SUPERCONDUCTING PROPERTIES OF MAGNETICALLY ORDERING (Me)1+x Mo6S8 MOLYBDENUM SULFIDES

(1)

HAL Id: jpa-00217577

https://hal.archives-ouvertes.fr/jpa-00217577

Submitted on 1 Jan 1978

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

SUPERCONDUCTING PROPERTIES OF

MAGNETICALLY ORDERING (Me)1+x Mo6S8

MOLYBDENUM SULFIDES

H. Freyhardt, K. Winzer, W. Schröter

To cite this version:

(2)

JOURNAL DE PHYSIQUE Colloque C6, mpplkment au

no

8, Tome 39, nofit 1978, Page C6-369

SUPERCONDUCTING PROPERTIES OF MAGNETICALLY ORDERING

(Me)l+xMo6S8

MOLYBDENUM SULFIDES

H.C. Freyhardt, K. Winzert andW. ~chrgtertt

I n s t i t u t f i i r MetaZZphysik

t I . PhtjsikaZisches I n s t i t u t

tt

I V . PhysikaZisches I n s t i t u t , Universitiit Gtlttingen, 3400 Gtittingen, Germany

R6sum6.- On a utilisd des mesures de susceptibilits magngtique entre 50 mK et 6 K pour Btudier la mise en ordre magngtique dans les composds supraconducteurs de sulfure de molybdsne du type (terre

rare)

-

Mo S 6 8'

Abstract.- Susceptibility measurements between 50 mK and 6 K were used to study magnetic ordering in superconducting (R.E.)Mo6S8 molybdenum sulfides.

The Chevrel phases of the rare earth molybde- an additional line occurs. This line is due to metal- num sulfides (R.E.) l+xM~6Se with x = 0, 0.2 become lic @-tin as can be deduced from the measured isome- superconducting /I/ except for Ce and Eu. The transi- ric shift. In addition we found evidence that excess tion temperatures Tc are generally smaller than 4 K ,

_sn

_{atoms can also occupy (0,0,0) sites. This would}

only the La and Yb compounds exhibit a Tc of -7 K and imply that compoun~s with a nominal

sn

content 'L8.5 K, respectively. Some of the No chalcogenides _20.2

are still understoich~ometric~ order magnetically /2,3/ at temperatures below T

.

In this contribution measurements of the suscept:- Magnetic susceptibilities~were measured for Pr, Nd, bility between mK and 6 were employed to study DY and Ho compounds with both x = 0 and 0.2 (figure the occurance or absence of magnetic ordering in the 1 and 2, table). In the investigated temperature superconducting state for R.E. = Pr, Nd, Dy and Ho. range above T all specimens exhibit a "Curie-Weiss From additional ~zssbauer effect investigations of law" :

X-'

a (T-El), with a negative Curie temperatu- Me,+xMo,Se (Me = Sn H O ~ - ~ , 1

>

y>O) using the re O

,

i.e. they behave antiferr~magneticall~. 11.

Y

23,8-keV irradiation of Snu 9 was ob- the temperature region above T the effective num- tained as to whether Me atoms occupy different lat- ber Bohr magnetons 9 - 7 DY12M06S8 tice sites. and for Ho Moss8 becomes appreciably larger than ₁₂

The specimens were prepared from the CornPo- _its_R,T._value. nents directly or from the Me sulfides by reacting

them in sealed quartz tubes under a protective argon atmosphere at 1340 K (48 hrs), followed by additional homogenization treatments. These ternary Mo chalcogenides are characterized by a slightly distordet ~ u b i c lattice of rhombohedral symmetry. Larger Me ions (e-g. Sn, Pb, R.E.) occupy the origin (0,0,0) and syuuuetrically equivalent sites, whereas the Mo6Ss clusters are found in the center of the rhombohedral unit cell. It was suggested / 4 / that excess Me atoms (x >O) occupy a crystallographically different posi- tion, (1/2,0,0), or that the occupancy of the origin (0,0,0) is larger than one. Our Miissbauer effect investigations /5/ at 77 K do not support any of these assumptions. Both Snl+xMosSa and ( S ~ , H O ) ~ + ~ M O ~ S ~

show one split line (anisotropic quadrupol splitting) Fig. 1 : Magnetic susceptibility versus temperature /5,6/ for x = 0, which is attributed to divalent Sn at different external fields for Ndl.0M06S8 ions at (0,0,0). With increasing Sn content(x=0.2, 0.4)

(3)

0.2

0

X - 0.2

- 0.6

TIK

Fig. 2 : Magnetic susceptibility versus temperature at different external fields for Dy1.2Mo6S8

While the Pr compounds do not show any sign of magnetic ordering for 50 mK<T<Tc a local maximum is superimposed on the (type I1 superconducting) dia- magnetic behaviour below Tc at nonzero magnetic fields for NdMo6S8

,

which can be ascribed to antiferromagnetic ordering below a critical temperature

Tm. For high enough magnetic fields

x

can become paramagnetic again at low temperatures. The ordering temperature Tm decreases slightly with increasing field. In the DylSo and Dylz sulfides ordering occurs already in zero field and xdrops abruptly below T _m' This behavior could imply the assumption that antiferromagnetic spin polarization decreases the pair breaking. The Ho compounds behave differently. They return to the state below Tm (see also 121) and the specimen becomes normal conducting al- though an antiferromagnetic behaviour occurs above Tc. For the Ho specimens both the transition and the ordering temperature are very sensitive to specimen

Transition temperatures (see Table) and upper critical fields BC2 (dBCP/dT)TC % 0.3 T/K for Pr and Nd) do not strongly depend on the R.E. content for x 5.0.2 (However, the normal/superconducting transitions become particularly sharp for x

2

0.2.). The T values in the (R.E.)l+xMo6S8 compounds must be gi- ven by a nonmagnetic contribution determined by the Mo 4d electrons and pair breaking effects due to the magnetic (R.E.13+ ions.

Transition temperatures T (50% transitions), magnetic ordering (Ti, in zerocfield) and Curie (0) temperatures of R.E. molybdenum sulfides.

To a first approximation the nonmagnetic part T O can be obtained by a linear interpolation /7/ between the T values of the La (7.0 K, R(L~~+) = 1.15

k)

and Lu (1,9 K, R ( L U ~ + ) = 0.93

i)

compounds where both trivalent ions are nonmagnetic. The ob-

served additional decrease in T :A T~=T:

-

Tc for Pr, Nd, Dy and Ho must be caused by pair breaking. If one tentatively applies the theory for paramagnetic impurities in superconductors (Abrikosov and Gorkov / g / ) A T~ should be proportional to N(E~)A'

(gL

-

112(5 + l)J (N(EF) : density of states, gL : Lande factor, A : exchange integral for the interac- tion between the spin ofc~nduction electrons and the total moment J of the R.E. ions). It could be shown, however, that this proportionality applies only if A is allowed to vary appreciably /7/, in particular for compounds of the light R.E. elements. Consequen- tly a better understanding is required for the effect of high concentrations of paramagnetic impurities, which are not statistically distributed and wnich can interact with each other.

References

/I/ Pelizzone, M., Treyvaud, A., Spitzli, P., and Fischer, 6., J.Low Temp.Phys.

2

(1977) 453. /2/ Ishikawa, M. and Fischer,

PI.,

Solid State Com-

mun

3

(1977) 37 and

g

(1977) 747.

/3/ Mc Callum, R.W., Johnston, D.C., Shelton, R.N., Fertig, W.A., and Maple, M.B., Solid State Com- mun

2

_-(1977) 501.

/4/ Sergent, M., Chevrel, R., Rossel, C., and Fis- cher,

@.,

1978, to be published.

/5/ Freyhardt, H.C., SchrGter, W., and Wagner, H., 1978, unpublished.

161 Kimball, C .W., Weber, L. ,van Landuyt, G. ,Fradin, F.Y., Dunlap, B.D., and Shenoy, G.K.,Phys.Rev. Letters

36

(1976) 412.

/7/ McCallum, R.W., Ph.D.Thesis, University of Cay lifornia,San Diego, 1977.

/8/ Abrikosov,A.A.and Gorkov,L.P.,Sov.Phys.JETP