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Recent progress in the field of permanent magnetsK. Hoselitz

To cite this version:K. Hoselitz. Recent progress in the field of permanent magnets. J. Phys. Radium, 1951, 12 (3),pp.448-458. <10.1051/jphysrad:01951001203044800>. <jpa-00234404>

RECENT PROGRESS IN THE FIELD OF PERMANENT MAGNETS

By K. HOSELITZ.

Sommaire. - Après un bref rappel des conditions techniques exigées pour la construction desaimants permanents, le rapport classe les alliages magnétiques en quatre types principaux : les aciersà aimants, les alliages durcissant par précipitation, les alliages durcissant par diffusion et les aimants enpoudres comprimées. Les aciers ne sont pas étudiés et quelques alliages à durcissement par précipitationseulement sont mentionnés : le « vicalloy » et les alliages fer-nickel-cuivre. On étudie, de façon plusdétaillée, les alliages durcissant par diffusion et, après un bref historique de leur développement,on décrit les plus récents et les plus utiles d’entre eux, comme l’alcomax et l’alnico V. On tente d’expli-quer le champ coercitif élevé et l’anisotropie de ces matériaux. Leur champ coercitif élevé est attribuéà l’hétérogénéité magnétique des alliages et les résultats expérimentaux sont en accord quantitatifraisonnable avec la théorie. L’origine de l’anisotropie réside probablement dans la forme des aggrégatshétérogènes, due à l’action du champ magnétique pendant le refroidissement. On indique les derniersprogrès réalisés avec la construction des éléments à cristaux orientés parallèlement, qui ont des

propriétés magnétiques supérieures à celles des aimants à cristaux orientés au hasard.

LE JOURNAL DE PHYSIQUE ET LE RADIUM. TOME 12, MARS 1951~ PAGE 448.

Technical Requirements. - In the majorityof applications of permanent magnet materialsthe salient requirement is that a high magneticfield should be maintained in an air gap of the

magnet system. The system must often be stableagainst external influences of changing temperature,mechanical vibration or shock or electromagneticfields and this should be achieved preferably witha minimum of magnet alloy. For this purposeit is necessary that the magnet system is correctlydesigned. The basic principles governing the designof permanent magnets working under static condi-

. tions, i.e. in systems where there are no considerable

.

external demagnetising fields, have been adequa-tely described by Evershed [1920]. From theseconsiderations it is clear that the most importantcharacteristic of a permanent magnet materialis its product and the flux density and

demagnetising field strenght corresponding to thepoint. The significance of the

point has been described extensively and variouslyin the existing literature and I do not propose toenter any further into this question.

If the magnet is applied in an instrument or

apparatus which involves either a varying externalreluctance or a varying external magnetising force,the question of the economic utilisation of thematerial suffers some slight modification and theminor hysteresis loops have to be considered, as

has already been pointed out by Evershed. For thecase of varying external reluctance only, the workingpoint of the permanent magnet should preferablybe chosen on that minor hysteresis loop on whichthe useful recoil energy is a maximum. A numberof workers have attempted to find algebraicalsolutions to the problem of determining the optimummagnet dimensions for a given application from thedemagnetisation curve of the permanent magnetmaterial. Such solutions usually rely on the

assumption that the major hysteresis loop in thesecond quadrant obeys an algebraical equation,usually of the second order and whilst such designprinciples are possibly of considerable practicalimportance they are our not thought to be of funda-mental scientific interest. Useful ,contributions tothe subject have been made by Hornfeck and

Edgar [I940], Desmond [1945, 1949] and San-ford

Even greater complications arise in the case ofexternal magnetising forces, arising for instance,from armature reaction in a generator or magneto.

These also have, to some extent, been dealtwith by the above authors. In the present accountdesign considerations are not included becausea superficial description would be inadequate and adetailed treatment would make this report into-

lerably long.&#x3E; In principle it is considered that the most impor-tant characteristics are those of the major hyste-resis loop, probably with the addition of the rever-sible permeability or recoil permeability. Consi-derable ambiguity is found in the values quotedfor recoil permeability of permanent magnet mate-rials by various authors and hence no figures aregiven in the present report.

In an efficient permanent magnet material in

general one desires a high coercive force, a highremanence and a high energy product. Further-more the manufacturing process will govern theexternal shape of the magnet and in some cases

the mechanical properties will influence the appli-cation.

There exists four main types of permanentmagnet materials :

Firstly magnet steels, which are essentially carbonsteels with additional elements. These are by nowstandardised and cannot be claimed in any way

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01951001203044800

449

to be classed as recent developments. They are,

therefore, not dealt with in this account, exceptfor the tabulation of their main properties inTable I.

°

TABLE I.

Composition, magnetic, physical, mechanical properties anelheat treatlnent of magnet steels.Nominal composition percent

(1) Unless otherwise specified; steels are available in Great Britain.(2) Equivalent to British steel, available under the name stiown in country inrlicated by letter G. : Germany : U. S. A. : United

States of America.

TABLE II.

Jfagnetic properties of precipitation Hardening klagnetic aLloJ"s.

Secondly there are various types of precipitationhardening permanent magnet alloys which can beworked by hot or cold working methods. Herewe have the conventional alloys such as iron-cobalt-molybdenum, iron-nickel-copper, and iron-cobalt-vanadium. Many of the precipitation hardeningalloys can be cold worked and after severe coldreduction a pronounced anisotropy in the rollingor wire drawing direction is observed. The manu-facture of these materials is usually expensive andthey are used only to a small extent. Their charac-teristics are quoted in Table II.

Thirdly there are the diffusion hardening magnetalloys (Table III) based on the iron-nickel-alu-minium system with additions of one or more ofthe elements cobalt, copper, titanium and niobium.By far the greatest number of modern permanentmagnets are made from one of the many good alloysof this group. Some of these alloys exhibit a pro-nounced magnetic anisotropy when heat treated in amagnetic field and these anisotropic alloys are the mostefficient modern permanent magnet alloys. Theiron-nickel-aluminium alloys are all brittle and hardand at present there is no way of making them workable.

450

MARLE III.

Magnetic properties of diilusion hardening Magnet alloys.

TABLE IV.

Magnetic properties of permanent magnet materials made from

Consequently these alloys have to be cast approxi-mately to shape and final accurate dimensions haveto be achieved by grinding.

Finally there is a large number of various per-manent magnet materials made by processes of

powder metallurgy, but it does not appear at presentas though their practical importance would justifya very extensive discussion. Some of these materialsare included in Table IV.The most notable discoveries along newer lines,

are based on the magnetic properties of ferro-

magnetic materials in a fine state of sub-division

(N6el, Weil and Aubry; 1942). Magnets composed

of powder particles each of which are smaller thana given critical size exhibit large coercive forcesand this fact is used in manufacturing permanentmagnets out of chemically produced colloidal powdersof pure iron or iron-cobalt.

Precipitation Hardening Permanent MagnetAlloys. ---- It was suggested by Seljesater and

Rogers [1931] and K6ster [I932 a, b] that preci-pitation of a second phase from a supersaturatedsolution could be utilised in producing materials of

high magnetic hardness for permanent magnets.The alloys investigated where those which form

451

extended solid solutions with iron at high temperatureswith decreasing solubility at low temperatures, i.e.

iron-molybdenum, iron-tungsten, iron-beryllium. andiron-titanium. In order to obtain a supersaturatedsolid solution these alloys have to be quenchedfrom high temperatures and subsequently preci-pitation hardening occurs during tempering at

temperatures between 4oo and 7ooo C. The coerciveforces obtained vary from 5o to 220 Oe, dependingon composition and heat treatment. None of the

binary alloys has better properties than 35 percentcobalt steel.

However, ternary alloys produced by the additionof either chromium or cobalt to some of the binaryiron alloys, but especially the alloys of iron, cobaltand molybdenum, show properties (Köster, 1932 a, b;Scheil, Bischof and Schulz, 1934) which were equalor even better than those of the best cobalt steel,at the same time promising cheaper manufacturingcosts. The alloy, which has found extensive appli-cation, contains 17 percent molybdenum and12 percent cobalt and is known under the name ofComol, Comalloy or Remalloy. This alloy is

quenched from I200~ C and precipitation hardenedat about 7ooo C. The magnetic properties (Table II)are about equal to those of 35 percent cobalt steel.In the soft (quenched) condition the alloy can bemachined and if impurities in the ingots are avoidedhot-rolling, whilst difficult, is common commercial

practice, especially in the United States.

Alloys of copper-nickel-iron and copper-nickel-cobalt seem to have been discovered by Dahl,Pfaffenberger and Schwartz [i9 5], Neumann [1935]and Dannoehl and Neumann [1938]. In both

ternary systems there exists a large area in whichprecipitation from supersaturated solutions occurs.An alloy with 18 percent iron, 20 percent nickeland 62 percent copper, quenched from 1000° C andtempered at 6ooo C had a remanence of 302o Gauss,a coercive force of 445 Oe and a x productof o.5 X 06. Even in the magnetically hardstate this alloy can be machined and worked.Cold working leads to the development of a pro-nounced magnetic anisotropy. After cold workingthe magnetic properties in the rolling or drawingdirection can reach considerable values, especiallythe BHI1l x product which is quoted as reachingvalues up to 1.85 X 106. The process of manu-facture, according to one source [Gould, Hoselitzand Edwards, 19 6] appears to be roughly as

follows : An alloy of 60 percent copper, 20 percentnickel and 20 percent iron is melted in a vacuum

high frequency furnace and cast into small ingotsof 7o mm diameter. These ingots are rolledto 35 mm octagon billets which are heated to 1000° Cand rolled to strips of 6 mm thickness. The stripsare now coiled and heated in hydrogen at 1040° Cfor 8 hours, then water quenched and tempered

for 3 hours at 65oo C. The material is now magne-tically hard and is further cold rolled to the finaldimensions where the reduction in area shouldbe about 80 to go per cent. The magnetic propertiesare somewhat spoilt during cold rolling, but furthertempering at 6ooO C restores the final magneticproperties so that a remanence of 600 Gauss,a of 1.3 X and a coercivity of

400 Oe is achieved.Scientific investigation of copper-nickel-iron alloys

has been carried out variously, but most recentlyby Sucksmith [1945] in conjunction with X-raywork by Daniel and Lipson [194 ~, 1944] the

alloy Cu4FeNiJ was chosen and quenched fromi i ooo C showing a single phase face-centred-cubicstructure. During heat treatment between 5ooand 6ooo C precipitation of a second face-centred-cubic phase takes place. Daniel and Lipson foundseveral stages of the process. At first concentrationdifferences are set up in the alloy at regular intervals,the wave length of which increases with time. The" amplitude " however, is a function of the tempe-rature of heat treatment and does not vary withtime. This diagnosis is reasonable and suggestsheterogeneous precipitation. The coherence of thelattice remains and the lattice becomes tetragonalbefore actually separating into its two equilibriumphases. Magnetic measurements show that thecoercive force goes through a maximum and the-reafter decreases ( fcg. i). The maximum of the

Fig. I. - Coercive force of Cu4FeNi3,quenched from 1100° C during tempering at.

1, 750° C; 2, 650° C; 3, 550° C.

coercive force is largely independent of temperingtemperature and appears to coincide with a perio-dicity of the concentration differences of over

I5o lattice parameters, i.e. 5 X IO-6 cm. The

magnetic hardness of the iron-nickel-copper alloyat the critical dispersion of the heterogeneousaggregates suggests that the periodicity of compo-sition variation extends in three dimensions on threesets of intersecting cube faces and that domainboundaries are more or less permanently locatedon these planes. The coercive force appears to

depend primarily on the size of the heterogeneousaggregates and the material probably consists

essentially of an assembly of single magneticdomains. Copper-nickel-cobalt alloys [Dannoehl andNeumann, 193 J] show magnetic properties similar

452

to those of copper-nickel-iron with rather lower

energy and higher coercive forces. They do not

appear to be so easily workable, but their applicationas sintered magnets has been described by Stei-nitz [1946].

Vicalloy, discovered by Kelsall and Nesbitt [1 ~40]is an alloy with 52 percent cobalt, I o to 15 percentvanadium, and 33 to 38 percent iron; its propertieswere described by Nesbitt [1946]. The alloy canbe machined and hot and cold worked and an alloycontaining 34 percent iron, 52 percent cobalt and14 percent vanadium (Vicalloy II) exhibits pro-nounced magnetic anisotropy after cold reduction.Figure 2 shows the effect of wire drawing on thedemagnetisation curve and the BH",", productof Vicalloy II according to Nesbitt.

Diffusion hardening Permanent MagnetAlloys. - Following on the original discovery byMishima [1931] of an alloy with 3o percent nickel,12 percent aluminium and 58 percent iron, whichwas said to have a remanence of g5oo Gauss and acoercive force of 43o Oe, it was attempted to producecommercial alloys of comparable but consistentlyreproducible properties. The alloys which were

evolved were found preferably to contain 24 to

32 percent nickel and about 12 percent aluminium,giving a remanence of 5 ooo to 6 ooo Gauss, a

coercive force of 5oo to 70o Oe with an energyof 1.25 X The addition of copper and cobaltto alloys of this type, discovered in Britain [Hors-burgh and Tetley, 1934], the U.S.A. [Ruder, 1933] ]and Germany [Krupp, 1933] about simultaneouslyin I g34, resulted in the Alnico series with about I 5to 20 percent nickel, 10 to 12 percent aluminium,5 to 4 percent cobalt and up to 6 percent copper.The Alnicos have a remanence of 6 ooo to 8 ooo Gauss,a coercive force of 45o to 65o Oe and energiesof r .5 to r . ~ X 1 06 G. O . In order to producesuch alloys satisfactorily, it was found that, contraryto the claims made originally by Mishima, theyhad to be heat treated, usually in two stages,the first a controlled cooling from a high tempe-rature such as 1250° C followed by a temperingat about 6ooO C for times of several hours.

Development work during the four years whichfollowed the invention of the Alnico series, resultedin no case in a substantial improvement or alterationin the general specification or properties. Thebeneficial effect of small additions of niobiumon the coercive force was discovered by Horsburghand Tetley in Simultaneous with the deve-

lopment of Alnico, Honda, Masumoto and Shira-kawa [1934] found that an alloy with 10 to 2o percentnickel, I o to 3o percent cobalt and 1 o to I 1 percenttitanium, with little or no aluminium, gave extre-mely high coercive forces and increased BH", ,x pro-ducts. This alloy forms the basis of a number

of high coercivity alloys available today all of whichare characterised by their substantial titaniumcontent and their properties are in general as fol-lows : Remanence 5 ooo to 6 ooo Gauss, coerciveforce 75o 1000 Oe, BHmax 1.6 to 2 X 106 G. 0.

It is known that the iron-nickel-aluminium

type magnet alloys are hard and brittle and cannotbe forged or rolled. The only machining operationapplicable is that of grinding and consequentlymagnets have to be cast approximately to shape.The addition of substantial percentages of titaniumincreases the brittleness and mechanical weaknessand even during grinding operations difficultiesof " spalling

"

are encountered. Consequently,although the high coercive force titanium alloysdo fulfill some technical requirements, their moreextensive application is limited by their poor mecha-nical properties.The significant experiment leading to substantial

improvement in magnetic properties was carriedout in 1938 by Oliver and Shedden, who discoveredthat the properties of Alnico could be improvedin one direction if the controlled cooling was carriedout in a magnetic field. The improvement in

energy was of the order of 20 percent.The result was interpreted by Oliver and Shedden

on lines similar to that given by Bozorth and Dil-linger [1935] to the effect of cooling in a magneticfield of iron-nickel-cobalt alloys. It was thoughtthat on cooling the alloy below the Curie pointmagnetostrictive stresses can be relieved by plasticflow, but on further cooling deformations are frozenin and the direction of domain magnetisation thusbecomes a preferred direction. In the absenceof a field applied during cooling the preferreddirections would be at random but the presenceof a field applied during cooling would align themthroughout the magnet. The improvement in BHn,,"was only small, but Oliver and Shedden stated thatit might be of some practical value. Researchworkers in Holland, working with alloys of a highercobalt content [Van Urk, 1940] demonstratedthat if the composition was chosen correctly theanisotropy could be very pronounced and the BH,naximprovement of the order of 150 percent comparedwith an isotropic alloy of the same composition.Other workers in other countries discovered similar

alloys and the resulting commercial grades of

permanent magnets are known under names suchas Alcomax, Ticonal or Alnico V. In general themagnetic properties of such alloys are as follows :Remanence 10 ooo to 13 ooo Gauss, coercive force 5ooto 85o Oe and BHn,;" 3 to 5.5. X 1 os G. 0.

The practical importance of the developmentof permanent magnet materials of such high energiesis mainly that it has enabled many energised magnetsto be replaced by permanent magnets with a conse-quent saving in cost, weight and stability, besides

453

opening up possibilities of employing permanentmagnets in completely new applications.The latest improvement of anisotropic permanent

magnet alloys has only been achieved in the lasttwo or three years. It was found that if magnetalloys of the anisotropic type could be producedwith unidirectional crystals, i.e. in which all the

crystals have one cube edge parallel to a givendirection, and if the direction of the magneticfield during cooling coincided with that direction,a further improvement in magnetic properties waspossible. From laboratory experiments so far avai-lable, it is known that the properties depend to someextent on the degree of alignment of the crystalsin,the magnet. In general however, if the alignmentis good unidirectional crystal magnets show a

remanence of about 13 5oo Gauss, a coercive forceof 600 to 800 Oe and an energy of 7 to 8 X 106 G. 0.

[Permanent Magnet Association, z g5o].The remarkable properties of some of the iron-

nickel-aluminium-cobalt alloys, especially the ani-

sotropic alloys, have been the subject of numerousinvestigations. From the point of view of manu-

facturing efflciency, composition and heat treatmenthave been investigated and whilst the results ofdifferent workers show minor differences the generalconclusions can be taken from a paper by Zum-busch [1942]. Zumbusch found that for productionof anisotropic properties the alloys must containat least 8 to 10 percent cobalt and should not containmore than 24 percent nickel. The maximum

permissible nickel content is also a function of thealuminium and copper additions and the more

aluminium and copper is added the lower mustbe the nickel concentration. Zumbusch foundthat the cobalt to nickel ratio was most importantin its influence in producing anisotropy, and he

concluded, that this was due to its effect on theCurie point of the alloys. The cobalt to nickelratio should accordingly be as high as possible,preferably in the region of 1.7. He recommanded

14 to 15.5. percent as the optimum nickel contentwith cobalt between 21.5 and 23.5 percent. Thealuminium content should be between 7.8and g. ~ percent and the amount of copper between 3and lE percent. Titanium is stated to be harmful,expecially if added in amounts of more than I percentand about 2 percent is claimed to destroy aniso-tropic properties. The conclusions by Zumbuschremain substantially correct, but it has been found[Hadfield, 1948] that small additions of niobiumincrease the coercive force whitout any change inenergy product.Heat treatment conditions have also been inves-

tigated by Zumbusch, who recommended that thealloy should be cooled from a temperature of 1250° Oat the average rate of approximately 10 C/s dow nto 5ooo C in a magnetic field of not less than

but preferably 4ooo Oe, followed by temperingfor about 1 o hours at 55oo C. Experience hasshown that the tempering time can be extendedwith advantage.

Fig. 2. - Effect of wire drawing (cold reduction)on demagnetisation curve of vicalloy

(34 percent Fe, 52 percent Co, 1 4 percent V).percentage reduction in area shown thus : 98.Wire diameter in mm shown thus : (o. r 5).

The effect of the strength of the magnetic field

during the controlled cooling on the value ofan anisotropic alloy is shown in figure 3, where itcan be seen that the maximum value quoted byZumbusch is about 25 percent too high.

Fig. 3. - Magnetic properties of anisotropic permanentmagnet alloy as a function of field applied during coolingfrom 12500 C.

Having established the most advantageous compo-sition and heat treatment for anisotropic permanentmagnet alloys it is of interest to investigate thereasons for the remarkable magnetic properties of suchalloys. The earliest investigation by means of X-raymethods appears to have been carried out byGlocker, Pfister and Wiest in Ig35 who foundthat at high temperatures iron-nickel-aluminium

alloys are solid solutions and at a lower tempe-rature a second phase is precipitated. It was

inviting, therefore, to attribute the high coerciveforce to a process of ordinary precipitation, but itwas shown already by Glocker, Pfister and Wiestthat during annealing the maximum of the coerciveforce is reached long before the diffraction patternshowed any signs of precipitation of a second phase.

454

Figure 4 shows the lattice parameter, line breadthand coercive force plotted against time of temperingat 6goo C of an alloy of 10 percent aluminium,22&#x26;5 percent nickel, 13 percent cobalt, rest iron.

- Coercive force, line breadth and lattice parameterof Fe-Ni-Al alloy (10 percent Al, 22.5 percent Ni, z 3 per-cent Co) quenched from 1 3 00° C during tempering.

Similar results were obtained by Burgers and Snoek. [1935]. Bradley and Taylor, during an exten-

sive X-ray study in 1937, found that in an

alloy Fe2NiAl separation of the high temperaturesolid solution into two body-centered-cubic phasestook place during very slow cooling. The lattice

parameter of the two phases differs only by I percent,but the magnetic properties are distinctly different,one being almost pure iron and the other approxi-mately FeNiAl which has a saturation intensity

only 3 of that of iron [Sucksmith, ig3g]. Duringthe controlled cooling required to obtain high coerciveforces in practice, Bradley and Taylor concludedthat the coherence of the high temperature latticeis maintained but small iron-rich islands are formedwhich have to conform to the original lattice dimen-sions and are consequently introducing largestresses.

Snoek [1938] also investigated the process of

magnetic hardening of iron-nickel-aluminium alloys,and measured the amount of non-magnetic or

weakly magnetic material interspersed betweenthe more strongly magnetic matrix. In an alloywith 26.5 percent nickel and I 2.3 percent aluminiumit was found that during cooling from a high tempe-rature the main increase in coercive force couldbe atributed to the processes occurring between gooand 8ooo C. During the cooling throught this

temperature interval, which took only 3 s, thecoercive force rose from 5o to 400 Oe. The magneticproperties of this alloy at various stages of heattreatment, preserved by rapid quenching, are

shown in figure 5. Varying the conditions of heattreatment it could be shown that the same maximumof coercive force of about 400 Oe can be obtained

in this alloy by different heat treatments resultingin structures with varying amounts of non-magneticor weakly magnetic component. Snoek was not

able to give a full interpretation of all the observedeffects, but he concluded that a finely dispersedprecipitate alone could not account for the highcoercive force, as is the case in precipitation harde-ning alloys. According to Bradley and Taylorand also to Snoek, magnetic hardening ~occurs inthese alloys by local variations in compositioncaused by atomic diffusion without the loss of acoherent lattice. Hence the name diffusion har-

dening alloys is given to this group of materials.

Fig. 5. - Magnetic properties of Fe-Ni-Al alloy quenchedat different stages during controlled cooling (~6.5 percent Ni,12.3 percent Al).

’

Thus until 1942 the explanation of the highcoercive force of iron-nickel-aluminium cobalt magnetalloys was incomplete. Significant contributionsto the subject were made in by Dannoehl[1943 a, b] who suggested that multiple precipi-tation was responsible for the hardening of iron-nickel-aluminium alloys. Dannoehl suggested an

extremely complicated process of precipitationbasing his study on experiments by himself [Dannoehl1942 a], Bradley and Taylor [ 1 g3 ~], Snoek [iQ83]and many others. However, since Dannoehl confinedhimself to the comparatively simple case of iron-nickel-aluminium alloys his conclusions are perhapsnot of the fullest general importance in explainingthe processes occurring in permanent magnetalloys, especially if it is remembered that all aniso-tropic alloys and the more important isotropicones contain substantial amounts of cobalt. Infact, it was shown by Jellinghaus [194~] that

processes in anisotropic iron-nickel-aluminium -

cobalt-copper alloys were slightly different from

455

the very complicated multiple precipitation proposedby Dannoehl. Jellinghaus investigated alloys with15 percent nickel, 2 3 percent cobalt, 3 percent copperand 5,7 to 16.6 percent aluminium, by means of

microscopic and magnetic measurements. He foundthat in alloys with up to about 7 percent aluminiuma face-centered-cubic phase (y) was precipitatedbut that the aluminium-richer alloys showed onlythe body-centred cubic solid solution {~). However,magnetic measurements suggest that alloys whichappear homogeneous under the microscope are,in fact, heterogeneous and Jellinghaus suggested thatthe two body-centered-phases called a and a’ were

present. Highest coercive force was found in alloyswhich were near to the ; phase boundary between; and 8 percent aluminium.

Oliver and Goldschmidt J made a thoroughX-ray study of some anisotropic permanent magnetalloys confirming that in the hardened state beforetempering a single body-centered cubic phase is

present. The X-ray reflections, however, showa side band structure, which can be attributedto periodic fluctuations in composition. Further-more X-ray back reflection photographs takenin the preferred magnetic direction and perpen-dicular, revealed a preferred direction in the progressof incipient phase transformations. During tem-pering a face-centred phase is precipitated in a

state of fine subdivision. Magnetic measurementsby Jellinghaus and Hoselitz and McCaig[1949 a] gave further information about the magne-tisation process and the origin of the magnetichardness of iron-nickel-cobalt-aluminium typemagnet alloys. In an alloy which has been hardenedbut without the application of a magnetic fieldthere exist fluctuations in composition which resultin regions of lower and high intrinsic magnetisationin each crystal. It is likely that the periodicityof the fluctuations of composition is of the orderof io-I to io-6cm. The alloys do not possessone sharp Curie point, but there is a more gradualdecrease in magnetisation with rising temperaturesuggesting continuously varying Curie points of theindividual aggregates.According to Neel [1946], heterogeneity of the

type found in these diffusion hardening alloysproduces high values of the coercive force. The

root mean square Is of the spontaneous magneti-sation is a measure of the magnetic hardness of sucha material. A very approximate Value of Js canhe obtained from measurements of the approchto saturation [Néel The magnetocrystallineenergy can be estimated from the reversible

energy of magnetisation if the material is free fromlarge stresses. Magnetostriction measurements byHoselitz and McCai 9 [1949 b] and McCaig [1949 1showed conclusively that the direction of domain

magnetisation in permanent magnet alloys coincides

closely with a cube axis and hence even if stressesand domain shape make a considerable contributionto the anisotropy energy the symmetry of the

energy distribution must remain essentially thatof the cubic lattice.. Measurements of the reversible

energy of magnetisation will, therefore, give thetotal anisotropy energy which may be due to oneor several elementary causes which from the pointof view of the magnetisation need not be separated.If the root mean square of the intrinsic magnetisationand the quasi crystalline energy K’ are introducedin the formula given by Neel [1946] the calculatedvalues for the coercive force correspond satisfac-

torily with the measured values [Hoselitz, 1950]and it is hence reasonable to conclude that the highcoercive force is due to magnetic heterogeneityof the alloys causing dispersed fields as suggestedby Neel.

Fig. 6. - Magnetisation curves of an anisotropic permanentmagnet alloy (alnico V).

Full lines : parallel; dotted lines : perpendicular to the

prefered direction.

The magnetic anisotropy of the alloys whichhave been cooled in a magnetic field is explainedby the fact that all domains are magnetised alongthat cube axis which includes the smallest anglewith the applied field and that mainly rotations ofdomain vectors are responsible for changes in

magnetisation. This is clearly shown by experimentsby Jellinghaus [ ~ g4s] who measured the minorhysteresis loops of anisotropic alloys parallel andperpendicular to the preferred direction as wellas by the measurements of initial susceptibilityby Hoselitz and McCaig [1949]. Some of thecharacteristic values measured for Alcomax II aregiven in Table V, and complete magnetisationcurves for an anisotropic alloy according to measu-rements by Jellinghaus are shown in figure 6.

Since the action of the field during cooling is to

456

TABLE V.

Some magnetic properties of alcomax II permanent magnet alloy.

select the cube axis including the smallest angle withthe field as preferred direction of domain magneti-sation. it remains to find the mechanism by whichthis additional uniaxal anisotropy energy termcould arise. It is possible that under the influenceof the magnetic field, the heterogeneous aggregatesacquire an elongated shape or that a system ofunidirectional magnetostrictive stresses is set upin the material or a combination of both theseeffects. Stresses alone, which have hitherto beenthought to be the only possible cause for the magnetichardness and anisotropy of these alloys, cannotaccount for the observed effects, as has been shownby experiments applying a unidirectional pressureto permanent magnet rods which only producedan insignificant change in the magnetisation curve[Hoselitz and McCaig, McCaig, On the other hand it is not immediately obvioushow the presence of a magnetic field of only a fewthousand Oersted could influence the shape of

heterogeneous aggregates, because the energy requi-red for diffusion is greater by several orders of

magnitude than that supplied by the magneticfield. Kittel, Nesbitt and Shockley [1950] assumethat thermal nucleation of a magnetically differentphase occurs at a high temperature (800 to 9000 C)in the shape of platelets. The surface energy ofthese nuclei is not very great and the presence of amagnetic field during cooling suppresses plates in aplane perpendicular to the field direction, owing totheir larger energy of demagnetisation. Further

precipitation along the planes of these nuclei takesplace at lower temperatures, and the final struc-ture is suggested to consist of plates and rods,of single domain dimensions. McCaig sug-gests independently that the interaction of magne-tostrictive stresses and magnetisation energy couldproduce agregates of anisotropic shape by selecting

certain 11 Magnetically favourable "

slip directions.A full discussion of the proposed mechanism is

given by McCaig.

Demagnetisation curves of anisotropic magnet alloy.~~, unidirectional crystals; B, random crystals.

Since the changes in magnetisation are causednot by boundary movements but by rotations ofdomain vectors which coincide very closely witha cube axis, a material in which all crystals haveone cube edge in common must represent an optimumassembly of heterogeneous aggregates and mustexhibit exceptional magnetic properties in the direc-tion of the cube axis. This is in fact borne out

by the most recent experiments in producing per-manent magnets with unidirectional crystals whichshow an almost completely square hysteresis curve

7).

Conclusions In some scientific investigations

457

magnetic alloys with considerably higher coerciveforces have been encountered, especially some

alloys of platinum and some alloys of manganese.Because of their cost or difficulty of preparationsuch alloys have not yet found any practical appli-cation, although they are of considerable scientificinterest. The coercive forces of the permanentmagnet alloys usually employed in practice are

of the order of 5oo to 100o Oe and the heterogeneityproduced by processes of precipitation or diffusioncan account in some way for the observed valuesin coercive force. However, the detailed mechanismsproducing the magnetic properties of modern com-mercial permanent magnet alloys are not yet fullyexplained and further work is required. Investi-

gations carried out so far given reasonable justifi-cation to the hope that the full explanations willhe fortheoming within the next few years.

Remarque de M. Went. - In the FeNiCoAl systemwith compositions as used for the Alcomax V,Ticonal V, Alnico V permanent magnets two phaseswith the approximative compositions NiAl and FeCocan be found. The non-ferromagnetic NiAl phaseand the ferromagnetic FeCo phase are both cubicbody-centered with pratically the same latticedimensions. This explains the fact that it is even

possible to grow a single crystal composed of thetwo different phases. From X-raym easurementsthe Ni -. Al, phase which is always ordered, can bedetermined separately by its superstructure lines.

Fir the explanation of the most importantproperties of Ticonal V two experimental factsfound by us are of interest :

1. X-ray measurements show that the two

phases are always present in roughly the expectedproportions, also after quenching from high tempe-ratures above the Curie temperature, and even afterquenching the melt in salt water.

2. In a single crystal the NiAl phase is situatedpreferentially along (100) planes. This is deducedfrom surface cracks along [100] plane directions.

In a quenched alloy, therefore, both phases arepresent but the coercive force is only about o,5-i Oe.During cooling through the temperature regionof the Curie point in a magnetic field a recrystalli-sation takes place in which the decrease of the

demagnetisation energy for long needles or platesof the FeCo phase compared with a more sphere-like shape, is the acting force. This energy gainmust compete with the increase of the, doubtlesssmall, surface energy, caused by the increasingsurface between the two phases. Large coerciveforces are found as a result of a new geometricdistribution with very fine particles of the ferro-

magnetic phase which are single Weiss domainswith a large shape anisotropy.

The large increase in values for largesingle crystal especially for crystals cooled in a

magnetic field in a [100] direction can be explainedby the fact that only then two of the three cubicplanes can be occupied by the non magnetic NiAl,phase, while in the third direction, the direction ofthe magnetic field during the heat treatment,the ferromagnetic phase can grow. Single crystalscooled in a magnetic field in a [111] direction donot show this anisotropy behaviour, because thenon magnetic phase is situated along three (100)planes. Small deviations of the magnetic fieldfrom the [111] direction immediately show pro-nounced anisotropy effects.

’

En outre sur des interventions de Hoselitzet 5hockey, M. Went répond. -- The coercive forcemeasured in the preferred direction is about twicethat in a direction perpendicular to this direction.The high values of Ire"’ of about o,95 measured

Isat

on polycristalline material is in fact larger thanshould be expected from the picture given above.

Remarque de M. Hoselitz. - I note with greatpleasure that the experiments by Mr Went appearto confirm the conclusions reached by Hoselitzand McCaig about the domain magnetisationin anisotropic permanent magnet alloys.

Remarque de Néel. -- Je constate avec intérêt

que la th6orie des alnicos trait6s dans un champmagn6tique s’oriente dans la voie que j’avaisindiqu6e en 1947 [N6el, 1947]? a une époque oumalheureusement on ne possédait encore aucune

precision sur le mode de s6gr6gation des deux

phases. Quels sont d’ailleurs les renseignementsexacts que l’on possède maintenant a ce sujet ?

Réponse de Shockley. - The situation regardingdirect observation of plate like precipitates inAlnico V is not clear. At Bell Telephone Labora-tories, R. D. Heidenreich has made a number ofelectron microscope studies of samples of givenvarious heat treatments, in and out of fields. Hehas not been able to find plate like structures oreven preferred orientation of some diffuse featuresof the pictures. On the other hand, an unpublishedreport received informally from Geissler at Sche-

nectady reports the finding of plates about 1000 Åthick. Based largely on magnetic data, which isnot so elegant as that of J. J. Went quoted earlier,our view has been that the plate like structuresare in somewhat better accord with the evidencethan are the needle shaped precipitates previouslydiscussed by Prof. N6el.

Remarque de M. Goldman. - In view of theinterest in the origin of the anisotropic propertiesof Alnico V and the mechanism of nucleation, I

458

should like to propose the f ollowi ng mechanismdue to myself and Smoluchoski and which is alludedin my report. If ones assumes that cobalt goesinto the ternary system Fe-Ni-Al as if it were

iron, the magnetic phase would have a compositionof approximately 3o per I oo cobalt. Single crystalmagnetostriction measurements on an alloy of thiscomposition yield the following results :

and

We, therefore, propose that nuclei with [100] inor close to the direction of the annealing field havea lower elastic energy and therefore grow preferen-tially. The anisotropy of the macroscopic samplemay then be a combination of crystal and shapeanisotropy of the precipitated magnetic phase.

Réponse de - The explanation putforward by Mr Goldman for the origin of orientedprecipitates in Alnico V is very inviting. I wouldlike to ask whether, in view of the high self dema-gnetising factor of the precipitate, the magneti-sation of the Fe,Co particles would be sufficientto show significant magnetostrictive deformation.It would also have to be considered whether a small

amount of highly magnetic precipitate would givethe required intensity of magnetisation of the

alloy as a whole.

Remarque de M. Shockley. -- As I recall the dataon remanence and magnetostriction in polycristallineAlnico V suggests that the orientation producedby the field is somewhat more complete than thatexpected from selection simply of the nearest [100]direction. If this is the case it constitutes an

argument against the simple (100) plates discussedby the Philips group.

Réponse de M. Hoselitz. - The nearest [ 100]direction is good enough to explain data.

Remarque de -- It should be pointedout that the observation of Nesbitt on the domainboundaries crossing crystal boundaries without

change of direction, where not on the same materialas that discussed by Dr Hoselitz. The formerwas on material of low coercive force, quenchedin a magnetic field, while the latter was also agedto give high coercive force.

REFERENCES.

BOZORTH R. M. and DILLINGER J. F. 2014 Physics, 1935, 6,279 and 285.

BRADLEY A. J. and TAILOR. 2014 Proc. Roy. Soc., 1937, 159,56. - See also Magnetism. Inst. of Physics, London, 1938,p. 91).

BURGERS W. G. and SNOEK J. L. 2014 Physica, 1935, 2, 1072.DAHL O., PFAFFENBERGER J. and SCHWARTZ N. 2014 Metall-

wirtschaft, 1935, 14, 669.DANIEL V. and LIPSON H. - Proc. Roy. Soc., 1943, 181,

368; 1944, 182, 378.DANNOEHL W. - Arch. Eisenhuttenwesen, 1942 a, 15, 321;

1942 b, 15, 379.DANNOEHL W. and NEUMANN H. - Z. Metallk., 1938, 30, 217.DESMOND D. J. - Journ. I. E. E., 1945, 92, 229; 1949, 96, 13.EVERSHED S. 2014 Journ. I. E. E., 1920, 58, 780.GLOCKER R., PFISTER H. and WIEST P. 2014 Arch. Eisen-

huttenwesen, 1935, 8, 561.GOULD J. E., HOSELITZ K. and EDWARDS A. - B. I. 0. S.

Final Report No. 717, 1946.HADFIELD D. - Brit. Pat. Spec., No. 634.686 and 634.700,

1948.HONDA K., MASUMOTO H. and SHIRAKAWA Y. - Sc. Rep.

Tohoku Univ., 1934, 23, 368.HORNFECK A. J. and EDGAR R. F. 2014 A.I.E.E. Tech.,

Paper 40-91, 1940.HORSBURGH G. D. L. and TETLEY W. F. - Brit. Pat. Spec.,

No. 431.660 and 439.543, 1934.HORSBURGH G. D. L. and TETLEY W. F. 2014 Brit. Pat. Spec.,

No. 496.774, 1937.HOSELITZ K. - Research, 1950, 3, 77.HOSELITZ K. and Mc CAIG M. - Nature, 1949 a, 164, 581;

Proc. Phys. Soc., 1949 b, Sect. B., 62, 163.JELLINGHAUS W. - Arch. Eisenhuttenwesen, 1943, 16, 247;

Z. Metallk., 1948, 39, 52.KELSALL C. A. and NESBITT E. A. - U. S. Pat. Spec.,

No. 2.190.667, 1940.

KITTEL C., NESBITT E. A. and SHOCKLEY W. - Phys. Rev.,1950, 77, 839.

KÖSTER W. - Z. Elektrochemie, 1932 a, 38, 553; Arch. Eisen-huttenwesen, 1932 b, 6, 17.

KRUPP (Fried Krupp A. G.). 2014 Brit. Pat. Spec., 1933425, 455.

Mc CAIG M. 2014 Proc. Phys. Soc., 1949, Sect. B., 62, 652;Nature, 1950, 165, 969.

MISHIMA, T. 2014 Brit. Pat. Spec., Nos 378.478 and 392.658,1931.

NÉEL L.2014. Ann. Univ. Grenoble, 1946, 22, 299; J. Phys. Rad.,1948, 9, 184; C. R. Acad. Sc., 1947, 225, 109.

NÉEL L., WEIL L. and AUBRY J. - French. Pat. SpecChambéry, 1942, 323, 7 avril 1942.

NESBITT E. A.2014 Metals Technology, 1946, 13, 1973.NEUMANN H. 2014 Metallwirtschaft, 1935, 14, 779.OLIVER D. A. and GOLDSCHMIDT H. J. 2014 E. R. A. Report,

1946, N/T 41.OLIVER D. A. and SHEDDEN J. W. 2014 Nature, 1938, 142, 209.Permanent magnet Association, 1950; Physical Society, Exhi-

bition of Scientific Instruments, London, April 1950.RUDER E. W. - U. S. Pat. Spec. No. 1.947.274, 1933; Brit.

Pat. Spec., No. 423.897, 1933.SANFORD R. L. - Cir. U. S. Bur. Stand., 1944, C. 448.SCHEIL E., BISCHOF K. and SCHULTZ E. H. 2014 Arch. Eisen-

huttenwesen, 1934, 7, 639.SELJESATER K. S. and ROGERS B. A. - Trans. A. S. S. T.,

1931, 19, 553.SNOEK J. L. - Probleme der Technischen Magnetisierungs-

kurve, ed. Becker, R., Springer, Berlin, 1938, 73.STEINITZ R. - Powder Metallurgy Bull., 1946, 1, 45.SUCKSMITH W. - Proc. Roy. Soc., 1939, 171, 525; E. R. A.,Report N. C. T. 26, 1945.

VAN URK A. T. - Philips Technical Review, 1940, 5, 29.ZUMBUSCH W.2014 Arch. Eisenhuttenwesen, 1942, 16, 101.