MAGNETORESISTANCE OF Sm1-xLaxS, Sm1-xYx S AND SmS1-xAsx ALLOYS

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MAGNETORESISTANCE OF Sm1-xLaxS, Sm1-xYx S AND SmS1-xAsx ALLOYS

P. Haen, F. Holtzberg, F. Lapierre, O. Peña, T. Penney, R. Tournier

To cite this version:

P. Haen, F. Holtzberg, F. Lapierre, O. Peña, T. Penney, et al.. MAGNETORESISTANCE OF Sm1- xLaxS, Sm1-xYx S AND SmS1-xAsx ALLOYS. Journal de Physique Colloques, 1980, 41 (C5), pp.C5- 181-C5-184. �10.1051/jphyscol:1980531�. �jpa-00219966�

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JOURNAL DE PHYSIQUE Colloque C5, supplément au n°6, Tome 41, juin 1980, page C5-181

MAGNETORESISTANCE OF Sm L a S , Sm Y S AND SmS As ALLOYS

1 -x x l - x x 1 - x x

P. Haen , F . H o l t z b e r g * , F . Lapierre"1", O. PeXa"1"0, T. Penney*, and R. Tournier4".

+Centre de Recherches sur les Tres Basses temperatures, CNRS, B.P. 166 X, 38042 Grenoble-Cedex, France. (Laboratoire associe a 1 'Universite Scientifique et Medicale de Grenoble)

* IBM Watson Research Center, Dept, of Physics, P.O. Box 218, Yorktown Heights, N.Y 10598, USA.

Résumé.- Nous décrivons des expériences de magnétorésistance des composés Smj_xLaxS,

^î-x^x3 e t SmSj-xASx dans la gamme de température 4,2 K - 1,5 K sous des champs allant jusqu'à 180 kOe. Le comportement observé diffère selon la présence de centres localisés de type Kondo et l'état, métallique 6u semiconducteur», du composé.

A b s t r a c t . - The m a g n e t o r e s i s t a n c e of Sm!_xLaxS, Sm^xy S and SmSi_xAsx a l l o y s h a s been measured a t 4.2 K and 1.5 K i n f i e l d s up t o 180 KOe. D i f f e r e n t b e h a v i o u r s a r e observed descending on whether or n o t Kondo c e n t e r s a r e p r e s e n t and wether semicon- d u c t i n g or m e t a l l i c phases o c c u r .

The e l e c t r o n i c t r a n s i t i o n i n SmS i n v o l v i n g a change i n t h e v a l e n c e s t a t e of Sm from

2+ towards 3+ , normally induced under p r e s - s u r e / l / can a l s o be induced a t a t m o s p h e r i c p r e s s u r e e i t h e r by r e p l a c i n g Sm by a t r i v a -

l e n t r a r e e a r t h ion / 1 - 3 / or by a l l o y i n g SmS w i t h SmP / 4 , 5 / SmAs / 6 / or SmSb /I/ . In t h i s p a p e r , we w i l l focus our a t t e n t i o n on t h r e e s y s t e m s :^{S m}i -^{x L a}x^S'^{S m}l - x^Yx^{S a n d}

SmS. As . The e l e c t r i c a l r e s i s t i v i t i e s p 1-x x and Hall c o e f f i c i e n t R„ of t h e s e a l l o y s a r e v e r y d i f f e r e n t from one system t o a n o t h e r and depend on t h e Sm v a l e n c e s t a t e .

Sm, La S forms a c o n t i n u o u s s e r i e s of 1-x x s o l i d s o l u t i o n s , t h e v a l e n c e b e i n g always i n t e r m e d i a t e between 2 and 3 / 2 / . A l l t h e a l l o y s of t h i s system e x h i b i t a m e t a l l i c c h a r a c t e r , w i t h R„ n e g a t i v e and approxima- t e l y independent of T / 8 / . T he c a r r i e r s a r e i n t r o d u c e d e i t h e r by t h e La, o t h e r t r i v a l e n t r a r e - e a r t h s or d e f e c t s which a r e a l s o p r e - s e n t i n pure SmS . For low La c o n c e n t r a t i o n s

(x < 0 .05) t h e r e s i s t i v i t y p a s s e s through a

Or

m i n i m u m near 100 K and b e l o w this increases as Log I . T h i s h a s b e e n interpreted as a

34-

K o n d o e f f e c t due to Sm w i t h localized m a g n e t i c moments acting as impurities / 8 / .

3+-

T h e concentration of Sm , deduced from m a - gnetization m e a s u r e m e n t s , w a s b e t w e e n 1%

and 3 .5% .

T h e systems Sm. Y S and S m S , A s

J 1-x x 1-x x

exhibit a large volume decrease and color change from black to gold, with increasing x.

In the Sm Y S system, stable black samples are obtained for 0 < x < 0 .15 and gold in-

° Present address : University of California Dept of ?hys. Riverside.. CA 92521, USA.

termediate valence (IV) samples for x ^ 0.15 / 9 / . All are metallic at room temperature

{RT) /10/ . However, only the gold samples with x > 0 .4 and the black samples show the normal metallic variation of p and negative R„ between RT and low temperatures . For

0 .15 < x < 0 .4 p and R„ display more compli- cated variations /10/ because cooling below room temperature converts the samples from gold to black / 9 / . These properties have been interpreted by a simple density of states model /ll/ .

In the SmS, As system, the alloys are semiconductors for x < 0 .05 and metallic at RT for higher.x / 6 / . They are IV forO.C7< x:

< 0 .5 and purely Sm for x > 0.5. There is a concentration range (0 .07 < x < 0 .15) where the alloys transform from metallic to semiconducting character on cooling below R T . The resistivity thus shows a simple metallic

variation for x :> 0 .15 (although R„ is temperature independent only for x> 0 .5) / 6 / .

Hence, it seemed interesting to compare the effect of a magnetic field on the resistivity of these different alloys . We have measured the magnetoresistance of typical alloys of each system in fields up to 180 kOe at 4 .2 K and 1-5K. However, we have elimina- ted from this comparison the alloys of the Sm, Y S and SmS. As systems which trans-

1-x x 1-x x J

form on cooling . T he resistivities at RT, 4 .2 K and 1 .5 K, of the samples studied are shown in table I . The measurements where made by the Van der Pauw method, with the magnetic field H perpendicular to the sample .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980531

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JOURNAL DE PHYSIQUE

The magnetoresistance data are also summa- rized in table I.They are obtained in per- cent, at each T , from the relation Ap/p =

( H = 0)

-

^P(R)] / ^p(H = 0)

.

Three different kinds of behavior have been observed.

1. Low La concentration alloys of the system Sml-xLaxS.- We have measured three samples in this concentration range

(x = 9.215, 0.022 and 0.05)

.

The variations of Ap/p vs H of these alloys are plotted on figure 1. Since the low temperature resistivities of these alloys exhibit a Kondo behaviour /8/, it seems very surprising that their Ap/p shows a positive maximum instead a negative variation.

Fig. 1 f l a g n e t o r e s i s t a n c e of t h r e e low c o n c e n t r a t i o n a l l o y s o f Sml-xLaxS. The c o n c e n t r a t i o n s x are g i v e n on t h e f i g u r e .

Thus, i't appears that Ap/p contains, supe- rimposed on the negative Kondo contribution, a positive contribution which increases rapidly at low fields and then increases more slowly or saturates above 50 to 100 k0e .The possibility that this positive contribution is due to rare earth impurities can prob&lybe excluded, because such a con-

tribution would saturate at a lower field, according to magnetic measurements /8/. On the one hand, this contribution is most pro- bably the normal positive magnetoresistance of the alloys, which is high, because we are dealing with semi-metals with low carrier concentrations (given by the La concentrations /8/)

.

On the other hand, the Kondo contribution due to the localized magnetic

~ m " is small because the value of T is low ( = 0 .05 K /a/) compared to the measuring temperature.Thus, the positive contribution can dominate at low fields and the Kondo contribution at high fields, leading to the maximum of Ap/p

.

2. Semiconducting alloys of the SmSl-xAsx system.- Two alloys of this phase, with nominal concentrations x = 0.01 and x = 0.04

(corresponding to the analyzed concentrations 1 .9% and 4 .5% in ref

.

/6/) have been studied

.

We have measured their resistivity between helium and room temperature, in more detail than in previous work / 6 / ) .These resistivities do not follow an activation law of the form p a exp (A/KP)

.

A plot of log p vs l/r does not show straight line behavior but a negative curvature. Taking at R T the tangents of these curves, one could get A= 75meV and 3 .7 meV for x = 0 .Ol and x = 0 .04, respectively. However a plot of log p against T - ~ / ~

,

as shown in figure 2, does give a good straight fit between -250K and % 30K for the SmSO -ggAso -O1~ample

.

^This

gives, for the hopping conductivity law

0 = A exp ( - B / r 'j4) a B = 44 K * / ~ .A similar behaviour (hopping conductivity in the range

lOOK

-

20K) was reported recently /12/ for the analogous alloy /4,5/ SmSO -9gP0 .Ol

.

^For

the SmSO .96A~0 .04 alloy, the ~ - 1 / 4 variation is not obeyed, or perhaps only over a short range of temperature : 1.5 $ T $3 .5 K, with B = 4 .9 R ~

.

/This hopping conductivity can ~ be explained by considering the model of localized Sm-ion collapse previously deve- loped for this system/6/.This model des- cribes the alloys as containing clusters of six Sm3 ions which are the nearest neigh- bours of each As. Each cluster is isolated from the others by sm2' ions. Many of the electrons, which are removed from the f

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2 + 3 +

states in the Sm -Sm transition, remain localized and non-conducting at low temperature.The clusters are coupled to form a spin glass with a T increasing from 'L 0.5K for x = 0.009 to % 9 4 K f o r x = 0.045 / 6 / . The T of the SmSO .96A~0 .04 alloy is in our

'3

resistivity measurement range, but it seems that this does notaffect the resistivity :

perhaps it may be responsible for the ab- sence of the T-~/* variation of log p below c 3 .5K.

the magnetic field, by alignment of the spins 3

+

of the Sm clusters, favours the hopping conduction process.

Fig. 3.- Magnetoresistance of two semiconducting SmS Asx alloys vs H (main curves).

InsA;% : plot of Ap/p vs the magnetization M of ref 6 (without correction for the demagnetizing factors)

-1/4 Fig.2.-Logarithmic plot of the resistivity vs T of the S m S O , g g A s O ~ O 1 and SrnS0.96A~0.04 alloys.

Figure 3 shows the magnetoresistance of these t w ~ * S m S ~ - ~ A s ~ alloys. Ap/p is stxon- gly negative and becomes roughly propor- tional to H above % 70 kOe. Using the magnetization data M, measured up to 150 kOe /6/

we have plotted Ap/p vs M in the insert of figure 3. This plot shows that A p is pro- portional to M and not to M ~ . Thus A p does not have a classical magnetic origin, as, for instance, the Kondo effect. A possible

3 .The alloys which exhibit a simple metallic resistivity

.-

^(x ⁰^.1 for SmlmxLa S,

X x

;

0.15 and x 0.4 for Sml-xyxS and x 2 0.15 for SmSl-.xl?sx).

These alloys have generally a low positive magnetoresistance, as is seen from the values of Ap/p at 150 kOe reported in table I.

Occasionnally Ap/p takes a low negative value (which can be accompanied by an increase of p from 4 .2 K to 1.5K, as for the Smo .3Lao .7S alloy)

.

Among these diverse values a positive magnetoresistance of the order of 20% at 15QkOe is observed at 4.2 and 1.5 K for the black alloy Smo .91Y0 .09S. This alloy is also remarkable for its low temperature specific heat /13/ :

it exhibits a normal y T + B T ~ behaviour, while other alloys of this phase, and SmS itself, show an additional anomaly .This anomaly has been attributed to the presence of sm3+ impurities with a localized magnetic explanation of this behaviour may be that moment. It seems clear then that this parti-

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JOURNAL DE PHYSIQUE

cular SmO .91Y0 .OgS a Table 1 Electrical resistivity at room temperature signr-f icant amount of such sm3 ⁺

.

^{This has} ^(RT),^4.2K and 1.5 K and magnetoresistance of va-

rious alloys of SmS. The values of Ap/p are given been confirmed by magnetization measurements _{for a}_field_of₁₅₀_kOe.

/8/.Thus we believe that a positive magnetoresistance represents the intrinsic behaviour for the alloys of this phase. This is also true for the other alloys considered in this chapter, particularly for the Sml-xLaxS alloys with x & Q.1, and for the Sml-xYxS alloys with x & 0.4. Among this last group, the Smo.lYo.gS alloy seems to be represen- tative with a positive A p / p of 2 ayd 8%, respectively, at 4. 2 and 1 .5 K . In the other samples, the positive intrinsic contribution

'I

is more or less cancelled by the ~egative contribution, caused by the sm3+ localized magnetic moments (in analogy to the alloys with low La concentration of the Sml-xLaxS

system)

.

The number of these ~ m * acting as magnetic impurities may vary from one sample to another, leading to the diversity of the results reported in #table I

.

Acknowledgments.- We would like to thank

Y . Cros for useful discussions about hop-

ping conductivity.

T h e magnetoresistance measurements were performed at the Service Mtional des Champs Intenses of the C N R S at Grenoble.

References

J ayaraman, A

.,

Dernier, P 9

.,

^and

Longinotti, L D

.,

^{High temp}

.

^High

Pressures

7

(1975) 1

.

.ALLOYS

Sm0.985La0.015S Smo.978La0.022S

Sm0.95 S

Sm0.4 S

Sm0.3 La0.7 S m ~ . l La0.9

'*0.93 0.07 Sm0.91

'

^0.09

Sm 0.6

'

^0.4

Sm0.3 '0.7 Sm 0.1 '0.9

Sm S o e g g X S ~ . ~ ~

Sm Asovo4

Sm '0.85 *'0.15

Holtzberg, F

.,

^{A .I .P}

.

Conf. Proc.

(1974) 478.

'4.2 (~12cm)

4 520

7 790 2 510 40.5 51.75 47.8

92.6 70.5 27.2 18.2 15.9

= 3.10'

2 570

84.0 O x T

4 610

5 000 2 560 77 73 96

'04 I o 3

63 46

3 670

1 I00

I3O

Gronau

,

M., and Methfessel, S., Phy- sics 86-88B (1977) 218.

Henry, D .C

.,

^Sisson,K J .,Savage, W .R

.,

Schweitzer, J .W

.,

and Cater, E D

.,

Phys

.

^Rev.

^g

(1979) 1985.

Pena, 2, 0

.,

Tournier, R., Senateur, J .P

.,

Fruchart, R

.,

to be published

.

Holtzberg, F

.,

^Pena,O2,

.,

^Penney,^T

.,

and Tournier, R., Valence Instabilities and Related Narrow Band Phenomena, R .D

.

Parks ed .(plenum, N.Y) 1977 p. 507

.

Beeken. R.B

..

^Savase.^W.R.. Schweitzer

1.5

4 650 9 370 2 565 39.8 54.5 47.7

92.6 70.3

18.OE 15.1

2 770 79.7

Chouteau, G

.,

^Pena,O'L

.,

Holtzberg,F Penney, T

.,

^Tournier, R., and Von Molnar, S

.,

^J

.

Physique Colloq

. ³⁷

C4 (1976) 283.

T ao, L J

.,

and Holtzberg, F

.,

^Phys

Rev. g (1975) 38.

A o I p ( 9 )

1

Penney, T

.,

and Holtzerg, F

.,

^Phys

.

Rev. Lett.* (1975) 322.

-4.2K See See See

' ⁰

+ 2.5

' 0

- ^0.5

~ 1 9 . 2

- ^0.2

+ 0.05 + 2

See See

- 0.8

Von Molnar, S

.,

^{Penney, T}

.,

^and

Holtzberg, F

.,

^J

.

Physique Colloq

.

37 C4 (1976) 241.

-

Morillo, J

.,

de Novion, C .H ., and Senateur J .P;

,

^J

.

Physique Colloq

.

40 C5 (1979) 348.

-

1.5K fig. I fig. I f i g . 1

+ 0.5

- ³

+ I

- 0.6 + 19.5

+ 0.7 + 8

fig. 3 fig. 3

- 1.25

Von Molnar, S

.,

and Holtzberg, F

.,

A .I2

.

^Conf

.

^Proc.2 (1976) 394

.

'

J .W

.,

and cater,

ED..,

phys. Rev.

(1978) 1334