LONG RANGE MAGNETIC ORDER IN THE SUPERCONDUCTING STATE OF HEAVY RARE EARTH MOLYBDENUM SULFIDES AND THEIR PSEUDOTERNARY COMPOUNDS

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LONG RANGE MAGNETIC ORDER IN THE SUPERCONDUCTING STATE OF HEAVY RARE EARTH MOLYBDENUM SULFIDES AND THEIR

PSEUDOTERNARY COMPOUNDS

M. Ishikawa, Ø. Fischer, J. Muller

To cite this version:

M. Ishikawa, Ø. Fischer, J. Muller. LONG RANGE MAGNETIC ORDER IN THE SUPERCON- DUCTING STATE OF HEAVY RARE EARTH MOLYBDENUM SULFIDES AND THEIR PSEU- DOTERNARY COMPOUNDS. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-1379-C6-1384.

�10.1051/jphyscol:19786575�. �jpa-00218066�

JOURNAL DE PHYSIQUE

Colloque C6, suppliment au no 8, Tome 39, aotit 1978, page C6-1379

LONG RANGE MAGNETIC ORDER I N THE SUPERCONDUCTING STATE OF HEAVY RARE EARTH MOLYBDENUM S U L F I D E S AND T H E I R PSEUDOTERNARY COMPOUNDS

M. Ishikawa,

0.

Fischer and J. Muller

De'pmtement de Physique de Za MatiBre Condense'e, Universite' de GenBve, 32 Bd drYvoy

-

1211 GenBve 4 Switzer Zwd.

Rbsum6.- On prdsente les observations exp6rimentales prowant l'existence d'un ordre magndtique B longue port6e 1 1'Btat supraconducteur dans les sulfures ternaires de molybdPne et de terres rares

(TR variant de Gd 1 Er). Parmi ces observations figurent des rdsultats dlexpQriences de diffraction neutronique dans certains cas. Les propribtbs magnstiques et supraconductrices de cette sQrie de composds sont discuthset des rdsultats prdliminaires concernant (k-Lu)Mo6S8 sont Qgalement exposds.

Abstract.- Various experimental evidences for long range magnetic order in the superconducting state of the heavy rare earth molybdenum sulfides (RE)No S8 (RE = from Gd to Er) are presented together with the results of recent neutron diffraction stdies on some of these sulfides. Their magnetic and superconducting properties along the rare earth series in the Periodic Table are discussed, and preliminary results on speudoternary compounds of Ho are also included.

Ever since Ginzburg studied / I / the problem called Chevrel compounds / l o / , various evidences of about twenty years ago, a great deal of effort has

been made in the quest for the coexistence of super- conductivity and magnetism 1.21. Earlier experiments were, however, carried out exclusively on dilute substitutional alloy systems and in such systems the superconductivity was often suppressed well liefore the concentration of magnetic ions became sufficien- tly high for a long range magnetic order / 3 / . Never- theless, some evidence of a long range magnetic or- der has been suggested in several ingeniously com- pounded alloys, but all of such magnetic orders tur- ned out ot be of short range ; either spin glass

-

or cluster

-

type with a correlation length shorter than 50

A

/ 4 / . Such a frustrating difficulty with dilute substitutional alloys has been overcome only after the recent discweries /5/ of superconducting stoichiometric rare earth compounds, which contaln as much as

7

to 10 at. % of magnetic rare earth

(RE)

ions distributed over regular lattice sites, about 6.5

A

apart. It is this property of such new com- pounds which provides us with a possible coexistence of superconductivity with a long range magnetic or- der. As a matter of fact, soon after these dlscwe- ries, a long range magnetic order was found almost at the same time in Ho M S

161

and ErRh4B4 /7/.

1.2 6 8

The long range magnetic order appeared in their superconducting state, but it was accompanied by a destruction of superconductivity at its onset. Soon later, however ,in some other

RE

molybdenum sulfides

181

and selenides / 9 / , (RE)Mo X (X = S or Se), so- 6 8

coexistence with a long range magnetic order have been reported : in (RE)Mo6S8 (RE = Tb, Dy and Er) /8/, for example, resistance anomalies in their su- perconducting state were found and interpreted as a result of a long range magnetic order with the aid of ac susceptibility and magnetization measurements.

The long range antiferromagnetic order was, in fact, confirmed for

RE

= Dy /11/ and Tb /12/ by recent neutron diffraction experiments performed at Brook- haven National Laboratory. Subseauently, a similar resistance anomaly was also found in Gd Mo

S .

1.2 6 8 Therefore, all these results on (RE)Mo S allow us

6 8

a systematic study of various magnetic and supercon- ducting properties across the series of the heavy rare earth ions in the periodic Table. As some de-

tails

about (RE)Rh B and (RE)Mo Se will be reviewed

4 4 6 8

by Dr. M.B. Maple elsewhere in this volume, experi- mental results regarding the coexistence problem only in the heavy rare earth molybdenum sulfides and their pseudoternary compounds are presented in this article.

First some of the previous results on (RE) Mo

S

are here recapitulated, before more specifics

6 8

of each compound are discussed : all of these com- pounds from Gd to Er become superconducting below about 2

K

and show amagnetic phase transition at a lower temperature, coexisting with the superconduc- tivity for W = 0, Th, Dy and Er, while for

RE

= Ho destroying the superconductivity at the onset of a ferromagnetic order. Accordingly it is extremely

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786575

JOURNAL DE PHYSIQUE

interesting to see what happens if some Ho ions in HOMO S are replaced by non-magnetic ions to sligh-

6 8

tly weaken the strong ferromagnetic interaction preventing the coexistence with the ferromagnetic interaction preventing the coexistence with the fer- romagnetism in this compound. Some preliminary re- sults on speudoternary compounds are also included in this article. Although the long range nature of these magnetic orders has been so far confirmed only for

RE

= Tb, Dy and Ho with neutron experiments, all magnetic orders are expected to be of long range

judging from similarities in their orther properties such as magnetization and temperature dependence of upper critical fields. This long range nature in these compounds, indeed, makes one of the most impor- tant differences from the magnetic states found pre- viously in dilute substitutional alloys.

The reentrant behavior of Ho Mo

S

161 1 - 2 6 - 8 is shown in figure 1, where its ac

susceptibilities,

Xac are plotted as functions of temperature ; the compound becomes superconducting below about 1.2

K

(This value for this particular sample is, however, much lower than more representative values, see fi- gure g ) , and returns to the normal state at 0.65 K where a long range ferromagnetic order establishes

itself.

Fig. 1 : ac magnetic susceptibility of Ho Mo S as a function of temperature, showing the re&bZrafit8be- havior (ref. 6)

.

The long range ferromagnetism was later confirmed by a neutron experiment /13/ : the Ho ions in the Che- vrel phase are aligned along the

D

¹ ternary axis

0

with a magnetic correlation length longer than 300k In the following, experimental evidences

.

for a Long range antiferromagnetic order in Er, Dy, Tb and Gd compounds are presented with an emphasis

on the particularities of each compound. In figure 2, the resistance anomaly of

Er

Mo

S

in the super-

1 . 2 6 8

conducting state is shown.

I . . . . I . . . . I . . . I . . . , . . . . , I

0 0.5 10 1.5 2.0 2.5

TEMPERATURE ( K )

Fig. 2 : Resistance vs temperature for Er,.,Mo,S, in magnetic fields between 0 and 2.14 kOe (Ref. 181).

This compound remains superconducting below 2.1

K

down to the lowest temperature investigated in zero magnetic field, but the weakening of the superconduc-

ting screening effect by the application of magnetic fields higher than about 800 Oe reveals an anomaly which is peaked at 0.16

K,

independent of the field.

This anomaly disappears and the full superconductivi- ty is restored at low enough temperatures. A very similar behavior in R vs T was found also in Dy

1 ' 2 Mo S as shown in figure

3.

6 8

1000 A 1250

o a5 10 1s 2 o ² 5

TEMPERATURE ( K )

Fig. 3 : Resistance vs temperature for Dyl *Mo

S

in magnetic fields between 0 and 2 kOe (Ref.

/8/7.

However, this compound shows a relatively abrupt rise of R at about 0.4

K

which is reflected in its anomalous upper critical field curve as discussed below. In figure 4, the spectra obtained by a recent elastic neutron diffraction experiment / I

I /

^are shown for the antiferromagnetic (upper spectrum) and the paramagnetic superconducting state (lower spec;

along the applied field direction. However, even at

SCATTERING ANGLE ( 2 8 )

Fig. 4

:

Powder neutron diffraction spectra (Ref.

11)

for Dyl 2M06S8 above (T

=

0.7 K) and below (T

=

0.05 K) its'magnetic transition. The inset shows the antiferromagneLic arrangement of moments along the ternary axis, 3.

The eight magnetic peaks (cross-hatched) in the range of scattering angles

(28)

up to

_63O

were successfully indexed on the basis of the simple cubic Chevrel lat- tice (with a slight rhombohedra1 distortion)

:

the inset shows a half of the tetragonal magnetic unit cell with the moments on {loo) planes alternately parallel and antiparallel to the [ll ternary axis.

These results conclusively confirm 1111 that the Dy ions in the main Chevrel phase, rather than in a se- cond phase develop a long range antiferromagnetic order. The extra Dy ions of 0.2 per formula unit are probably the cause of a few extra small magnetic peaks in the spectra 11 11. A similar long range anti-

ferromagnetic order was confirmed also for

Tb

I.iMDSS 8

by another neutron experiment 1121. This antifer- romagnetic order below its magnetic ordering tempe- rature of

1

K is demonstrated by the low values of magnetization for H

<

1.5 kOe, in figure

5.

'L

Fig. 5

:

Magnetization vs applied magnetic field for Tb Mo6S8, showing an antiferromagnetic beha- vior at'$

=

0.1 K (Ref. / 141).

In higher fields, the magnetic,moments flip over

18.5 kOe a complete saturation of the moments could not be attained. This unusually slow saturation pro- cess, which is not well understood at the moment, seems to be characteristic of these compounds 1141.

In figure

6,

xaC of Gd Mo S as a function

1 - 2

6 8

of temperature is shown for H

=

0 and 400 Oe. It should be remarked that such a pronounced peak of 'ac in zero applied magnetic field was found only for this compound in this series. This anomaly in xac

may be explained as a result of a stronger exchange scattering effect in this compound than in the others which is reflected also in its large suppression of T and upper critical fields as shown in figures

8

and

9.

Because an almost identical M-H curve to that shown in figure 5 was found /14/ for this compound, the magnetic state below 0.95 K appears to be also long range antiferromagnetic.

Fig.

6 :

ac susceptibility

6

temperature for Mo6S8. The arrow indicates the ordering temperature where x deviates from the Curie-Weiss law.

ac

From the resistance measurements in magnetic fields, upper critical fields are determined as re- ported earlier 181. The results are summarized in figure 7 and 8 for RE

=

Ho and Dy, and Gd and Tb, respectively in order to show the apparent importance of specific magnetic interactions in characterizing these anomalous H curves. The curves down to their

c2

magnetic ordering temperatures, Tm were successfully fitted as shown by the solid lines in the figure and interpreted as the spin polarization effect in the framework of the additive pair breaking theory as described elsewhere 181. Below their Tm's, however, a sudden decrease of H is accompanied Qor these compounds

;

this decrease at T is very abrupt in

m

both Dy and Ho compounds and seems to be due to a

rather strong ferromagnetic interaction in these com-

C6-1382 JOURNAL DE PHYSIQUE

pounds. In fact, the magnetization measurements

Temperature ( K

Fig.

7

: Superconducting upper critical field vs temperature for Dyl Mo

S

a n d H ~ ~ . ~ M o ~ S ~ , showing an abrupt decrease 2g

~ . : T

higher than about 1.3 kOe. On the other hand, in the Tb and Gd compounds, there is a broad peak around T (indicated by an arrow), where the moments

m

are still aligned antiferromagnetically in the cor- responding fields

1 1 4 1 .

The peak is followed by a sharp rise at lower temperatures, as shown in figure 8. This sharp rise is a result of the fast recovery of superconductivity in the well ordered antiferro- magnetic state as evidenced in their

x -

and 9-T

ac curves. One notes that the variation of the H

C2 below Tm for the Dy compound is much smaller. This apparent difference may be, however, simply due to its much lower

T

compared with T for this compound.

m

In figure

9

are summarized the magnetic 'ordering temperature, T and the superconducting

m

transition temperature, Tc, of all the compounds discussed above

1 1 5 1

in order to show their general trend across the heavy rare earth series in the Pe- riodic Table. Magnetic orderings in metallic subs-

"0 0.5 1.0 1.5

Temperature ( K

Fig. 8 : Upper critical field vs temperature for Tb Mo S m d G d Mo S8. The arrows shows their rnailiztig 8rderin&'$em$eratures.

showed

1141

that the magnefic moments in the Dy com- pound are also ferromagnetically aligned in fields

Fig.

9

: Tm, T decrease of Tc, -ATc,de Gennes factor, G and

6'13

for heavy rare earth molybdenum sulfides (Ref.

1 1 4 1 ) .

tances, in general, result from a complex combination of interactions of various origins

/ 1 6 / ,

such as an indirect exchange interaction mediated by conduction electrons and an anisotropic interaction caused by crystal fields to which magnetic ions in crystals are subjected. In addition to our still poor under;

s t a n d i n g of t h e d e t a i l e d e l e c t r o n i c and c r y s t a l l o - g r a p h i c p r o p e r t i e s of t h e s e complex t e r n a r y compounds, t h e small i n t e r a c t i o n e n e r g i e s involved i n t h e s e ma- g n e t i c t r a n s i t i o n s a s evidenced by t h e i r low orde- r i n g temperatures make t h e d i s c u s s i o n extremely d i f - f i c u l t . T h e r e f o r e , some p o s s i b l e mechanisms f o r t h e magnetic o r d e r i n g s i n t h e s e compounds a r e b r i e f l y d i s c u s s e d and t h e d e t a i l s must be l e f t f o r f u t u r e s t u d i e s .

I f t h e RKKY t y p e i n d i r e c t exchange i n t e r a c - t i o n 1171 i s predominantly r e s p o n s i b l e a s i n pure heavy r a r e e a r t h m e t a l s T would v a r y w i t h t h e de m Gennes f a c t o r , G , d e f i n e d a s ( g - ~ ) ~ J ( J + I )

.

I n f a c t , t h e molecular e x p r e s s i o n f o r T i s given by 1181

m kBTm = CNX --(L

~ r ~ .

1 2 4

C i s t h e c o n c e n t r a t i o n of magnetic i o n s , Xo i s t h e paramagnetic s u s c e p t i b i l i t y of t h e conduction e l e c t r o n s and

r

^{i s} t h e exchange i n t e r a c t i o n c o n s t a n t

1 +

+

d e f i n e d by Hex =

- - v

C ¹ (g-1)T Ji. S, where N i s t h e

₊

t o t a l number of atoms p e r u n i t volume, J i s t h e t o t a l a n g u l a r momentum of t h e RE i o n and S i s t h e e l e c t r o n

+

s p i n . As c a n b e seen i n t h e f i g u r e , however, T f o r m t h e s e compounds v a r i e s approximately w i t h G~~~ a s found i n many b i n a r y a l l o y s of t h e r a r e e a r t h m e t a l s

/

191. Furthermore, a s p o i n t e d o u t e a r l i e r 1141, t h e

exchange c o n s t a n t , u s i n g t h e above formula based on t h e RKKY i n t e r a c t i o n . The v a l u e of

r

can be a l t e r - n a t i v e l y e s t i m a t e d from t h e d e c r e a s e of T c ' -ATc due t o s p i n exchange s c a t t e r i n g 1211, u s i n g t h e f o l l o - wing formula 1221

~ 1 2

kBhTc =

3-

C N ( O ) G T ~

where N(0) i s t h e e l e c t r o n i c d e n s i t y of s t a t e s p e r eV-atom-spin. For t h i s c a l c u l a t i o n , t h e T 's of hy- p o t h e t i c a l non magnetic compounds a r e r e q u i r e d . How- ever f o r t h e s e dense magnetic compounds, i t i s d i f f i - c u l t t o s e p a r a t e t h e s p i n exchange e f f e c t from o t h e r chemical e f f e c t s . Hence, La Lu Mo S 1231 have been

1-x ^X 6 8

t e n t a t i v e l y chosen a s non magnetic compounds. Thus chosen -AT ' s v a r y approximately w i t h G a s expected from t h e formula ( s e e f i g u r e 9 ) . The v a l u e s of

r

determined w i t h t h e two methods f o r t h e Dy compound, f o r example, were 50 and 25 meV, r e s p e c t i v e l y . It was found (141 t h a t e i t h e r method y i e l d s a reasona- b l y c o n s t a n t

r

a c r o s s t h e heavy r a r e e a r t h s e r i e s , although t h e v a l u e s determined from Tm where appre- c i a b l y l a r g e r t h a n t h o s e from -AT

.

These v a l u e s of

r

f o r t h e Chevrel compounds a r e c o n s i d e r a b l y s m a l l e r than t h o s e of o t h e r RE compounds 1241, a s expected from t h e f a c t t h a t t h e RE i o n s i n t h e Chevrel com-

0

pounds a r e w e l l s e p a r a t e d (about 6.5 A a p a r t ) . As s t a t e d i n t h e b e g i n n i n g , f o r our under- z i g z a g v a r i a t i o n of T s u g g e s t s t h a t t h e i n t e r a c t i o n

m s t a n d i n g of t h e problem r e g a r d i n g a p o s s i b l e coexis- i s modulated by an a n i s o t r o p i c i n t e r a c t i o n such a s

t e n c e of ferromagnetism and s u p e r c o n d u c t i v i t y , i t i s t h e c r y s t a l l i n e f i e l d e f f e c t p a r t i c u l a r t o each RE

v e r y important t o s t u d y s l i g h t l y d i l u t e d Ho compounds i o n , p o s s i b l y a c t i n g d i f f e r e n t l y on t h e Kramers (Gd,

with non magnetic i o n s . For t h i s purpose, Dy and E r ) and t h e non Kramers i o n s (Tb and Ho)

.

H o ~ . ~ - ~ L u ~ M o S (0

5

x

5

0.2) have been chosen : t h e Other mechanisms such a s a s u ~ e r e x c h a n e e in t e r a c t i o n

-

6 8

v i a s u l f u r atoms between t h e RE i o n s and t h e c l a s s i - c a l magnetic d i p o l a r i n t e r a c t i o n could b e a l s o r e s - p o n s i b l e f o r t h e s e low temperature o r d e r i n g s .

It should be a l s o p o i n t e d o u t t h a t t h e d i r e c t i o n of t h e magnetic moments i n b o t h f e r r o - and a n t i f e r r o m a g n e t i c s t r u c t u r e s c o i n c i d e s w i t h t h e unique c r y s t a l l o g r a p h i c t e r n a r y a x i s of t h e Chevrel phase 111-131. T h i s might suggest t h e importance of t h e a n i s o t r o p y caused by t h e s u l f u r atoms around each RE i o n i n determining t h e e a s y d i r e c t i o n of t h e moments i n t h e s e compounds. I n f a c t , a cube formed by t h e e i g h t s u l f u r atoms surrounding a RE i o n i s compressed a l o n g t h e t e r n a r y a x i s 1201.

I n g p i t e of t h e s e r e s e r v a t i o n s concerning t h e magnetic o r d e r i n g s i n t h e s e compounds, i t seems worthwhile t o e s t i m a t e t h e o r d e r of magnitude of t h e

d i l u t i o n w i t h Lu r e s u l t e d i n a S l i g h t d e c r e a s e of T m' a s expected, £rum 0.75 K f o r x = 0 t o 0.65 K f o r x = 0 . 2 , b u t w i t h o u t any a p p r e c i a b l e change of T

.

^This

f a c t i n d i c a t e s t h a t t h e s p i n e f f e c t i s n o t s u f f i c i e n t t o p r e d i c t a change of T i n t h e s e compounds.

T h e r e f o r e , a t t h e moment i t i s d i f f i c u l t t o make a comparison w i t h t h e o r e t i c a l p r e d i c t i o n s 1251. I n r i g u r e 10, t h e dc r e s i s t a n c e of Ho Lu No S 1.15 0 . 0 5

6

8 i s shown a s a f u n c t i o n of temperature and a p p l l e d magnetic f i e l d . Although t h i s sample showed a v e r y

s i m i l a r

x

a c -T curve t o t h a t of Ho 1 . 2 6 8 No S

,

t h e l i n e a r d e c r e a s e of R w i t h temperature i n z e r o magne- t i c f i e l d s u g g e s t s t h a t t h e sample would e v e n t u a l l y become superconducting a t v e r y low temperatures. I f t h e magnetic s t a t e i s f e r r o m a g n e t i c a s i n Ho lvro S 1 . 2 6 8 t h i s may be t h e f i r s t example of t h e c o e x i s t e n c e

JOURNAL DE PHYSIQUE

C6- 13 84

References

w i t h ferromagnetism. I n o r d e r t o c l a r i f y t h e s e ques-

111

Ginzburg V.L., Sov- Phys. JETP

4

(1957) 153- t i o n s , more experiments a r e i n p r o g r e s s t o g e t h e r / 2 / S e v e r a l good review a r t i c l e s o n t h i s problem a l -

r e a d y appeared i n t h e l i t e r a t u r e . See, f o r exam- w i t h some o t h e r p s e u d o t e r n a r y compounds of Ho. p l e , P a r k s R.D., i n S u p e r c o n d u c t i v i t y , e d i t e d by Wallace P.R. (Gordon and B r e a c h ) ,

2

(1969) 625 and F i s c h e r

0.

and P e t e r M., i n Magnetism, e d i t e d b y Rado G.T. and Suhl H. (Academic p r e s s ) , V

(1973) 327. F o r a more r e c e n t review, s e e R&. 4.

/ 3 / Maple M.B., Appl. Phys.

9

(1976) 179.

/ 4 / Roth S., Appl. Phys.

15

(1978) 1 .

/5/ F i s c h e r

O . ,

Treyvaud A . , Chevrel R. and S e r g e n t M., S o l i d S t a t e Comun. (1975) 721, S h e l t o n R.N., McCallum R.W., and Adrian H., Phys. L e t t . 56A (1976) 213, and M a t t h i a s B.T., Corenzwit E.,

-

Vandenberg J . M . , and Barz H.E., Proc. Nat. Acad.

S c i . USA

74

(1977) 1334.

/6/ Ishikawa M. and F i s c h e r

0.,

S o l i d S t a t e C o m n . 23 (1977) 37.

-

0 05 10 15 2 0 2 5 / 7 / F e r t i g W.A., J o h n s t o n D.C., DeLong L.E., McCallum

T IKI R.W., Maple M.B. and M a t t h i a s B.T., Phys. Rev.

L e t t .

38

(1977) 987.

Fig. 10

i

d c r e s i s t a n c e v s t e m p e r a t u r e f o r /8/ Ishikawa M. and F i s c h e r

PI.,

S o l i d S t a t e Commun.

H o l . 1 5 L ~ 0 . 5 M ~ S8. Note t h e l i n e a r d e c r e a s e on t h e

c u r v e f o r

%

⁼

6

Oe

-

24 (1977) 747.

/9/ McCallum R.W., J o h n s t o n D.C., S h e l t o n R.N., B e f o r e t h i s a r t i c l e i s concluded, a few ge- F e r t i g W.A. and Maple M.B., S o l i d S t a t e Commun.

24 (1977) 501.

n e r a l remarks p e r t i n e n t t o t h e problem o f c o e x i s t e n c e

-

/ l o / General p r o p e r t i e s of Chevrel compounds have i n t h e s e C h e v r e l compounds a r e made i n t h e f o l l o w i n g . been r e c e n t l y reviewed by F i s c h e r

O.,

Appl. Phys.

Long r a n g e magnetic o r d e r s i n t h e i r s u p e r c o n d u c t i n g

-

16 (1978) 1 .

s t a t e s have now been proved i n t h e s e compounds w i t h / I

11

Moncton D.E.. S h i r a n e G., ~ h o m l i n s o n W . , ~ s h i k a w a M. and F i s c h e r

@.,

t o b e p u b l i s h e d .

t h e a i d of n e u t r o n d i f f r a c t i o n experiments. However,

/12/ Thomlinson W., S h i r a n e G . , Moncton D.E., Ishikawa more e x p e r i m e n t s must b e c a r r i e d o u t i n o r d e r t o M. and F i s c h e r

0.,

t o b e p u b l i s h e d .

b e t t e r u n d e r s t a n d t h e m i c r o s c o p i c n a t u r e o f t h i s /13/ Lynn J . K . , Moncton D.E., Thomlison W., S h i r a n e c o e x i s t e n c e . A t t h e p r e s e n t s t a g e , t h e p r e p a r a t i o n G . , and Slielton R.N., S o l i d S t a t e C o m n . ( t o b e

p u b l i s h e d ) . of v e r y homogeneous and p u r e samples of t h e s e com-

1141 Ishikawa M. and Muller J . , S o l i d S t a t e C o m n . pounds i s an e x t r e m e l y d i f f i c u l t t a s k . The e x p e r i - ( t o b e p u b l i s h e d ) .

ments have been s o f a r performed e x c l u s i v e l y On ca- /15/ For t h e d e t a i l s about t h e d e f i n i t i o n s of Tm and r e f u l l y p r e p a r e d s i n t e r e d powders, and t h e r e f o r e t h e Tc, s e e Ref. 14.

m i c r o s t r u c t u r e c o u l d n o t be p r o p e r l y i n v e s t i g a t e d . f16/ E x c e l l e n t a c c o u n t s on v a r i o u s magnetic o r d e r i n g s and r e l a t e d t o p i c s i n RE m e t a l s a r e found i n T h i s q u e s t i o n would b e p a r t i c u l a r l y i m p o r t a n t f o r c h a p t e r s 2 and 6 of m a g n e t i c p r o p e r t i e s of r a r e s t u d i e s of d i l u t e p s e u d o t e r n a r y compounds. With r e - e a r t h metals- e d i t e d by E l l i o t t R.J.,(Plenum

P r e s s ) 1972

.

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1 0 r metry around e a c h RE i o n would b e i n d i s p e n s a b l e , a s (1957) 893.

1181 De Gennes P.G., C.R. Hebd. Sdan. Acad. S c i . , w e l l as a check on a p o s s i b l e s l i g h t change of sym-

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247 (1958) 1836, Abrikosov A.A. and Gorkov L.P., metry a t t h e magnetic phase t r a n s i t i o n . N e v e r t h e l e s s , Sovi Phys. JETP

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p r i o r t o p u b l i c a t i o n , and D r . M. S e r g e n t f o r the 1221 Although t h e formula was o r i g i n a l l y d e r i v e d f o r a d i l u t e system, i t i s assumed t o b e a l s o v a l i d f o r p r e p a r a t i o n of t h e p s e u d o t e r n a r y compounds. our d e n s e magnetic systems i n t h e

r e g i o n .

1231 McCallum R.W., Ph. D. t h e s i s (1977).

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1251 Maekawa S. and T a c h i k i

M.,

(To b e p u b l i s h e d ) .