THE INFLUENCE OF Sm3+ IMPURITIES ON PROPERTIES OF PURE SmS AND ITS ALLOYS WITH LaS

(1)

HAL Id: jpa-00216563

https://hal.archives-ouvertes.fr/jpa-00216563

Submitted on 1 Jan 1976

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.

THE INFLUENCE OF Sm3+ IMPURITIES ON

PROPERTIES OF PURE SmS AND ITS ALLOYS

WITH LaS

G. Chouteau, O. Pena, F. Holtzberg, T. Penney, R. Tournier, S. von Molnar

To cite this version:

(2)

JOURNAL DE PHYSIQUE Colloque C4, suppliment au no 10, Tome 37, Octobre 1976, page C4-283

THE INFLUENCE OF Sm3

+

IMPURITIES ON PROPERTIES

OF

PURE SmS AND ITS ALLOYS WITH LaS

G. CHOUTEAU ("), 0. PENA (") F. HOLTZBERG ("*), T. PENNEY (""")

R. TOURNIER ("") and S. VON MOLNAR (**")

RBsum6. - Les composes du type SmS presentent un certain nombre d'anomalies inexpliquees. La rksistivite de SmS et des alliages S ~ I - ~ L ~ ~ S montre une anomalie de typeKondo avec un minimum bien marque suivi dans quelques cas d'une variation en log T

a

basses temperatures. Ceci ne peut btre une propriCt6 intrinsbque du materiau car les niveaux excites sont, dans ce cas, trop loin en energie au-dessus du niveau fondamental singulet pour btre

a

l'origine d'une rQistivitemagnktique. Recemment la presence d'ions Sm3+ a kt6 detectbe par resonance electronique dans la region de temperature ou l'effet Kondo est observe : entre 1,2 et 20 K. On a pu estimer, d'aprks les mesures de R. P. E., que la concentration de ces ions, probablement dus aux defauts, est comprise entre 0,s

et 5 %. Les mesures de chaleur specifique entre 1 et 4 K ont permis de detecter la presence. d'anomalies non intrinskques aux materiaux dans SmS pur et quelques alliages avec Y. Nous avons Btudie l'aimantation jusqu'a 150 kOe & 1,4 K des Bchantillons qui avaient kt6 utilises pour la mesure de la resistivite. Les ions Sm3+ porteurs d'un petit moment sont satures. A trbs basses temperatures, jus-

qu'8 70 mK, dans un champ de 3 kOe, on sature les autres impuretQ parasites ayant un plus grand

moment. Nos mesures permettent de determiner le mbme ordre de grandeur de concentration en Sm3+ que les experiences de resonance electronique.

Nous concluons que ces composes contiennent des ions Sm3+.-11s representent un nouvel exemple d'impurete de terre rare donnant l'effet Kondo. L'analyse des resultats de chaleur specifique per- met de conclure raisonnablement que l'anomalie due aux ions Sm3+ est un effet a une impurete plut6t qu'un effet d'interaction entre impuretks.

Abstract.

-

Compounds similar to SmS exhibit a number of unexplained anomalies. The resistivity of SmS and the alloys S ~ I - , L ~ ~ S show a pronounced Kondo type resistivity minimum and in several cases a Log T behaviour at low temperature. This behaviour can not be intrinsic for the excited levels are too far in energy from the singlet ground state to be the origin of the magnetic resistivity. Recently the presence of Sm3+ has been detected by electron paramagnetic resonance in the temperature region where the Kondo effect is observed : between 1.2 and 20 K. The number of Sm3+, which are probably due to crystalline defects or vacancies, is estimated from the EPR to be between 0.5 and 5 %.

Measurement of the specific heat in pure SmS and alloys with Y, between 1 and 4 K, have shown the presence of non-intrinsic effects. We have studied the magnetization to 150 kOe and 1.4 K i n the same samples as those used for the resistivity. The Sm3+ which have a small moment are saturated. At very low temperature, 70 mK, and 3 kOe field, the other impurities with a larger moment are saturated. Our measurements give the same order of magnitude for the Sm3+ concentration as the EPR measurements.

We conclude that these compounds contain Sm3+. They are a new example of a rare earth impurity which causes a Kondo effect. Analysis of the specific heat results show that the Sm3f anomaly is a single impurity effect and not an effect of interaction between impurities.

Magnetic rare earths generally give a magnetic resistivity independent of T above the ordering temperature. The mixing of the 4f levels with the Fermi sea is generally unimportant and an ionic description o f the magnetic properties is therefore possible.

(*) Service National des Champs Intenses, Centre National de la Recherche Scientifique, B. P. 166 Centre de Tri, 38042 Gre- noble-Cedex, France.

(**) Centre de Recherches sur les Trks Basses Tempkratures, Centre National de la Recherche Scientifique, BP 166 Centre de

Tri, 38042 Grenoble-Cedex, France.

(***) IBM Research Center, Yorktown Heights, N. Y. 10597,

U. S. A.

The observation of the Kondo effect on the resistivity behaviour of alloys containing rare earths h a s been strictly limited up t o now t o Cerium, Ytterbium and very recently to Praseodymium [I].

SmS is known t o be a semiconductor [2] which becomes metallic a t a pressure of 6.5 kbar. A large increase of the resistivity [3] has been observed in the collapsed phase between 50 K and 2 K. Furthermore, a

low temperature specific heat anomaly, in addition to

the large linear term, is seen [3]. Similar effects are

observed in Sm, _,Y,S [4].

We present here the resistivity of samples i n t h e

(3)

(3-284 G . CHOUTEAU et a[.

Sm, -,La$ system which contain well pronounced minima. The concentrations studied are degenerate n-type semiconductors with a measured Hall constant approximately independent of the temperature, indicating a metallic behaviour. The carriers can be introduced either by La, other trivalent rare-earths or defects which are present in pure SmS depending on the sample preparation. We will argue that the resistivity anomaly is attributable to the Kondo behaviour of Sm3' impurities. The presence of such Sm3+ impurities has been previously recognized by EPR measurements [5] of pure SmS a t low temperature between 1.4 and 20 K. The samples contain a concentration 0.5 %-5

%.

The fondamental ground state of Sm3+ is the doublet T7. The mean experimental g value for all directions of the crystal is

g

= 0.70

1

0.02. This ~and-5 factor is apparently enhanced by a factor 1.5 compared to the theoretical value. This is probably due to the exchange coupling between Sm3' moment and its Sm2' nearest neighbours. Thus the expected value for the saturation magnetization in the ground state is 0.35 Bohr magne- ton.

We shall also present here some transport properties of SmS. Some of these materials appear to be metallic and others have a semiconductor behaviour. Two exam- ples of each are given. In addition a systematic study of magnetization allows us to confirm the presence of Kondo impurities.

1. Transport properties. - In figure 1 we have plotted the resistivity of different specimens of Sm, -,LaXS measured between 4 K and 300 K. When the concentration x decreases the number of conduction electrons decreases and the resistivity increases. As in Kondo systems the temperature of the resistivity minima change very little with concentration. The resistivity and Hall effect of 10 SmS samples with different properties have been studied. Some are semiconductors. Their resistivity and their Hall constant vary strongly with temperature. Several samples are metallic with a well pronounced resistivity minima and a Hall constant varying little with the temperature. One example is given in figure 2. The metallic behaviour of Sm,-,La$ is also indicated on the same figure (Sm,.,7~Lao.022S) for which the Hall constant is completely independent of the temperature.

2. Interpretation of the transport properties.

-

The two metals YS and LaS show a normal temperature dependence of the lattice resistivity. They are consi- dered as simple metals. Their Hall constant is consistent with one electron per rare earth ion. From it, we derive a carrier concentration of 2.8 x ~ m - ~ and 2.5 x 10'' cmW3 for YS and LaS respectively, while the X-rays densities give values of 2.4

x

lo2' ~ r n - ~ and 2.3 x 10'' respectively. It has been shown [6] by adding YS to SmS that the conduction electrons are introduced by YS and sup- port a one-band model. We interpret our results in

FIG. 1. - Kondo resistivity of ( S ~ I - , L ~ ~ ) S .

5000 COOO- 3000- metal-li ke Srn S I I I I I

.

P ( p R . c m )

f

.

_.

.

_.

*'..

'197% La0.022s * a .

.

-

FIG. 2.

-

(a) Resistivity versus temperature for metal-like SmS.

(b) Hall constant versus temperature for metal-like SmS (circles)

(4)

INFLUENCE OF Sm3f IMPURITIES ON PROPERTIES OF PURE SmS AND ITS ALLOYS WITH LaS C4-285

the same manner. The negative Hall constant of Sm,~,,,Lao~022S gives an electron concentration approximately two times smaller than the Lanthanum concentration.

FIG. 3. -(a) Total resistivity of two S ~ I - ~ L ~ ~ S samples. By considering that the straight line corresponds to the log T behaviour (b) of the Kondo resistivity, we obtain for the lattice

resistivity the curves in figure 4.

The behaviour of the resistivity plotted versus log T is given in figure 3b. We see the possibility of log T

behaviour. Assuming that this behaviour exists up to 300 K, the Matthiessen rule is obeyed, and taking the difference between the total resistivity and this log T term, we obtain a lattice resistivity which takes the Bloch Gruneisen form. If we superpose the T term of this resistivity above 50 K and the LaS resistivity, the scale factor is approximately the Lanthanum concentration. Therefore we conclude that the resistivity contains a log T term and the concentration of conduction electrons is given by the Lanthanum concentration (Fig. 4).

We estimate a lower limit of the concentration c of Kondo impurities by taking the resistivity at 4 K and introducing in it the following formula for the residual resistivity of Kondo impurities at T = 0 K. [73,

FIG. 4. - Lattice resistivity of two Sml-xLaxS samples. Resisti- vity of LaS (notice the change in vertical scale).

In the free electron model this formula becomes 181

z is the number of conduction electrons per atom.

6, is the phase shift of the non magnetic 4f virtual bound state. As Coqblin did for Cerium impurity we assume that

In the approximation the width A,,, of the virtual

bound state is assumed to be small compared to its distance in energy to the Fermi level.

We neglect the resistivity of Lanthanum impurities compared to the contribution of the Kondo impurities.

z is taken equal to the Lanthanum concentration. We obtain c = 0.007 for x = 0.015, c = 0.026 for

x

= 0.022, c = 0.042 for x = 0.05, c = 0.035 for

x

= 0.1, c = 0.028 for x = 0.2.

All c values for Sm3 + concentration are in the range

(5)

Cl-286 G. CHOUTEAU et al.

shows the Kondo effect in the same range of temperature due to the proximity of the magnetic excited states which are populated in this temperature range.

3. Magnetization study and interpretation.

-

In order to determine the Sm3+ contribution to the total magnetization of Sm, -,La,S alloys we have perfor- med magnetization measurements up to 150 kOe, down to 1.5 K.

Due to the Kondo effect and the small moment per Sm3+ we expect that a very large external field is necessary to reach saturation. The precision of this measurement is sufficiently good to determine the end of the small curvature in the magnetization above 70 kOe (Fig. 5). We extrapolate the slope X H , of the

FIG. 5.

-

Magnetization curves up to 150 kOe fields and T = 1.5 K for different alloys (saturation UHF and magnetiza-

tion dope XHF are defined in the text).

be deduced after extrapolation of the slope

x,,

at H = 0. a,, is smaller than a,,.

x,,

is larger than

xHE.

All the quantities for the different alloys are given in the table.

a,, is the saturation magnetization of rare earth impurities which carry a large moment and are paramagnetic without Kondo condensation. The fact that XLF is higher in some cases than the high field slope

x,,

indicates that the Kondo impurities are far from saturation, have lost their magnetic moment and contribute to the slope of the magnetization in a

3 kOe field. Thus, T = 0.05 K is a temperature lower than or of the order of TK, the Kondo temperature.

FIG. 7. -

xc

T versus T diagram for Smo.gssLao.ol5S.

We have also studied the initial part of the magnetization in low fields from T = 0.05 K to 4 K. Above T = 0.5 K the susceptibility follows (Fig. 7) the Curie law within the experimental error.

C

magnetization in high field down to zero field and we

xi=^+

Xo. obtain a total saturation a,, which contains the contri-

bution of Sm3+ atoms plus other accidental rase earth XO is in most cases equal to XHF except for the metallic impurities. In order to separate the two contributions SmS- Since we expect that

we have measured the magnetization (Fig. 6 ) between _CRE _{C ~ m 3} +

0 and 3 kOe down to 0.05 K. A saturation a,, can Xi =

-

_T

+

--- _{T + T K}4- Xo

.

C

74 s s i d - i k

/

5 0 v Srn S sern~conductor

4 4 74

'

Sm.91 ".09

Then TK must be at least an order of magnitude lower

than the lowest temperature for which the Curie law is obeyed. So

T'

2i 0.05.

x0

=

xHF

is the Van Vleck part of the susceptibility independent of temperature except for the metallic SmS which probably has a higher Kondo temperature. The fact that

x0

=

xH,

proves that the magnetization of Sm3+ in all alloys is saturated in a 100 kOe field and that the analysis we have presented is self-consistent except for the metallic SmS.

Thus we are able to attribute d o = cHF

-

0 'and ~ ~

AX =

xLF

-

xHF

to Kondo impurities. In figure 8 we have established a linear relation between the two

FIG. 6.

-

Magnetization curves at very low temperature and low quantities, the initial susceptibility AX measured fields (saturation ULF and magnetization slope XLF are defined around = Oeo5 and the Sm3'

(6)

INFLUENCE OF Sm3+ IMPURITIES ON PROPERTIES OF PURE SmS AND ITS ALLOYS WITH LaS _C4-287

seen from the low field saturation at low temperatures. The curve shown for Smo~glYo,ogS measured to

3 kOe is essentially a straight line and has the same slope as the curve measured to 150 kOe. This indicates that the concentration of Sm3+ in this sample is too low to be detectable in the magnetization measurement and is therefore negligible. The specific heat of many samples of SmS and its alloys shows an anomaly between 1 and 4 K which has been attributed to an impurity effect [4, 51. For those samples on which we have measured both the specific heat and the saturation magnetization we find that when there is a specific anomaly we observe a saturation whereas in the specific case of Smo~g,Yo,,gS where is no detectable saturation there is no observable specific heat anomaly in the

FIG. 8. - Kondo susceptibility AX of Sm3+ ions as a function

of its saturation magnetization do. temperature range of interest. We conclude that the

impurity is Sm3+. effect of Sm3+ present in Sm, -,La,S alloys and exclu-

des the existence of a spin glass ordering between Sm3 moments.

The concentration of Sm3+ impurities deduced from the saturation magnetization Aa is indicated in the table. The samples on which we have measured the resistivity and the magnetization are not exactly the same but come from the same crystal growth. The concentration of Sm3 + determined by the two methods

are in accord. The table and figure 6 both show two families of compounds prepared at different times from different rare earth sources. The two upper curves of the Sm, -,La.$ system show the presence of a least ten times the amount of accidental rare earth impurities compared with the lower group of curves as can be

For the metal like SmS the high field susceptibility

xHF

is almost the same as that of semiconducting SmS (see table), consequently we think that in both cases we have the same type of impurity. In the case of metal like SmS,

xo

is almost independent of temperature between 1 and 4 K. This means that the Kondo

temperature in this material is perhaps of the order of

1 K or more.

In conclusion, we have detected the presence of Sm3+ impurities in the Sm,-,La$ system by resistivity and magnetization experiments. We confirm the EPR measurements of Walsh et al. [6] which have established that Sm3+ is present in many samples with different concentrations below 5

%.

We have shown for the first time that Sm3+ is a new rare earth Kondo impurity.

Magnetization data for Sm,-,La$ alloys. For comparison a Smo~g,Yo~og sample is included

All data are given in emu/g.

xHF

: Susceptibility measured between 70 and 150 kOe at 1.5 K.

CTHF : Saturation magnetization deduced from the high field measurements. xLF : Susceptibility measured between 1.5 kOe and 3 kOe below 1 K (see text).

oLF : Saturation magnetization deduced from the measurements below 1 K.

(7)

C4-288 G . CHOUTEAU et al.

References

[I] LETHUILLIER, P., HAEN, P., Phys. Rev. Lett. 35 (1975) 1391. VON MOLNAR, S., PENNEY, T. and HOLTZBERG, F. (These

[2] BUCHER, E., NARAYANAMURTI, V. and JAYARAMAN, A., J. proceedings).

Appl. Phys. 42 (1971) 1741. [5] WALSH, W. M., BUCHER, E., RUPP, L. W. and L O N G I N O ~ , L.

D., A. I. P . Conf. Proc. 24 (1975) 34.

JAYARAMAN2

*.'

NARAYANAMURT19 V" BUCHERy E' and _[6]_PENNEY,_{T. and HOLTZBERG,}_{F., Phys. Rev. Lett.}34 (1975)

MAINES, R. G., Phys. Rev. Lett. 25 (1970) 1430. ₁₇₇_---.

[31 S. D.3 PHILLIPS, N. E. and McWmN, D. B.9 Phys. _[7]_KONDO,_J.,_{Solid State Physics}_{(Academic Press, New York)}

Rev. B 7 (1973) 4686. _1969,_Vol._XXIII.

[4] VON MOLNAR, S. and HOLTZBERG, F., A. I. P. Conf. Proc. 29 [8] COQBLIN, B., MAPLE, M. B. and TOULOUSE, G., Intern. J.