Magnetic structure studies at Brookhaven National Laboratory

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Magnetic structure studies at Brookhaven National Laboratory

L.M. Corliss, J.M. Hastings

To cite this version:

L.M. Corliss, J.M. Hastings. Magnetic structure studies at Brookhaven National Laboratory. Journal

de Physique, 1964, 25 (5), pp.557-562. �10.1051/jphys:01964002505055701�. �jpa-00205827�

557

D’autres _corps tels _que MnEr03 possèdent égale-

ment un faible ferromagnétisme ^et probablement

une structure antiferromagnétique analogue ^{en ce} qui ^{concerne les atomes de Mn.}

En somme MnY03 possède ^un antiferroma-

gnétisme ^« triangulaire », couplé ^{avec un faible} ferromagnétisme. Ce corps ^est également ^ferroélec- trique. Nous ^ne doutons pas que la série des ^com-

posés MnTO3 (T

^{Y ou terre} rare) poseront ^au physicien ^d’autres problèmes intéressants.

BIBLIOGRAPHIE

’

[1] ^YAKEL (H.), ^KOEHLER (W. C.), ^BERTAUT (E. F.) ^et

FORRAT (F.), ^Acta Cryst., 1963, 16, ^957.

[2] ^BERTAUT (E. F.), ^FORRAT (F.) ^{et FANG} (P.), ^{C. R.}

Acad. Sc., 1963, 256, 1958.

[3] ^BERTAUT (E. F.) et MARESCHAL (J.), C. R. Acad. Sc., 1963, 257, 867.

[4] ^BERTAUT (E. F.), ^J. Appl. Physics, 1962, 33, 1138 ; cf. C. R. Acad. Sc., 1961, 252, 76 ; ^J. Phys. ^Chem.

Sol., 1961, 21, ^{256 et notamment} appendice p. ^279.

[5] ^BERTAUT (E. F.), ^J. Physique Rad., 1961, 22, ^{839 et}

aussi dans Treatise of Magnetism, ^vol. III, Éditeurs Suhl et Rado, ^Academic Press, ^1963.

[6] DZYALOSHINSKI »I. E.), ^J. Phys. ^Chem. Sol., 1958, 4,

241.

[7] ^MORIYA (T.), Phys. Rev., 1960, 117, 635.

[8] ^BERTAUT (E. F.), ^PAUTHENET (R.) ^{et MERCIER} (M.), Phys. Letters, 1963, 7, 110, (~ ^5, 2 7).

[9] SMART (J. S.), Treatise of Magnetism, ^vol. III, Éditeurs Suhl et Rado, ^Académic Press, ^1963.

MAGNETIC STRUCTURE STUDIES AT BROOKHAVEN NATIONAL LABORATORY (1) By ^L. M. CORLISS and J. M. HASTINGS

Chemistry Department, Brookhaven National Laboratory, Upton, ^L. I., ^{New York.}

Résumé.

La communication présenté rapporte les résultats de plusieurs ^études de structures

magnétiques et de transitions magnétiques, actuellement en progrès ^{ou récemment} complétées ^au

Laboratoire National de Brookhaven.

1° Le composé intermétallique Ni5Er possède ^{une structure} ferromagnétique uniaxiale ; ^la

diffraction magnétique ^{est en accord avec un moment} essentiellement nul du nickel comme les

mesures d’aimantation le suggèrent.

2° Le composé MnSn2 possède ^une transition brusque ^{d’une structure} antiferromagnétique ^vers

une configuration ^{où les} amplitudes ^{des moments,} couplés antiferromagnétiquement, ^{sont modu-}

lées sinusoïdalement. L’effet d’une substitution partielle de l’étain par l’indium est décrit.

3° Les résultats d’une étude des réflexions antiferromagnétiques ^de MnSe2 ^en fonction de la

température, ^étude entreprise ^{sur la} suggestion ^{de J. O.} Dimmock, ^sont comparés ^{avec ses} pré- dictions, ^{basées sur la} théorie des transitions de second ordre de Landau-Lifshitz.

4° La structure magnétique ^de Cr2O3 ^{a été} réexaminée sur des poudres ^{et sur un} monocristal.

Elle est discutée en relation avec les idées et suggestions ^{actuelles de la science.}

Abstract.

The present communication reports ^{the results} of several investigations ^of magnetic

structure and magnetic transitions currently ⁱⁿ progress ^or recently completed ^{at Brookhaven}

National Laboratory.

1° The intermetallic compound Ni5Er possesses ^a uniaxial ferromagnetic structure ; the magne- tic scattering ^is consistent with the assignment ^of essentially ^zero moment to the nickel, ^as sug-

gested by measurements of the saturation magnetization.

2° The compound MnSn2 has been shown to exhibit a sharp transition from an antiferromagnetic

structure to one in which the magnitude ^{of the} antiferromagnetically coupled moments exhibits a

sinusoïdal spatial modulation. The effect of partial substitution of tin by ^{indium is described.}

3° Results of a study ^{of the} temperature dependence ^{of the} antiferromagnetic reflections of

MnSe2, undertaken at the suggestion ^{of J. O.} Dimmock, ^are compared ^with predictions ^{based on}

the Landau-Lifshitz theory of second-order phase transitions.

4° The magnetic structure of Cr2O3 ^{has been} reinvestigated, using ^both single crystals and pow- dered specimens, ^{and is discussed in relation} to current ideas and suggestions ^{in the} literature.

LE JOURNAL DE PHYSIQUE ^TOME 25, ^MAI 1964,

N’5Er

Intermetallic compounds having ^the Cu,5Ca structure, ^{in which the} components ^{are rare earth}

and transition metals, have been investigated

(1) ^Research performed ^{under the} auspices ^{of the U. S.}

Atomic Energy Commission,

extensively ^{in recent} years by Wernick and Geller [1] ^and by ^{Wallace and coworkers} [2].

These hexagonal compounds, B5A, in which B is

an iron-group ^{element and A a} lanthanide, belong

to space group with A in (a) : 0, 0, 0, ^{2BI in} (c) : -;~ (1 j3, 2/3, 0), ^{and 3RII in} (g) : 1 j2, 0, 1/2 ; 0, 1 j ~, ~ ~2 ;

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505055701

--

’hhe Cu,Ca ^structure.

The magnetic measurements of Nesbitt et al. [3, ^4]

indicate that compounds formed between cobalt and the heavier rare earths are f errimagnetic,

with the cobalt substructure antiparallel ^{to that}

of the lanthanide. This model has been recently

confirmed at Grenoble by ^{the neutron} diffraction

experiments ^{of J ames et al.} [5] ^{for the case of} Co5Ho. ^The Ni5R compounds, first obtained by

Wernick and Geller [1], appear ^to exhibit a diffe- rent magnetic structure. Nesbitt et al. [6] suggest

that in these compounds ^{the nickel behaves as} though ^it possessed ^zero moment, ^{the three valence} electrons of the rare-earth metal filling ^{the holes}

in the 3d band of the nickel. This hypothesis ^is

based on saturation moment data as well as on the absence of compensation points ^{in the} ferromagne-

tic compounds ^{and of} antif erromagnetism ⁱⁿ N’5y’

Neutron diffraction studies of powdered Ni5Er

lend support ^{to this} supposition. The best fit of

intensity ^{data taken at 4.2 OK was obtained for}

-

0 and _03BCEr

7 .7, ⁱⁿ agreement ^{with the} macroscopic magnetization. The moments are

directed along ^{the c axis at helium} temperatures.

The low value of the saturation moment is thus

probably ^to be associated with a low value for the atomic moment of erbium. The departure

of the erbium moment from the theoretical value of is not unusual f or the second half of the rare-earth series and may indicate ^{that the ex-}

change interaction and crystal ^{field effects are of} comparable magnitude.

MnSn2

Recent measurements by Kouvel and Harte- lius [7] ^{of the} magnetic and electrical properties

of the intermetallic compound MnSn2l ^indicate

a number of unusual features. The magnetic susceptibility passes through ^{a weak maximum}

at 325 OK, ^{continues to rise on} cooling ^{to a} pro- nounced peak ^{at 86} OK, ^{and then} abruptly ^decre-

ases at 73 °li to a low vale which is maintained

down to helium temperatures. The salient fea- tures of the magnetic ^{behavior are} paralleled by

anomalies in the electrical conductivity ^and

volume.

MnSn2 ^{has the} body-centered tetragonal ^C16

structure (fag. 2), consisting ^of alternating layers

of Mn and Sn perpendicular ^{to the c axis. Each}

Mn atom is surrounded by eight ^{Sn atoms which}

connect it to its second and third nearest neighbors through angles ^of approximately ^and 146~, respectively. Nearest-neighbor ^{Mn atoms have}

no intervening ^{Sn atoms and are} separated by ^a

distance of 2.72 A in the ^c direction.

FIG. 2.

The C16 structure.

Cu Al2 type, ^{b. c.} tetragonal, ^M

4, I4/mcm

a = 6.647, ^c

^{5 .434. x}

^0.160.

Neutron diffraction studies show that above the abrupt transition at 73 OK, MnSn2 ^{is antifer-} romagnetic ^{with a Néel} temperature ^{in the} neigh-

borhood of 325 OK, ^the temperature ^of the upper

susceptibility ^{maximum. The} superstructure ^lines

characteristic of the antif erromagnetic ^{state appear,} however, ^{to saturate in} intensity ^near nitrogen temperatures ^{and show no} anomaly ⁱⁿ passing through ^the region ^{of the} large susceptibility

maximum at 86 OK. The magnetic ^structure

consists of ferromagnetic ¹¹⁰ planes which alter- nate along ^{the direction} perpendicular ^{to the}

sheets. Individual moments are oriented parallel

to the direction of alternation and reach a magni-

tude of 2.35 _{yB at} 77 oK. The abrupt ^transition

at 73 OK is marked by ^the disappearance ^{ot the} high-temperature antif erromagnetic diffraction pat-

tern and the appearance of a new pattern ^charac-

teristic of a modulated spin ^{structure below this} temperature. ^The low-temperature ^{structure is}

a simple modification of the spin arrangement

above the transition, ^{in which the} magnitude ^of

the moments is no longer ^constant but varies

sinusoidally along ^{the direction} perpendicuar ^to

the f erromagnetically aligned ^{sheets. The obser-}

ved modulation has an amplitude ^{of 3.11} (.LB

and a wavelength ^{of 18.8} A, wich is twice the

559

basal plane diagonal. ^{For a} particular ^{choice of}

the experimentally indeterminate phase ^{of the} modulation, ^{the structure reduces to the} simple

double sheet arrangement ^{+ + --.}

Partial substitution of indium for tin [8] ^lowers

the Neel point ^{and raises the} temperatures ^{of the}

lower susceptibility maximum and of the sharp transition, ^while maintaining ^{the same} general shape. ^The temperature ^interval between the

abrupt transition and _Xmax increases from about 130 in lVfnSn2 ^{to about 300 for 20} percent indium substi- tution. Neutron measurements for MnSn1.8In.2

confirm the lowering ^{of the Neel} point ^and

show that the antiferromagnetic structure at intermediate temperatures and the modulated structure at low temperatures remain the same.

The temperature range ^over which the sample

transforms from the antiferromagnetic ^{to the}

modulated structure is broadened and raised,

however. Whereas in MnSn2 the transition _appe-

ars to start at the break in susceptibility ^{and end}

at lower temperatures, ⁱⁿ MnSnl.gIn’2 ^it appears to begin ^{in the} neighborhood ^{of Xm.tx and end where}

the- susceptibility drops abruptly. ^{The coexis-}

tence of the two magnetic phases ^of the indium- substituted compound ^{can be seen in} figure 3,

FIG. 3.

Temperature dependence ^{of the 310 antiferro-} magnetic reflection and the 310(+) ^satellite.

where the proportion ^of high-temperature ^antifer- romagnetic ^{structure is indicated} by ^the 310¡inten- sity and that of the modulated structure by ^the

310 satellite. Experiments involving further subs- titution of indium are currently in progress.

MnSe2

The low-temperature antiferromagnetic ^struc-

tures of the homologous series, MnSe,, ^and MnTe2, ^were reported [9] ^{a number of} years ago.

These compounds crystallize ^{with the} pyrite structure, ^{a NaCl-like} arrangement ^{of M} and X 2

groups with the ^axes of the X2 groups parallel ^to

the various body diagonals. ^While the location of Mn ions relative to the X 2 groups ^{is the same as}

in MnO or oc-MnS, ^the presence ^of nearly ^tetrahe-

dral Mn-X-Mn linkages ^{suggests that nearest-} neighbor interactions might predominate ^and this,

in fact, ^{is what is} observed. The disulfide exhibits

ordering ^of the " third " kind, ^the ditelluride, ordering ^of the " first " kind, ^{and the} diselenide,

an arrangement ^which is intermediate between the

two, having ^{the same first} neighbor correlations

as the other members of the series but differing ⁱⁿ

the second neighbor environment. The magnetic

structures at helium temperatures ^{are sllown in} figure ^4.

FIG. 4. - Magnetic structures of lVInS2, MnSe2, ^and MnTe2.

Positive and negative orientations of the dipoles ^are

shown by ^{black and white} spheres. The direction of the magnetic axis is shown by ^an arrow ; in the ^case of the ditelluride the orientation within the plane is unspe- cified.

Recently, ^Dimmock [10] ^has pointed ^{out that} according ^to the Landau-Lifshitz theory, ^the MnSe2 structure cannot exist in the immediate

vicinity of the transition point between the para-

magnetic ^and antiferromagnetic phases, provided

there is a single transition and that it is of second

order. The reason for this is that the spin ^distri-

bution does not transform as a basis function for

a single irreducible representation of the space group ^{of the} paramagnetic phase, ^as required ^for

a second order transition. Dimmock proposed ^a decomposition ^{of the moment} density ^observed

at 4. 2 0 K into two parts ^{with different} temperature dependences, ^{such that the} initial transition would take place ^{to a modulated structure} which, ^{as the} temperature ^was lowered, would transform conti-

nuously ^{over into the observed} low-temperature

structure. Experimentally, ^{this results in a divi-}

sion of the antiferromagnetic reflections into two groups (corresponding ^{to the} splitting ^{of the mo-}

ment density ^{into two} parts) ^{with different} tempe-

rature dependences.

Investigation of the behavior of the diffraction

pattern ^with temperature reveals that the tran- sition is in fact very sharp. ^{At 51 OK the} pattern

is well-developed ^and corresponds ^{to 85 percent}

of the moment at 4.2 OK. Furthermore, ^the

relative intensities of the two classes of reflections

are essentially ^{the same as at helium} temperatures.

However, ^{at 56} OK, long-range magnetic ^{order has}

been replaced by short-range order, whereas the

expected ^Néel temperature, ^{based on a Brillouin} extrapolation ^{from the 4.2~ and 51 OK} points,

would be 90 °1(. These results are summarized in

figure ^{5. It is} difficult, from these observations, ^to

FIG. 5.

Sublattice magnetization ⁱⁿ MnSe2- conclude that the transition is actually ^first order, although this may very ^{well be true.} However,

the radical departure ^from Brillouin behavior makes it extremely unlikely ^{that the} theory ^of

second order transitions would be applicable ^to

this case. Work is in progress ^to further charac- terize the nature of the transition,

Cr2o3

The magnetic ^{structure of} CrO, ^{shown in} figure 6, ^was proposed by Brockhouse [11] ^about

FIG. 6.

The magnetic structure of Cr,O,.

ten years ago ^on the basis of preliminary ^data

which he ^was not able to extend at the time. The results were sufficiently precise ^{to allow a choice}

to be made among the three simple ^structures

which did not require enlargement ^{of the unit} cell,

but no assignment ^{of the} spin direction could be obtained. This information was subsequently supplied by ^the susceptibility measurements of McGuire and co-workers [12] ^{and more} directly by ^the polarized ^{neutron studies ot Nathans et}

at. [13] who found the spins ^{to lie} along ^{the rhom-}

bohedral axis. More recently, Cr103 has been the

subject of detailed experimental and theoretical

investigations which include the measurement of the magnetoelectric ^effect by ^Astrov [14] ^and by

Rado and Folen [15], ^the high-field antiferroma-

gnetic ^resonance experiments ^{of Foner} [16] ^{and a}

theoretical study ^{of the} magnetic ^and optical properties by ^{Pratt and} Bailey [17]. These inves-

tigations ^{have raised a number of} questions ^concer- ning ^the precise spin arrangement ⁱⁿ Cr203’ ^One

would like to know the sublattice magnetization

at low temperatures ^{as well as its} temperature ^de- pendence up ^to the Néel point, ^whether spiralling

or canting ^{of the} spins occurs, and whether any unusual features appear in the paramagnetic scattering ^above the transition point. ^Investi- gations currently in progress, in collaboration with R. Nathans and G. Shirane, may provide ^some

of the answers to these questions.

Examination of a very pure powder sample ^of Cr20l [18] ^{reveals that the Cr+2 moment is} appro-

ximately eight percent lower than the spin-only

value at helium temperatures. ^In addition, ^the

sublattice magnetization departs markedly ^from

a Brillouin temperature dependence ^{as shown in} figure ^7. These results were obtained from the

temperature variation of the 110 reflection, ^after

subtracting the nuclear contribution remaining

above ^{the Néel} point (approximately ^{15 percent}

561

FIG. 7.

Temperature ^variation

of the sublattice magnetization ⁱⁿ Cr 20 3"

of the total at low temperatures). Experiments performed ^on single crystals ^{indicate the} presence of weak diffracted intensity ^{at a number of reci-} procal lattice points ^{to which the} Cr2o3 ^structure

makes ^no contribution. These results cannot be

interpreted until several possible spurious ^effects

have been eliminated. Polarized neutron studies

are capable ^at present ^of eliminating ^{canted struc-}

tures in which the perpendicular component ^is

ordered either in the sequence +-+- (Cr203)

or + + - - (Fe203) along ^the rhombohedral axis.

Discussion

’

Pr RUNDLE. - Si vous avez un ordre a courte distance au-dessus de la temperature ^{d’ordre a} longue distance, pouvez-vous ^encore parler ^de

transition brusque ? ^Peut-on appliquer ^{la th6orle}

de Landau-Lifschitz a l’ordre a courte distance ? Dr CORLISS.

La transition brusque ^dont je parle ^{ici est} la transition conduisant a un ordre à

longue distance. Je ne pense pas que la th6orie de Landau et Lifschitz soit facile a appliquer ^{au cas ou}

l’ordre a courte distance est tr6s prononc6, ^bien qu’alors ^on puisse penser 1’appliquer ^{a une 6chelle} microscopique.

Pr OPECHOWSIKI.

Dans 1’6tude de MnSe2, ^vous prétendiez _{que la} ^brutalite de la transition rend

1’application de la théorie de Landau-Lifschitz

impossible. Je n’ai pas très bien compris pourquoi

vous mettez 1’accent sur « brutalite _o, ou bien voulez-vous dire que la manière dont Dimmock

applique ^{la théorie de} Landau-Lifschitz ^ne conduit nulle part ?

Dr CORLISS.

On peut supposer que la théorie des transitions du second ordre s’applique lorsque 1’6nergie libre varie lentement au voisinage ^{de la} temperature ^de transition. Dans une transition

brutale, l’intervalle de temp6rature ^est trop ^faible

pour permettre ^le genre de mesures envisag6es ^ici.

Pr PAOLETTI.

Je demande si les mesures

faites avec des neutrons polarisés ^sur Cr,O,

donnent des r6sultats identiques ^{a ceux de Nathans}

et Shull.

Dr CORLISS.

Les mesures originales ^{ont été} reprises par ^{Nathans et} Shirane sur le meme cristal que celui ayant servi a la pr6sente ^6tude.

Dr PICKART.

Les mesures les plus ^r6centes

ont été faites avec une resolution bien meilleure que pr6c6demment.

Pr BERTAUT. - La situation de MnSn2 ^est exceptionnelle, ^{car une situation} modul6e sinu- soidalement se trouve ici loin de Trr ^et pr6c6d6e

d’une structure colin6aire antif erromagnetique. ^La

modulation sinusoidale observ6e dans les terres rares est d’une nature différente, ^{elle corres-} pond ^{4 un ordre de} spins ^{dans une direction} (TN plus grand ^{selon c} que selon a ou b). Est-ce que dans MnSn2 ^on n’aurait pas de super6change superpose ^au couplage Rudermann-Kittel, ^{car ce}

corps ^est

je le suppose

^un conducteur ! Dr CORLISS.

Nous n’avons aucune explica-

tion de la structure antiferromagnétique ainsi que de la ^structure modul6e. Le modele que ^vous pro- posez peut ^bien jouer ^{un role assez} important.

Dr SPARKS.

~.~ Les variations des parametres

cristallins de MnSn2 ^{ont-elles été étudiées en}

fonction de la temperature ^{dans la} region ^{de la}

transition brusque ?

20 Est-ce que le ^meme échantillon de MnSn2

utilise _{pour les} mesures de susceptibilités ^{a servi}

a l’étude par diff raction neutronique ?

Dr CORLISS.

Les mesures de dialatations fai- tes par Kouvel ^sur des 6chantillons polycristallins

montrent une faible variation de volume au point

de transition. Les variations des param6tres ^cris-

tallins étaient trop ^faibles pour ^6tre observ6es sur

notre diagramme enregistr6 ^aux petits angles. ^Des

mesures de suseeptibilit6s magn6tiques ^{ont été} entreprises ^sur de nombreux 6ehantillons parmi lesquels ^celui utilise lors de notre experience. ^Le broyage ^subi par 1’echantillon pour ^cette exp6-

rience 61argit ^neanmoins quelque ^{peu la transi-}

tion.

Pr WALLACE. - Je voudrais faire remarquer que Bleaney ^{a fait des} calculs de champ ^cristallin

pour le compose ANI ’5 ^et AiVi2. ^Les r6sultats de ses

calculs _pour ErNi5 ^{sont en} bon accord av ec votre moment observe pour Er dans ErN’5’

Pr WALLACE. - L’expérience ^{dans notre labo-}

ratoire a montre que l’impuret6 d’oxyg6ne ^existant

a quelques parties pour mille peut ^{dans le cas des} compos6s AB changer ^les temperatures ^{de Curie de} quelques centaines de degr6s ^et même alterer le

couplage depuis I’antiferromagn6tisme jusqu’au ferromagnétisme. ^{Avez-vous une} indication ^sur votre impureté d’oxyg6ne ^dans ErNi5 ?

Dr CORLISS.

Nous n’avons _pas d’indication

sur l’oxyg6ne ^{contenu dans nos} 6ehantillons.

Quoiqu’ll ^en soit, puisque ^les propriétés magn6- tiques ^sont les memes que celles des 6chantillons que ^{vous avez} prepares (avec ^de grandes pr6eau-

tions pour éliminer 0,) ^on peut penser que la contamination O2 ^est n6gligeable, ^ici.

Pr JAMES. - Dans notre travail sur YC05 ^et HOC05, ^nous observons de petits pics d’oxydes

ternaires parce que nous avons utilise des nacelles

d’alumine, alors que Lemaire et ses collaborateurs

pr6parent actuellement leurs echantillons par ^une

technique ^de levitation modifiée qui ^{donne des}

6chantillons de plus grande pureté.

Dr PRINCE.

Rado et Folen ont fait ressortir que la ^structure de Cr203 peut ^{avoir deux} types

de domaine qui peuvent ^{montrer des eff ets d’inter-}

férences destructives si les domaines sont suffit- samment petits, ^{même si les deux} structures son

en principe indiscernables (a grande distance).

Avez-vous vu des eff ets qui pouvaient s’expliquer

par ^un ph6nom6ne ^{de ce} types

Dr CORLISS. - Nos mesures ne permettent pas d’inf6rer un tel effet quoique ^{ce soit} peut-être

difficile a voir. Dans le cas extreme on pourrait

s’attendre a un 61argissement ^des raies ; ^ceci

n’a pas ^été observe.

,

REFERENCES

[1] ^WERNICK (J. H.) and GELLER (S.), ^Acta Cryst., 1959, 12, 662.

[2] ^NASSAU (K.), ^CHERRY (L. V.) and WALLACE (W. E),.

J. Phys. ^Chem. Solids, 1960, 16, 123 ^and 131.

[3] ^NESBITT (E. A.), ^WERNICK (J. H.) ^{and CORENZWIT} (E.),

J. Appl. Physics, 1959, 30, ^365.

[4] ^NESBITT (E. A.), ^WILLIAMS (H. J.), ^WERNICK (J. H.)

and SHERWOOD (R. C.), ^J. Appl. Physics, 1961, 32,

342 S. See also ref. 2.

[5] ^JAMES (W.), ^LEMAIRE (R.) and BERTAUT (F.), ^{C. R.}

Acad. Sc., 1962, 255, ^896.

[6] ^NESBITT (E. A.), ^WILLIAMS (H. J.), ^WERNICK (J. H.)

and SHERWOOD (R. C.), ^J. Appl. Physics, 1962, 33,

1674.

[7] ^KOUVEL (J. S.) ^{and HARTELIUS} (C. C.), Phys. Rev., 1961, 123, 124.

[8] ^KOUVEL (J. S.), Private communication.

[9] HASTINGS (J. M.), ^ELLIOTT (N.) ^{and CORLISS} (L. M.), Phys. Rev., 1959, 115, 13.

[10] ^DIMMOCK (J. O.), Phys. Rev., 1963, 130, 1337.

[11] BROCKHOUSE (B. N.), ^{J. Chem.} Physics, 1953, 21, ^961.

[12] ^McGUIRE (T. R.), ^SCOTT (E. J.) and GRANNIS (F. H.), Phys. Rev., 1955, 90, 1562.

[13] ^NATHANS (R.), ^RISTE (T.), ^SHIRANE (G.) ^{and SHULL} (C. G.), ^{M. I. T.} structure of solids group, Technical

Report ^{No. 4, 1958.}

[14] ASTROV (D. N.), ^Soviet Phys. JETP, 1961, 13, ^729.

[15] ^RADO (G. T.) ^{and FOLEN} (V. J.), J. Appl. Physics, 1962, 33, 1126.

[16] ^FONER (S.), Phys. Rev., 1963, 130, ^183.

[17] ^PRATT (G. W.), Jr. and BAILEY (P. T.), Phys. Rev., in press.

[18] ^{We are indebted} to Mr. R. A. Stephens ^{of C. K. Wil-}

liams Co. for making ^this sample ^{available to us.}