation of the disturbance. Valvo G.m.b.H. 38, 3B3, 3B5. 3D3367, 3HIFigure 7 shows the change of initial permeability as a 37 3function oftime for a typical MnZn ferrite. HollandThe disaccommodation factor Dp is defined by N. V. Philips (Ferroxcube) 3B, 3B3, 3B5. 3D3

3B7, 3H1

DIAl -

'2 (5) -al 2 loglo (t2/tl) Japan

Fuji Electrochemical Co. Ltd.where pu and J.2 are the initial permeabilities measuredat times t, and t2, respectively, after the disturbance. Hitachi Metals Ltd. 1A. 1B, iC,IF

Residual loss factor. It has been noted previously that Nippon Ferrite Ltd. VL-71, VL-74, AL-3, CL-81the phase difference between the sinusoidal B and H FQ-2, GP-5,in a ferrite core tends, at vanishingly small amplitudes, GP-3, GQ-25SB-S. FB-5to a finite value called the residual loss angle, di. This is Sony Corp. FBM, FBI, 503, 403, FBI.often expressed as a residual loss tangent, tan 5,. Referring FB4, FB4A FB2

TDK Electronics Co. Ltd. 1-15A,HIA. H;A. QIB. Q2Dto E~q. (3),H6 11BHA

tanS, = ' (6)lAs Tohoku Metal Industries Ltd. Neferrite C, 801 F

It is convenient in the application of ferAte to inductors Super Nefer-

to use this quantity normalized with respect to the initial rite C

permeability; that is, (tan 8,)/)A. This is called the residual United Kingdomloss factor. When multiplied by the effective permeability Aladdin Components Ltd.of a gapped core it gives the value of the residual loss ITT Components Group SA502, SA503, SA401. SA403tangent ofthat core. SA500L

Hysteresis coefficient. The phase angle between B and Mullard Ltd. (Ferroxcube) Al, A5, A13 AIOH due- to hysteresis (see Fig. 5) is designated &ih and is Neosid Ltd. P10, F7, F8A F11often expressed as the hysteresis loss tangent, tan ah. Siemenys UK. Ltd. (Siferrit) N22 N2, N29, M25. M33To a first approximation this quantity is proportional to N32

SEI Ltd. P, Q S

United States

FIGURE 7. Change of initial permeability of a typical MnZn Allen-Bradley Co.ferrite as a function of elapsed time after disturbance by Arnold Engineering Co.a saturating ac field. Fair Rite Products Corp. 71, 72, 73 33, 43zc0

Ferroxcube Corp. (Ferroxcube) 3B7, 3B9 3D3

Z,ZL Indiana General Co. (Ferramic) TC9, TC12

Magnetics Inc. C, D, G ANational Moldite Co. Inc.Stackpole Carbon Co. (Ceramag) C24. C26 C5N, C7D,

M

E | i C27A

1 IO 102 D 0 16 0. M. Steward Mfg. Co. F-112Time after disturbance. mninutes * As far as is known, classes I to IV are basically MnZn ferrites and V

46 IeEE spectrum JANUARY 1972

III IV v VI VIl Vill lx x xi

1500-10 000 1000-3000 > 1000 500-1000 150-500 70-150 30-70 10-30 < 10

Wide-band High-B8at applica- Wide-band HF wide-band Antenna rods, Inductors, an- Inductors Inductors Inductorsand pulse tions, TV and and pulse and power HF power tenna rods,transformers power trans- transformers transformers trans- HF power

formers formers transformers

LF-200 MHz < 100 kHz 1-300 MHz 100 kHz- 500 kHz- 2-30 MHz 10-40 MHz 20-60 MHz > 30 MHz300 MHz 5 MHz

T4, T6 B30, B42, B50 H20 H30, H32 H50 H52, H602003, 2004 3001, 3002 1101 1102 1122, 1103 1124, 1104 1125, 1105

3E1, 3E2, 3E3 3C2, 3C6, 3C8 4A1, 4A3, 4A4 4B1 4C1, 4C6 4D1, 4D2 4E1, 1Z2

DlSl, DlSl1 C21, C22, C23, E2, E3 E4 E5 E6 E7C2

Fl F2 FlOb F20 F40 F100T26, N30, T37, N27 M11 KI K12 U17, U60T38

417, 421 407, 417 503 606, 612 602 704 818 8143E1, 3E2, 3E3 3C2, 3C6, 3C8 4A1, 4A3, 4A4 4B1 4C1, 4C6 4D1, 4D2 4E1. 1Z2

3E1, 3E2, 3E3 3C2, 3C6, 3C8 4A1, 4A3, 4A4 4B1 4C1, 4C6 4D1, 4D2 4E1, 1Z2

H32, H20, K4 Li, Q12, B1 Q33, Q61 Qs, Qni, Q5.D1, Ml

3A, 3B, 4A, 4B, lA, 1B, 1C, 1F,4C 2A, 2B, 2C

VL-71, VL-74, SB-5, FB-5, FB-3, L-84, L-85, T-314, QL-400, L-81, QM-101, MH-81, VH-40, VH-50, VH-100, VH-300FB-3, GP-5, YL-7 TH-100, CL-81 QM-051, QM-201 VH-5OB, VH-150,GT-7 L-82 MH-90, IT-1 VH-200

204, FBL, FC2, 304, 307 4B1 5A5 5A2 7A1, 6A6 7A2, 6A7 7A3, KH51, 7A4, KH75FC4 KH72

H5C2. H5B. H7A. H7A. H3S, H3V, L6 L5 K5, QIB, Q2B, K6A, Q5B, Q3C, MSB. M8C, M11 M5. M5C, M5DH5B2, H5D, H4K QIC, Q2D, QIE M8L, M9HP4000, H5A, DIB, Dic,DP5000 DIE, D3B,

D84000H, 7000H, 1300B, 2000B, 2000L 600L 250L, 400L 80L, 10OL, 40L 20L 10L12000H 3000B 150L

Rl R2 R4, R5 R6, R8 R9 RIOSA500T, SB700 SB600 SB500 SB400 SB300 U17, U60SA601,SA611A7, A8, Al5 A3, A9, A16 B1 B2 B1O B4 B5PlO, F8A F8B F13 F14 F16, F18 F25 F22 F29T5, T5T SF2, NW29, NW25 NW1O, H32

T26, N30, T37, NW26 N27 Mil Kl K12 U17, U60T38

T R K2 K4 K6 K8

W-5 W-5 R-02AK16, AK20, AK16, AK20, AK04, AM20 AM12 AM04AK30 AK3072, 73, 74, 75 73, 74 62, 64 61, 65 63

3B7, 308, 3E2A, 3B7, 3CB 4A6 4A 4B 40, 4C4 4D 4E, 4E2, 1Z23E3

05, 06 05 PR6200, PR6400, H TC4, Q1 Q2 Q3PR6500, 06

C, D, G,H, J D,P N1MM2.-

024, C24H, 0248 C5N, C7B 09 C11 012 014 C14AC24K, 026,028F-124, F-130 P4S-i P7-23, P7-21 F-220 P6-23 P6-21

to XI are basically NiZn ferrites. Exceptions are 1Z2, a hexagonal structure, and AL-3, Cl-81, and YL7, which are NiZn ferrites.

Snelling-Ferrites for linear applications 4

H and B and, for the same reason that was given in the The resistivity is of particular importance in the case ofcase of residual loss, it is convenient to normalize it with MnZn ferrites because at the higher application fre-respect to permeability. This gives rise to a hysteresis quencies the value often is not high enough to preventcoefficient nB as proposed by the IEC1 1: some eddy-current effects.

Curie point. The temperature above which the orderingtan ah (7) of the domain magnetization breaks down and the mate-

,13rB rial becomes paramagnetic is referred to as the Curie

where Jir is the relative permeability measured at peak point 0k. The value depends strongly on the proportionflux density B. of zinc used in the composition and it can be varied in

Saturation flux density. Saturation flux density Bsat iS MnZn ferrites from room temperature to 300°C. Thean important parameter in high-power applications. It actual value is usually above 130°C.generally refers to the static flux density measured at a Other parameters. There are, of course, other parame-standard value of field strength that takes the material ters that may be relevant to soft ferrite applications, suchwell beyond the knee of the B-Hcurve. as permeability at high flux densities, permeability in thePower loss density. The power loss density Pm is of presence of a static field, and harmonic distortion. How-

prime importance when ferrites are used at higher flux ever, such data, if given, are usually for guidance only;densities. Expressed as the power loss per unit volume, the control and specification usually depend on some orit is usually given as a function of frequency and peak all of the previously defined parameters.flux density. When quoted as a material parameter it isusual to omit any contribution of eddy-current loss in Survey of compositions and gradesthe ferrite, since this loss depends on the size and shape Compositions. As stated earlier, virtually all the com-of the test core. mercially available linear ferrite materials are eitherFor a given core type one can refer to the total power MnZn or NiZn ferrite, and only these compositions

loss (in watts, for example) and this figure would, of will be considered here. Before a brief description of thecourse, include any eddy-current loss. main characteristics of these compositions is given,

Resistivity. The resistivity p, measured by dc tech- perhaps passing reference should be made to the smallniques, is usually quoted. Since ferrites are semicon- but growing tendency to use lithium zinc or magnesiumductors, the resistivity falls as the temperature rises. manganese ferrite for television deflection yokes. ThisThe bulk resistivity is also dependent on frequency, is because, though magnetically soft, these ferrites haveparticularly for the MnZn ferrites. This arises from the very high resistivities and thus the deflection coils maygranular structure. At low frequencies the high-resistivity be wound directly onto the ferrite without additionalgrain boundaries play a dominant role in determining insulation.the bulk resistivity. At high frequencies these boundaries MnZn ferrites. Of the two basic compositions underare shunted by their capacitance, so the bulk resistivity consideration, MnZn ferrites are characterized by havingapproaches the lower values appropriate to the grain lower residual and hysteresis losses, higher permeabilities,interior-10-3 m for a typical MnZn ferrite. and lower resistivities. The higher the permeability the

I. Typical values of parameters for some grades of soft ferrites

Principal Application Categories

Measuring Conditions* II III IV VilIMF In- Wide-Band High-Flux- HF In-

Flux ductors, Pulse Density ductorsDensity, LF Antenna Trans- Appli- and Power

Parameter Frequency mT Inductors Rods formers cations Transformers Units

Initial permeability ,i < 10 kHz < 0.1 800-2500 500-1000 1500-10 000 1000-3000 70-150

Saturation flux densityBsat 350-500 at -400 at 300-500 at 350-520 at 250-420 at mT

H=1 H=1 H=1 H=1 H=4 kA-m-

Residual loss factor 10 kHz < 0.1 0.8-1.8 1-10 10-6(tan 5,)/pi 100 kHz 1.5-10 5-15 4-60 10-6

1 MHz 10-40 20-50 10-610 MHz 60-120 10-6

Hysteresis coefficient qjj 10 kHz 1-3 0.3-1.3 0.48-1.9 0.1-1.3 1.6t-48 mT'- X 10-

Power loss density P,m 16 kHz 200 80-190 pAW-mm-3Curie point 0, < 10 kHz < 0.1 140-210 200-280 90-280 180-280 350-490 °C

Temperature factor t, < 10 kHz < 0.25 0.5-1.5 1.0-3.0 0-10 OC t X 10-6from 5 to 55°C

Disaccommodation factor < 10 kHz < 0.25 1-3 2-10DF from 10 to 100 min

FResistivity p dc 0.5-7 1-20 0.02-0.5 0.2-1.0 > 103 QrnApprox. compositionMnO 27 34 27 30 mo[% .NiO 32ZnO 20 14 20 15 18Fe,03 53 52 53 55 50

* The measuring temperature is 25°C unless otherwise stated.t This low value of hysteresis loss is obtained on specially heat treated nickel zinc ferrites.

48 IEEE spectrum JANUARY t972

lower the frequency of the ferrimagnetic resonance; see cores may haye passbands extending up to about 200Eq. (1). Since this resonance is accompanied by a large MHz, provided that the lower limit of the passband liesrise in losses and a dispersion of the permeability, it below the frequency at which the dispersion of the per-follows that the higher the permeability the lower is the meability occurs.upper limit of the frequency range in which they can be The application of MnZn ferrite is also somewhatused in applications requiring low loss angles. In practice limited by eddy currents. Since the electrical conductivitythis means that, as cores for high-Q inductors, the MnZn is not negligible, eddy currents can give rise to significantferrites are generally restricted to frequencies below about power loss, and even permeability dispersion, if the2 MHz. For some other applications this limit may not frequency or the core cross section, or both, are excessive;apply; for example, wide-band transformers using MnZn the power loss density due to eddy currents is propor-

tional to (Bfd)2/p, where dis the smallest dimension of thecore cross section.NiZn ferrites. In contrast, the NiZn ferrites generally

have higher losses and lower permeabilities, but muchhigher resistivities. It follows from the foregoing discus-

500 I 1|1I|1|1 sion that these ferrites find application mainly in high-Qinductors above about 2 MHz.Through the appropriate choice of Ni/Zn ratio these

ferrites may be made with initial permeabilities ranging400 l ll from about 2000 to 5. Thus, in practice, a series of grades

is available, giving a graduated range of applicationfrequencies from 2 MHz to about 70 MHz respectively.Eddy-current effects are usually negligible at all fre-quencies in this range.

300 i l | Application classification. Although there are manyE ~~~~~~~~~~~~~~~gradesof soft ferrites currently available, nearly all of

FIGURE 10, The real (inductive) component.'and imag-inary (loss) component ,u" of the series complex perme-ability as functions of frequency for typical ferrites.

Field strength H, kAm-110FIGURE8. B-H loops of typical MnZnferritesat20°C.

FIGURE 9. Initial permeability as a function of tempera-ture for typical ferrites.

102 102

10~~~~~~~~~~~~~~0 0 0

50 0 100 200Frequency, hertz

Temperature, IC

Snelling-Ferrites for linear application 49

them may be placed into relatively few categories accord- table, some omissions are inevitable and the authoring to the principal application for which they are in- apologizes in advance for these.tended.Manganese zincferrites: Typical properties

I. Inductors for frequencies up to about 200 kHz. Table II lists the parameters defined earlier in thisII. Inductors for a frequency range of about 100 kHz article and specifies the usual measuring conditions.

to 2 MHz. Antenna rods for medium- and long- Typical values of these parameters, as measured onwave broadcast bands. toroidal samples, are given for five of the more important

III. High-permeability applications-in particular, application categories previously defined, the first fourwide-band transformers (lower cutoff frequency referring to MnZn grades and the fifth referring to anup to about 10 MHz) and low-power pulse trans- NiZn grade. The approximate compositions are alsoformers. indicated.

IV. Applications requiring high saturation flux density It should be emphasized that material properties areand low loss at high flux densities-in particular, not a very reliable guide to the performance of com-television-line-scanning transformers, deflection ponents made from those materials. The reason is thatyokes, and power transformers for about 700 Hz the core geometry in general influences the homogeneityto 100 kHz. of the pressed core and its exposure to firing conditions

Nickel zincferrites: and these in turn affect the properties. ComponentV. yL > 1000. Wide-band transformers for about properties are usually specified as such.

1 to 300 MHz. Pulse transformers for short-dura- Figures 8-12 illustrate a few of the more importanttion pulses. relations that characterize the performance of soft ferrites.

VI. 500 ( ,Uf < 1000. Wide-band transformers forabout 5 to 300 MHz. Antenna rods for medium-and long-wave broadcast bands. Power transfor-mers for about 100 kHz to 1 MHz.

VII. 150 ( ,Ut < 500. Antenna rods for medium- and FIGURE 12. Power loss density as a function of frequency,long-wave broadcast bands. Power transformers with flux density as a parameter, for an MnZn ferritefor about 500 kHz to 5 MHz. suitable for high-flux-density applications.

VIII. 70 K pi < 150. Inductors for about 2 to 20 MHz. 103Antenna rods for short-wave broadcast bands.Power transformers for about 2 to 30 MHz.

IX. 30 ( pA <70. Inductorsforabout 10 to40 MHz.X. 10 ( pA < 30. Inductors for about 20 to 60 MHz.XI. pA < 10. Inductors for frequencies above about

30MHz.A survey has been made of all the principal manu-

facturers of ferrite components known to the writer and102

whose data were readily available at the time of writing. 102With the assistance of the manufacturers, which is grate-fully acknowledged, a list of grade numbers has beencompiled and arranged in accordance with the foregoing lapplication classification. Grades not primarily intended EEfor any of these applications have been omitted.The result of this survey is shown in Table I. Although .

great care has been exercised in the preparation of this 0E

Z'I0 1

FIGURE 11. Residual loss factor as a function of fre-quency, derived from the data of Fig. 10.f.

io-_ ||!4 1

10660 06l-I 10310410610

Frequency, hertzFrequency, hertz

50 IEEE spectrum JANUARY 1972

Individual curves relate to typical grades, the identifica- port in this is greatly appreciated. The author would also like totion numbers referring to the application categories thank Jan van der Poel of Ferroxcube Corp., Saugerties, N.Y.,and Prof. Saito, University of Tokyo, for their assistance in thepreviously defined. updating of parts of Table I, and two of his colleagues at Mul-

lard Research Laboratories, David Annis for the preparation of

Future developments Fig. 1 and Paul Rankin for valuable discussion of the text.

Because the understanding of magnetization processes REFERENCESin ferrites has increased greatly in recent years, pro-in fi h n s rl r1. Hilpert, S., "Genetische und Konstitutive Zusammenhange ingressive improvements in properties have resulted. Efforts den magnetischen Eigenschaften bei Ferriten und Eisenoxyden,"have been made to optimize the properties in relation to Ber. Deut. Chem. Ges., vol. 42, p. 2248, 1909.the requirements of specific application areas. In practical 2. Snoek, J. L., New Developments in Magnetic Materials. Newterms there have been small but important composition York: Elsevier, 1947.

3. Polder, D., "Ferrite materials," Proc. IEE (London), vol. 97,changes, usually by additions or substitutions of metal pt. II, p. 246, 1950.ions, but the main improvements have come from more 4. Smit, J., and Wijn, H. P. J., Ferrites. Eindhoven: Philips, 1959.advanced manufacturing processes leading to more 5. Sugimoto, M., Kobayashi, I., Yamagishi, I., and Ishii, R.,accurate process control. These trends will continue. "Growth and properties of large manganese zinc ferrite single

crystals," presented at the Internat'l Conf. on Ferrites, Japan,Specific areas of improvement will now be considered July 1970.very briefly for three main classes of application. 6. Takada, T., and Kiyama, M., "Preparation of ferrites by wet

Inductor ferrites. There will be further decreases in method," presented at the Internat'l Conf. on Ferrites, Japan,residual loss and hysteresis loss and, in the case of MnZn July 1970.

7. Broese van Groenou, A., Bongers, P. F., and Stuyts, A. L.,ferrite, this will make it desirable to further increase the "Magnetism, microstructure and crystal chemistry of spinelresistivity. In order to utilize the potentially higher values ferrites," Mater. Sci. Eng., vol. 3, 1968-69.of Q resulting from these improvements, better control 8. Ross, E., and Hanke, I., "The microstructure of ferrites withof temperature factor (that is, closer tolerances) wil be high permeabilities and its influence on magnetic properties,"

Z. Angew. Phys., vol. 29, pp. 4, 225, 230, 1970.required and it will be necessary to make further reduc- 9. Bozorth, R. M., Ferromagnetism. Princeton, N.J.: Van Nos-tions in the disaccommodation factor. Recent reports'3"4 trand, 1951.indicate that significant progress toward these objectives 10. Chikazumi, S., Physics of Magnetism. New York: Wiley,is possible by the substitution of some of the iron by 1964.

titanium or tin. Moereisc11. "General classification of ferromagnetic oxide materials andtitanium or tin. More precise composition control will definition of terms," Internat'l Electrotechnical Commission Pub.probably result in further improvements. 125, Geneva, 1961; also Amendment No. 1, 1965.However, the future of the ferrite-cored inductor is 12. Snelling, E. C., Soft Ferrites, Properties and Applications.

threatened by a number of competitive technologies, London: Butterworth, 1969.' 13. Stijntjes, T. G. W., Broese van Groenou, A., Pearson, R. F.,

notably mechanical filters, inductance simulation by Knowles, J. E., and Rankin, P. J., "Magnetic properties and con-active circuits, and the development of communication ductivity of Ti substituted MnZn ferrites," presented at the Inter-systems that avoid the use of critical frequency-selective nat'l Conf. on Ferrites, Japan, July 1970.circuits.'5',6 This competition will grow, but the ferrite 14. Matsubara, T., Kawai, J., and Sugano, I., "Disaccommodationin MnZn ferrites," presented at the Internat'l Conf. on Ferrites,core, which is well established and capable of further Japan, July 1970.development, will probably not be superseded soon or 15. Orchard, H. J., and Sheahan, D. F., "Inductorless bandpasssuddenly. filters," IEEE J. Solid-State Circuits, vol. SC-5, pp. 108-118, June

Lo-o -rtasfre ferrites. Avalable permeabili- 1970.Low-power transformer ferrites. Available permeabili- 16. Moschytz, G. S., "Inductorless filters," IEEE Spectrum, vol. 7,ties have increased dramatically in recent years and after pp. 30-36, Aug 1970; pp. 63-75, Sept. 1970.some consolidation there is no reason why this trendshould not continue. Higher permeabilities permit smallercores and thus lead to higher flux densities. In the case of Reprints of this article (No. X72-012) are available tosignal transformers, therefore, the hysteresis will have readers. Please use the order form on page 8, which givesto be reduced accordingly if harmonic distortion is to be information and prices.kept down to acceptable levels.

Transformer ferrites, being applicable to pulse as wellas signal transformers, are less vulnerable than inductorferrites to competitive technologies. E. C. Snelling studied electrical engineering at the

High-power transformer ferrites. The saturation flux University of London and graduated in 1948. Hedensities achieved in high-power transformer ferrites then joined Mullard Research Laboratories, thealready approach the limits inherent in currently available British constituent of the Philips International re-ferrite compositions. However, some increases may be search facilities. He first worked on the develop-possible.There ismoescot. ' . . ment of FDM carrier telephony equipment and

possible. ihere1s more scope in the reduction of hysteresis acquired broad experience in the design of induc-losses at high flux densities. It would also be desirable tors and transformers and their application to com-to increase the resistivity in order to reduce the ap- munication networks, particularly channelingpreciable eddy-current losses that can occur in large equipment. In 1969 he became leader of the Ferritecores at high frequencies. It is important that these im- Applications Section, with responsibility for a wide

relat to heanicipted orkin temera- range of projects concerned with the properties andprovements reaet h nilae okn epr- applications of linear ferrites. In 1969 he publishedtures ofthe materials. "Soft Ferrites," a book that deals extensively with

the technical properties of these materials and thedesign of inductors and transformers using ferrite

Some of the material in this article is based on parts of the cores. He has also published numerous technicalauthor's book, Soft Ferrites, Properties and Applications. The articles and isaFellow of thelInstitution of Electricalpermission of the publisher, Butterworth's of London, is gratefully Engineers.acknowledged. The manufacturers listed in Table I readily co-operated by supplying details of their ferrite grades and their sup- -___________________________

Snelling-Ferrites for linear applications 51