VOLUME 19, 1957/58, No. 7-8 pp. 209-244 Published 10th February 1958

Philips Technical RevieWDEALING WITH TECHNICAL PROBLEMS

RELATING TO THE PRODUCTS, PROCESSES AND INVESTIGATIONS OFTHE PHILIPS INDUSTRIES

CRYSTAL-ORIENTED FERROXPLANA

by A. L. STUIJTS and H. P. J. WIJN. 621.318.13 :538.213

The magnetically soft materials ''ferroxplana'', on which an article recently appeared inthis Review, have a permeability which remains constant up to frequencies far above 100 Mc]«,It appears that the crystals of these materials can be aligned. As a result the permeability isappreciably increased, with only a slight drop in the limitingfrequency. Moreover, this producesa material with an tinisotropie permeability, for which there arc special applications.

Introduetion

A short time ago some new groups of ferromagne-tic oxides were discovered in the Philips Laboratory,Eindhoven, which, like the earlier described materialferroxdure, possess a hexagonal crystalline struc-ture. These materials were described in an articlein this Review 1), which we shall henceforth referto as I.The composition of these compounds can berepresented in a triangle diagram, the vertices ofwhich are formed by three oxides. This triangle isshown in fig. 1. Me represents a divalent ion fromthe series Mn, Fe, Co, Ni, Cu, Zn, Mg, or a mixtureof these ions. On the sides of the rriangle are foundrhecompound B (BaFe204), the group ofcompoundsS (MeFe204, which includes ferroxcube) and thecompound M (BaFe12019'main constituent of ferrox-dure). The new groups of compounds discussedin I are represented in the triangle by the pointsW (BaMe2Fe10027)' v (Ba2Me2Fe12022) and Z(Ba3Me2Fe24041)' Later, the new groups of corn-pounds X and U were discovered 2); If, in the Z

1) G. H. Jonker, H. P. J. Wijn and P. B. Braun, Ferroxplana,hexagonal ferromagnetie iron-oxide compounds for veryhigh frequencies, Philips tech. Rev. 18, 145-154, 1956/57,(No. 6); referred to in this article as 1.

2) See P. B. Braun, thesis, Amsterdam 1956, also Philips Res.Rep. 12, 491-548, 1957 (No.. 6); G. H. Jonker, Ferri-magnetic iron oxide compounds with hexagonal crystalstructure, paper presented in June 1957 to 16e Congrèsintern. Chimie pure et appl., Paris.

group, for example, the Co ion is chosen for Me, thiscompound can be briefly denoted by C02Z, indic-ating that the structural unit (this is not the unitcell, see I, ref.7) and P: 153) contains two Co ions.As in the case of ferroxcube and ferroxdure, these

materials are prepared by sintering at high tempera-

S=MeF~04. -(ferroxcube)

M=8afe/20/9(ferroxdure}

B=8aFe204.

Ta

Fig. 1. The composition of the materials W, Y, Z, X and U canhe represented by points in the triangle BaO-MeO-Fe203'D,S and M represent the known materials BaFe204' ferroxcubeand ferroxdure, respectively.

....-----...,.....,.~----------------------~~. ,._,. -----_-- - _.

210 PHILIPS TECHNICAL REVIEW VOLUME 19

tures, which gives rise to a reaction between theconstituent oxides and the atmosphere. The ceramicproduct so produced has a high resistivity (104-1010ohm cm).It has been found that in all compounds of the

Y group, and also in those of the Zand W groupswhich contain more than a certain content of Co,the crystals exhibit a preferred plane for the magnestization; these compounds have been given the na:n;teferroxplana. This preferred plane is perpendicularto the (hexagonal) c-axis, which we have calledthe "abhorred" direction. The magnetic anisotropyfor rotations out of this plane is generally large,which is to say that the magnetization is verystiffly bound to this plane, and therefore the per-meability in the direction perpendicular to thepreferred plane is small. However, the magnetiza-tion can fairly easily be rotated in the preferredplane itself (relatively small anisotropy for rota-tions in the preferred plane) so that the permeabilityin directions parallel to this plane can be muchhigher 3). In polycrystalline material, in which thepreferred planes lie in arbitrary directions, there isthen a certain average isotropic permeahility. Itwas shown in I that, owing to the markedly dif-fering anisotropies of the crystals, the product ofthe permeability and the frequency above whichthe losses show a sharp increase (limiting frequency)can attain a much higher value than with ferrox-cube. For example, in the case of C02Z the permea-bility is found to be about 10 at a limiting frequencyof about 400 Mc/s, while ferroxcube 4E with apermeahility of 12 has a limiting frequency ofabout 90 Mc/s.In this article we shall first discuss how the per-

meability of ferroxplana can be influenced by ananisotropic distribution of the orientations of thecrystallites in the material (texture). We shall thendescribe a method of, crystal alignment differingfrom a method described for ferroxdure in an earlierarticle in this Review 4),' henceforth referred to as11. It will be shown that by the alignment of ferrox-plana crystals the permeability of the material can

• be considerably increased in certain directions; theexplanation of this phenomenon will he discussed.The limiting frequency, is only slightly reducedby the alignment.

3) Where permeability and susceptibility are referred to,the relative initial permeability and susceptibility arealways meant (in #0 units in the rationalized Giorgisystem), i.e. the value measured in the demagnetized stateupon application of a very weak magnetic field.

') A. L. Stuijts, G. W. Rathenau .and G:H. Weber, Ferrox-dure Il and Hf, anisotropic permanent magnet materials,Philips tech. Rev. 16, 141-147, 1954/55;referred to in thisarticle as n. .

Anisotropic ferroxplana materials

The most anisotropic material is the single crystal.The anisotropic magnetic properties of a ferrox-plana single crystal are exemplified in fig. 2, where

-H

° 5000 10000 15000 oersted

'b II

Wb/m2 / r+-0,3 I+""'" 30001/ gauss

0,25 I' -PoM 4nM

, t ) t0,2 2000

0,15 /0,1 / 1000

fJ,05 /0 00 0,5 1,0 H 1,5Wbjm2

--'flo 94237

Fig. 2. The magnetization M of a COzZsingle crystal as a func-tion of the applied magnetic field H; a) preferred plane atright angles to H; b) preferred plane parallel to H.

the magnetization M for a single crystal of C02Zis plotted as a function of the applied inductionf-toH in a direction perpendicular to the preferredplane (curve a) or parallel thereto (curve b). Itcan he seen from the figure that the inductionf-toH needed to saturate the material is in the firstcase about 1.2Wh/m2 (fieldstrength 12 000 oersteds),while in the second case a much smaller inductionsuffices for saturation.

In a normal polycrystalline specimen, the crystalshave a random orientation, and therefore the speci-men possesses isotropic magnetic properties. Textu-res are possible, however, in which anisotropicmagnetic properties occur in the pol)'crystallinespecimen. These textures are the following:

1) The basal planet' of the crystals, which' coincidewith the preferred planes, are all parallel to oneline, as illustrated in the upper half of fig. 3a (fantexture). In thè lower half of this figure the' orien-tation of the crystals is illustrated as they are seenwhen. a cut is made perpendicular to the commonline of the basal planes (the coaxes of all crystals

1957/58, No. 7-8 CRYSTAL-ORIENTED FERROXPLANA 211

lie in the plane of this figure). It should be borne inmind in this connection that the crystals are gene-rally in the form of platelets, with the smallestdimension in the direction óf the c-axis, The perm-eability will now be greater in the direction of thecommon line than in the isotropic material, whereasthe permeability in the perpendicular directionswill be smaller.2) All basal planes are mutually parallel, as

illustrated in fig. 3b (foliate texture). In this casethe permeability in all directions parallel to thebasal planes is greater than that in the perpendicu-lar direction. .

As is known, the permeability is caused by the-+

fact that an applied field H changes the direction-+

of the spontaneous magnetization Ms in each Weiss-+ -+

domain if the couple floHX Ms differs from zero. Inferroxplana the magnetization is so strongly bound

-+to the preferred plane, and hence the rotation of Msout of this plane is so slight, that this rotation makesa negligible contribution to the permeability. Forthis reason, the only contribution to the permeability

-+is made by the component of H that lies in thepreferred plane of a given crystal. In a specimenin which the crystals are isotropically oriented, thepermeability is therefore smaller than in a speci-

-+men with aligned crystals, in which H is made tolie parallel to the preferred planes.

For the case where there is no magnetic interac-tion between the crystals, the alignment causes therotational susceptibility Xi= fli - 1 to increase by

933+6 •

bFig. 3. Anisotropic ferroxplana. Above: basal planes in per-spective; below: orientation of crystals in a section perpendic-ular to the basal planes. a) Fan texture; b) foliate texture.

a factor 1.5. The interaction, however, is by nomeans negligible, as appears from the idealized caseillustrated in fig. 4. This figure shows a number ofcrystals of a specimen, with the preferred plane ofthe centre crystal assumed to be parallel to thehatching and perpendicular to the plane of thedrawing. The preferred planes of all the other cry-stals are assumed to be parallel to the plane of thedrawing. In this case the lines of force of the applied

93349

Fig. 4. Demagnetization due to a wrongly oriented crystal inferroxplana, The preferred planes of the surrounding crystalslie in the plane of the drawing; that of the central crystal isperpendicular to the plane of the drawing and parallel to thehatching. The lines of force must bend round the "cross-wise" crystal.

field must bend to the left and right around the"crosswise" crystal, so that this crystal has a strongdemagnetizing influence. Ás a result the permeabili-ty is smaller than that of a specimen in which thepreferred planes of all crystals are in parallel align-ment.

It will be shown below that both mechanismshere described are effective, and that therefore thepermeability can be increased by the process ofalignment by a factor greater than 1.5.

Methods of aligning crystals of magnetic materials

There are various. known methods of producing amagnetic material with a preferred orientation.

In the case of the metallic magnetically softmaterials fernicube (50Ni-50Fe) and "grain-orient-ed" silicon-iron (3% Si, balance Fe) a crystallinetexture is obtained by rolling and subsequent re-crystallization. With fernicube the preferred direc-tions of the c:rystals are aligï:ï.edat .right angles to

~.;.J.

the direction of rolling, so that in the latter direction

-----------,.---,------~----------------------- ~ ., .. _- ._

212 PHILIPS TECHNICAL REVIEW VOLUME 19

the relation between the permeability and the ap-plied field becomes more linear; with silicon-ironthe. permeability is increased in the direction ofrolling, because the preferred directions of the cry-stals are aligned parallel to this direction.- Rolling cannot be applied in the same way to

ceramic materials because they cannot be plasti-cally deformed. Use might he made, however, of thespecial form of the particles (which, as stated, aremostly flat with the. smallest dimension in thec-direction). It has been found possible with ferrox-dure to obtain a crystalline texture by compressingthe powdered material in a steel tube and thenrolling. The effect is slight, however, and of the sameorder as the texture obtained by pressing the pre-heated powder in a die.With some magnetically hard materials (ferrox-

dure, MnBi) the fact that the magnetization isstrongly bound to a preferred direction can he putto very good use for aligning the crystals and there-by substantially increasing the energy product(BH)max' (For ferroxdure, see II.) In ferroxplanathe magnetization is strongly bound not to a pre-ferred direction but to a preferred plane (as aconsequence of which ferroxplana is magneticallysoft). Here, too, it has been found possible to alignthe crystals by making use of a magnetic field. Todo this we start with a uniform magnetic field--*H in which a number of ferroxplana crystals areplaced, each of which, it is assumed, are free to

--*rotate. The magnetization Ms of each particle is

--* --* --*drawn in the direction of H by a couple Ilo H X Ms= ItoHMs sin a (see ,fig. 5), where a is the angle

--*between the magnetization Ms and the field direc-tion. The crystal anisotropy field HA binds the. --*spontaneous magnetization Ms to the preferredplane with a couple which p:roves to be equal to

H

Fig. 5. Rotation of a ferroxplana crystal in an external field H.Owing to the couples PoH[l.1. sin a and PoHAM. sin e coseacting on [1.1•• the preferred plane tends to take up a positionparallel to H.

--* -+lloHA X Ms = IloHAMs sin e cos e, where o is theangle between the magnetization' and the preferredplane. If the particle is free to move, it will thenrotate until in a state of equilibrium a = e = 0,the preferred plane then being parallel to H. Theplane itself can be in an arbitrary direction so thatwe have obtained here a fan texture.To produce the foliate texture we subject the

powder alternately to two magnetic fields whoselines of force are at right angles to each other, forexample by energizing alternately two crossed elec-tromagnets. The only stable state for the orienta-tion of the particles is now that in which the pre-ferred plane is parallel to both field directions, i.e.the foliate texture. Naturally, this state of equili-brium occurs only if the motion of the particles isdamped, but in practice this condition is alwayssatisfied. Instead of using two alternate, mutuallyperpendicular fields, we can use a magnetic fieldcontinuously varying in direction but lying in oneplane, that is a rotating magnetic field which can,for example, be produced by mechanically rotatinga yoke magnet. A rotating magnetic field can alsobe produced with a stationary magnet with the aidof the three phases of the electric mains. An electro-magnet on this principle has been developed byU. Enz in the Philips Research Laboratories inEindhoven, and has been found very suitable foralignment processes.To produce good results a number of conditions

must be fulfilled. The basic material is a magneticpowder that is obtained by pre-sintering and subse-quent grinding. In the first place the preferred planesof the crystals in one particle must he mutuallyparallel (see II). Use is preferably made of a sus-pension of the powder in a liquid in order to pro-duce a state in which the particles can readily berotated. In: order to be able to make a well finishedceramic product the particles, after orientation,should be stacked as compactly, as possible. Forthis reason the process of aligning the particles iscombined with a pressing treatment. This' means,however, that the particles obstruct each other, sothat an ideal state of alignment can never heachieved.Moreover, the method can only be employed if

the principal anisotropy field'HA is not too weak.For materials with a composition with which HAis weak, as in certain mixed crystals of C02Z withother Me2Z compounds, the result is therefore poor.Itmay be said of these materials that they show theeffect typical of ferroxplana to a much lesserdegree.Fig. 6 shows two micrographs taken with an

1957(58, No. 7-8 CRYSTAL-ORIENTED FERROXPLANA 213

a Fig. 6. Electron-microphotographs of aligned ferroxplana with foliate texture. In (a)the preferred planes are parallel to the plane of the paper, in (b) they are perpendicularthereto.

b

a Fig. 7. Photographs of a specimen with fan texture; the direction to which the preferredplanes are parallel lies in (a) in the plane of the paper (roughly vertical) and in (b) per-pendicular to the plane of the paper.

b

electron microscope; the foliate texture of thecrystals is here clearly visible. In fig. 6a the pre-ferred planes (the basal planes of the crystals) lieparallel to the plane of the paper; in fig. 6b they lieperpendicular thereto. The latter micrograph givesan idea of the platelet shape of the crystals. Fig. 7shows two micrographs of a fan texture. In fig. 7athe direction to which all preferred planes are

parallel lies in the plane of the paper; in fig. 7bit is perpendicular thereto.

During sintering, anisotropic shrinkage occurs inaligned ferroxplana as it does in aligned ferroxdure.In the direction of the hexagonal axis of the crys-tals, the shrinkage is much greater than in thedirection perperidicular thereto, as can be seen infig·8.

PHILIPS TECHNICAL REVIEW

Fig. 8. Anisotropic shrinkage occurs during the sintering of analigned PI essed material. Right: an nnaligned specimen; left :an aligned specimen with foliate texture. In the latter casethere is considerable shrinkage in the direction of the coaxesof the crystals, but less at right angles to that direction. Beforesintering, the dimensions of both specimens were identical.

Properties of aligned ferroxplana

The degree of alignment

The crystal-anisotropy energy in a single crystalcan be found by determining the magnetizationcurve both in the preferred direction and in the"abhorred" direction, as in fig. 2. The anisotropyenergy is equal to the area between the two curves.

For a material with a foliate texture it is pos-sible, since the orientation of the preferred planeis known, to measure the magnctization curves in the"abhorred" direction as well as parallel to the prefer-red plane. Ifwe neglect the anisotropy in the plane,we can in this way also determine the cryst al-aniso-tropy energy for an ideally aligned material. If thematerial is not ideally aligned, the two curves willbe closer to each other and will bound an area whichis a fraction f of the area between the curves forthe single crystal. We shall call this fraction f thedegree of alignment of the anisotropic material. It isnot possible to deterrnine this for materials withfan texture since the preferred planes are notparallel.

The degree of alignment of specimens so treatedis found to reach 90% or more.

Penneability

To ascertain how the permeability depends uponthe degree of alignment, we made a number ofC02Z specimens with foliate texture. The degree ofalignment can be varied from specimen to specimenby regulating the current producing the rotatingfield. The density was approximately the same forall specimens. In Jig. 9 the permeability of thesespecimens is plotted against the correspondingdegree of alignment. The figure shows clearly the

VOLUME 19

considerable gain in permeability, particularly witha high degree of alignment. A compound CoO•SZn1•2Z,which has a permeability of 22 in the unalignedstate, is found to have a permeability of 55 afteralignment. There are indications that, with a morecomplete alignment than has hitherto been possible,an even higher permeability could be obtained.During the measurement of the permeability the

applied field is naturally taken parallel to the pre-ferred planes. If we measure at right angles to thisdirection, we can expect a lower permeability thanin the isotropic material. In fact, a permeabilityof only 2.5 was measured in this way on the C02Zspecimen with f = 0.91. A value of 1.3 is calculatedfor an ideally aligned specimen.Apart from its greater permeability, the new

material also offers advantages, owing to the aniso-tropic character of the permeability, when used ina magnetic circuit with an air gap. In that case thecomponent of the stray field perpendicular to thedirection of the flux will be small owing to the lowpermeability in this direction. A useful applicationthat comes to mind is, for example, around the airgap of a recording head in magnetic recorders,where the stray field should be concentrated in thesmallest possible space; another useful applicationwould be for obtaining a more uniform field in anarr gap.When aligned specimens are made with a fan

texture a greater permeability is found in the mostfavourable direction than in a corresponding un-

P-

t30

O_L-----~--~~----~~----~----~o 0,4 0.6 0,8 1,0_. f 93351

Fig. 9. The initial permeability ft of C02Z materials plottedagainst the degree of alignment f

aligned specimen. The increase, however, is alwayssmaller than in the case of a foliate texture. Weassume that this is due to the fact that the degreeof alignment of a fan texture will always be smallerthan that of a foliate texture. In the latter we find,

1957/58, No. 7-8 CRYSTAL-ORIENTED FERROXPLANA 215

as in ferroxdure (see Il) a marked increase in' thealignment percentage during sintering at hightemperature. This appears clearly from Table I; thedegree of sintering is varied from specimen tospecimen by changing the sintering temperature.

Table I. The influence of the sintering temperature on thedegree of alignment f and the permeability ,Lt ofCo2Zspecimens.

Sintering f ,Lttemperature

H80°C 0.65 10.51200°C 0.65 11.31220°C . 0.72 12.41240°C 0.77 18.51260°C 0.89 32.0

What probably happens is that the properlyoriented crystals grow at the expense of the wronglyoriented ones, as described in Il. It is not probablethat the textural improvement needed for obtaininga high degree of alignment will occur in a materialin which the basal planes are still in a fan-typedistribution. As described, it is not possible to checkthis directly since f cannot be measured in a materialwith fan texture.

The stress anisotropy, too, can cause a differencein permeability between the materials with the twotextures. Owing to the anisotropic coefficient ofexpansion of the crystals, more stress will ariseduring cooling in a material with fan texture thanin one with foliate texture. If the magnetostrictionis large, this is attended by a smaller permeability.

We have seen that, owing to the alignment, thepermeability increases by much more than the factorof 1.5 expected for ideal alignment, owing to thefact that the external field can he applied parallelto the preferred planes. As stated, this additionalgain is very probably attributable to the muchsmaller average internal demagnetization inaligned specimens; see fig. 4.. We shall now examinethis in more detail.

The internal demagnetization factor

Ferroxplana, like ferroxcube, is a sintered productcontaining pores. Consequently internal demagnetiz-ing fields occur which reduce the permeability, Agood impression of the demagnetization due to poros-ity can be obtained by measuring the so-calledideal curve. For this purpose an alternating mag-netic field, whose amplitude gradually decreasesfrom a high value to zero, is superimposed on aconstant ext~'ffial magnetic fieldH.The curve whichindicates the relation between the magnetization,

produced in this way and the intensity of the con-stant magnetic field is known as the ideal magneti-zation curve. If no demagnetization is present, thefirst part of the curve coincides with the magneti-zation axis, i.e. (dH/dM)H=o = O. Owing to de-magnetization, the internal magnetic field Hi isnot equal to the externally applied magnetic fieldH, and the ideal curve will make an angle a withthe magnetization axis, determined by tan a =dH/dM = Ni, where Ni is a so-called internal de-magnetization factor (see jig. 10). It appears thafbetween Ni and the porosity p of some types to

M

H 93352

Fig. 10. The ideal magnetization curve. a) No internal demag-netization; b) internal demagnetization factor Ni = tan a.

ferroxcube there exists a relation as represented bythe curve a in jig. 11. As long as Ni is small, ft willnot be limited by the' internal demagnetization butby other anisotropies; in this region, therefore,small and large permeabilities are found side byside. When Ni is large, however, the internal de-magnetization prevents the permeability from reach-ing a high value; indeed, the values found for ftdecrease monotonically with increasing Ni.If we determine in the same way the relation

between Ni and p for various unaligned ferroxplanamaterials we get an entirely different picture, ascan he seen from curve b in fig. 11. Even with verydense materials a fairly large Ni factor is foundwhich, moreover, increases rapidly with the porosity;this curve as compared with that of ferroxcube isseen to have shifted some way to the left. The cor-responding permeabilities are small. This anomalousbehaviour can be understood by referring to fig.·4;the crystals lying cross-wise influence the lines offorce in the same way as an air pore. The dernagnet-izing influence is greater still: in the surroundingcrystals the lines of force must continue to run paral-lel to the preferred plane, i.~. in the plane of the

216 PHI!-IPS TECHNICAL REVIEW VOLUME 19

drawing. Thus, they can only bend round to theleft and right and not forwards and backwards.It is understandable, then, that the internal dernag-netization in a fairly dense, unaligned ferroxplanamaterial is just as great as in a ferroxcube materialhaving a porosity of about 30%.

-p 93353

Fig. 11. The internal demagnetization factor Ni as a function ofporosity p: a) for ferroxcube; b) for unaligned ferroxplana;c) for aligned Co2Zwith various degrees of alignment f.

Wethen measure Ni and p on aligned ferroxplanaspecimens, for which the relation between f1 and fis given in fig. 9. The results of this measurementare shown in fig. 11, together with the appropriatedegrees of alignment. We see that Ni decreasessharply with increasing degree of alignment f, justas expected: the ideal aligned state is approachedat which, as.in the case of ferroxcube materials, Niis determined solely by the porosity. With well-aligned materials, the greatest gain in permeabilityis obtained by avoiding the demagnetizing influenceof wrongly oriented crystals.

Frequency-dependence of the permeability

For application at very high frequencies a higherpermeability is only of value if it does not entail asevere drop in the limiting frequency. In the case ofspecimens with a composition C02Z or CoO•SZn1•2Z,therefore, the complex permeability f1 = f1' - j,u"(,u" is a measure of the losses: tan c5 = p" / ,u')was measured as a function of the frequency. The

results are set out in figs 12 and 13; the curves forunaligned materials are shown for comparison. Itis noticeable in both figures that the frequency

1

t<,

~00,

I~lP \

lP ./ ,'t-o./

,20 pH V ,

~ r- ;) ,1\ itn,a" -....- -.. -jo--

, r'\. T=è-

5 "'11" ,, '\__ - i\

2__

'\

1 ~

5-

2

0,5

D,2

10 2f)0,/

fUJOMcfs50 100 2IJO 500 1000 sx»_f

Fig.12. The quantities /L' and p" as a function of frequency f;full curve for aligned CozZ,broken curve for unaligned Co2Z.

at the peak of the p" curve is only slightly reducedby the alignment, in any case by a smaller factorthan that by which the permeability is increased.The same applies to the limiting frequency fn if thatis defined, for example, as the frequency at whichtan c5 = 0.1, i.e. the frequency of the point of inter-section of the appertaining ,u' and ,u" curves.This can be explained as follows. Compared with

an uilaligned specimen an ideally aligned specimen,if the anisotropy field H! for rotations in the pre-

20

.I/

I ;I~i""

,H .L

~I-" , I '\__

1-= e-: .. '_- I'....k f'l

,

.,.

50pHt

2f)

100 10

p't 50 5

2

10

5 os

2 0,2

1la

0,12f) 50 100 2f)() 500 1CXXJ 2{)()() Mcjs

_f Q3Qb2

Fig. 13. The quantities /L' and u" as a function of frequency ffor COO•SZnl•2Z; full curve for aligned material, broken curvefor unaligned material.

1957/58, No. 7-8 CRYSTAL-ORIENTED FERROXPLANA

,ferred plane is unaltered, will show a susceptibility1.5 times as large for a field applied parallel to thepreferred plane as. for a field perpendicular to thisplane. As we have seen above, however, this aniso-tropy field H~ becomes smaller by reducing theinternal demagnetization; this additionally enhancesthe susceptihility. The large anisotropy field H~for rotations out of the plane is not changed by thealignment, leaving out of account relatively smalleffects that might be caused by changes in the shapeanisotropy. If we give all quantities of the unalignedmaterial the suffix 1 and those of the aligned materialthe suffix 2, we can write:

From I we know that the limiting frequency fr is'proportional to iH~ H!. Since (H~h = (H~)2' itfollows that:

From the permeabilities #1 and f.tz we calculate forC02Z the ratio fr2/fn = 0.77; for CoO•SZn1•2Z wefind fr2/fr1 = 0.80. It can be seen from figs 12 and13 that this is in good agreement. with the experi-mental data.

Thus, by the alignment of the crystals of ferrox-plana we can now obtain materials with a permeabi-lity of 30 to 50 and showing low losses up to fre-quencies of 200 and 100 Mcfs respectively.

Summary. Since the magnetization of ferroxplana materialsis strongly bound to the preferred plane, the particles of apowdered specimen can be aligned in an external magneticfield. In a uniform field all preferred planes are parallel to thedirection of the field (fan texture); in a rotating field all pre-ferred planes are more or less mutually parallel (foliate tex-ture). As a result the permeability becomes anisotropic andis large in certain directions; this gain in permeability is largelydue to the avoidance, by alignment, of the strong demagnetiz-ing effect' of wrongly oriented crystals. In this way the per-meability of some aligned specimens is increased by a factorof 2.5 to 3. For use at high frequencies it is important that thelimiting frequency should not be greatly reduced; in alignedspecimens this is about 0.8 times that in unaligned specimens.This effect can be explained quantitatively. The anisotropicnature of the permeability can also be put to good use incertain cases.

217