CeCu5: Another Kondo lattice showing magnetic order

Journal of Magnetism and MagneUc Materials 63 & 64 (1987) 37-39 North-Holland, Amsterdam

37

CeCus: A N O T H E R K O N D O LATTICE SHOWING MAGNETIC O R D E R E B A U E R , E G R A T Z and C S C H M I T Z E R

Insatute of Experimental Physics, Techmcal Umvers~ty Vienna, Austria The temperature dependence of the electrical reslstlwty, thermal conductwlty, thermopower and specific heat of CeCus will be presented CeCus behaves as a Kondo latUce system with magnehc ground state properties (transmon temperature ~3 9 K) The valency of Ce m this hexagonal compound Is close to 3 and the overall crystal field sphttmg estimated from our results is about 200 K

1. Introduction Recently, in C e - C u mtermetalhc compounds a variety of phenomena such as heavy fermlon and K o n d o lattice behavlour as well as superconductivity have been found For example, C e C u 6 is known as a nonmagnetic heavy fertalon system showing an anomalous large 3'value of the specific heat [1], a giant thermopower [2] and a structural transition from the orthorhomblc to a monochnic type around 220 K [3] Most of the physical properties appear to be strongly anisotropic CeCu2Si2 was the first c o m p o u n d which was identified as a heavy fermlon system, characterized by a superconducting transition at about 0 5 K [4, 5] CeCu2 has been classified as a K o n d o lattice, exhibiting antiferromagnetism below 3 5 K [6] We have studied the CeCus c o m p o u n d which crystalhzes in the hexagonal CaCu5 structure (P6/mmm) The valency of the Ce-ion m CeCus, as deduced from a study of the lattice parameters in the R E C u s series, corresponds to a 3+ state [7]

LaCu5 in fig 2 While A(T) of LaCu5 shows a p r o n o u n c e d maximum in the low temperature region, typical for materials with minimal Impurity scattering, the thermal conductivity of CeCu5 shows no maximum and is considerably smaller over the whole temperature range Recently we have shown that the latUce thermal conductivity for good metallic conductors IS negligible [8] If we therefore neglect this contrlbutlon, the low lying A(T) curve m CeCu5 must be attributed to scattering processes of the heat carrying electrons with the magnetic moments of the Ce 3+ ions T o study the quantity of th~s contribution we have compared the thermal resistivity ( W = l/A) of CeCu5 and LaCu5 This result is shown in fig 3 in a semilogarlthmlc plot as A W T vs T {AW = W ( C e C u s ) - W(LaCus)} Two regions,

/,0

." ..o..O/ 30 ¸

'

~ -

2. Results and discussion For purpose of comparison the electrical reslstlwty p ( T ) of CeCus as a function of temperature is displayed together with those of the Isostructural nonmagnetic LaCu5 in fig 1 The reset shows a kink in p ( T ) at = 3 9 K, indicating a magnetic phase transmon m CeCu5 The thermal conductivity A(T) as a function of the temperature is plotted for CeCu5 and

CeCus LaCu5

,

,

J

20.

10'

T[K] 50

I00

150

200

250

300

Fig 1 The temperature dependence of the electrical resistivity p of CeCu5 and LaCu5 The reset shows details of the magnetic transmon of CeCu5 around 3 9 K

0304-8853/87/$03 50 O Elsevier Science Publishers B V (North-Holland Physics Publishing Division)

38

E Bauer et al d

CeCus

/

another K o n d o lamce showing magneac order

h [mW/cmK]

500 •

,~00

~

300

LaCu5 CeCus

200

too

?[K}

O"

50

100

150

200

250

300

Fig 2 The temperature dependence of the thermal conductwxty h of CeCus and LaCus

where A W T is proportional to ( - I n T), separated by a m a x i m u m around 1 9 0 K are observable This figure includes also the t e m p e r a t u r e dependence of Pspd (magnetic contrtbutlon to the electrical resistivity) Note, m both A W T vs T and p~vo vs T the same qualitative behavlour can be found T h e different slopes of the two ( - l n T ) regmns and the m a x i m u m which separates them can be understood in the scope of a model introduced by Cornut and Coqbhn [9] A c c o r d m g to this model the current results are characteristic for a K o n d o system with an overall crystal field sphttlng t e m p e r a t u r e around

~,wr f 1 0 2 C m l ~ ' ' w ]

oOsp d Qa.,~cm] 8

200 K However, it should be noted that the position of the m a x i m u m t e m p e r a t u r e depends sensitively how the electron phonon contribution has been defined [10] In fig 4 the temperature dependence of the t h e r m o p o w e r S ( T ) of C e C u s is drawn For higher temperatures a saturation tendency ~s visible from this m e a s u r e m e n t After changing the sign, S ( T ) of CeCu5 reaches a minimum near 25 K, followed by a small m a x i m u m around 7 K A very similar S ( T ) behaviour has recently been published for the tetragonal heavy fermlon system CeCu4Als [ I 1 ] Apart from the low t e m p e r a t u r e peak these results resemble those of CeAI2 and CeCu2 [12, 6] which are regarded as typical K o n d o latttces showing magnetic ground state properties However, the absolute value of the t h e r m o p o w e r minimum of C e C u s ts somewhat smaller than those reported for CeAI2 and CeCu2 We believe, that this distinct S ( T ) behavtour confirms the K o n d o lattice property of C e C u s as also deduced from the electrical and the electronic thermal reslstWltles Followm g a discussion of Brandt and Moshchalkov [13] for K o n d o systems with TK < A< v ( A c t - overall crystal field splitting) the lowering of the temperature from T >>Acv to T ~ At ~ might result m a change of sign m the t h e r m o p o w e r This seems to reflect the crossover from the '1 = 5/2' state, realized for T >> ACF, to the '1 = 1/2' situanon for T < ACF at finite temperatures A m o n g Ce based K o n d o systems the sign inversion is

7 6 5

61 s ;pV/Kl

51

E

4 3

3'

2

2¸

t o-

J

5

,o

2'o ~o so r [K]

,oo

~oo ~oo

1

r [K]

-0

~,~¢so

7o0

~o

20o

25o

aoo

-1

Fig 3 Pspd and A W T vs In T for CeCu5 (A = Psr~, O = A W T ) P~paand A W denotes the magnetic contribution to the electrical and the electromc thermal reszstw~ty

Fig 4 The temperature dependence of the thermopower S of CeCus

E Bauer et a l /

CeCu5 another Kondo latace showing magnetic order

also observed in CeCu2SI2 (TK ~ 8 K, ACF ~ 140 K), CeAI2 (TK ~ 10 K, A c r e 9 0 K), CeAI3 (TK~SK, A C F ~ 5 0 K ) (ref [13] and refs thereto) and CeCu2 ( T K ~ 1 0 K AcF~200K) [6] In various K o n d o compounds (e g CeAl2, C e C u 2 S 1 2 , C e C u 2 ) the t h e r m o p o w e r m l m m u m appears at about Tmm-~ 2 TK If we estimate the K o n d o t e m p e r a t u r e from this S(T) measurement, we obtain a value around 12 K In fig 5 the specific heat of CeCu5 is plotted as cp/T vs T F r o m the posihon of the p e a k the magnetic phase transition has been derived as 3 9 K A high t e m p e r a t u r e extrapolation ( T > 10 K) of the cp/T vs T 2 data yields a y-value of ( 1 0 0 ~ 10) m J / m o l K 2 T h e y-values of CeAl2, CeCu2, CeSix 8 and UCu5 have been found in the same order of magnitude, which seem to be typical for magnetically ordered K o n d o c o m pounds T h e magnetic contribution to the specific heat Cm plotted as reset lU fig 5 has been obtained by substracting the electronic and the lattice part from the total heat capacity (cm[mJ/molK] = co - 1 0 0 T - 0 8 2 T 3) T h e m a g netic entropy Sm as a function of temperature is also shown in this inset T h e value attributed to the magnetic phase transition amounts to 4 2 J / m o l K This ts approximately 70% of the value one would expect for a c o m plete removal of the twofold spin d e g e n e r a c y of the crystal field ground-state doublet ( A S = cp/T

EJ / m o l e

K 2]

c~ I r [J/mote K22

Sm [J/mole KJ

o 4

g

4

3

)

•

2

g °°

r [K! o

f

\ °°°a°°°°°°°°~°~°o°°°°

2

° ° ° ° ° ¢~ ° ° ° ° ° ° ° ° ° ° ° °

T/K./

z;

Fig 5 A cv/T vs T plot of CeCu5 The inset shows cm/T vs T and Sm vs T (Cm, Sm denotes the magnetic contnbutlon to the specific heat and the magnetic entropy, respectwely)

39

R In 2 = 5 76 J/molK) We beheve that this substantlal reduction of the magnetic entropy cannot be referred to short range order effects in the paramagnet~c t e m p e r a t u r e range, since Cm vanishes for T > 1 0 K In comparison with CeAl2 and C e C u 2 w e therefore conclude also for CeCu5 a K o n d o - d e r i v e d reduction of the C e - m o m e n t s Finally, it should be noted, that in the c J T v s T and also in the p v s T d a t a j u s t below the magnetic phase transmon temperature an anomalous behavlour has been detected which possibly is inferred from an instability of the magnetic structure Analogous observations were made in CeAI2 and CeB6 below the m a g netic ordering t e m p e r a t u r e [14] We thank the "Hochschuljubilaeumsstlftung der Stadt Wien" for financial grant Part of the work was supported by the "Fonds zur Foerderung der Wlssenschafthchen Forschung" (proJect n u m b e r 6104)

References [1] G R Stewart, Z F~skand M S Wlre, Phys Rev B 3 0 (1984) 482 [2] A Amato, D Jaccard, E Walker and J Flouquet, Sohd State Commun 55 (1985) 1131 [3] E Gratz, E Bauer, H Nowotny, H Mueller, S Zemlrh and B Barbara, J Magn Magn Mat 63&64 (1987) 312 [4] F Steghch, J Aarts, C D Bredl, W Lleke, D Meschede, W Franz and H Schaefer, Phys Rev Lett 43 (1979) 1892 [5] F Steghch, Physlca 130B (1985) 145 [6] E Graiz, E Bauer, B Barbara, S Zemlrh, F Stegllch, C D Bredl and W Lleke, J Phys F 15 (1985) 1975 [7] D Glgnoux, F Glvord, R Lemalre, H Launols and F Savetat, J de Phys 43 (1982) 173 [8] E Bauer, E Gratz and G Adam, J Phys F 16 (1986) 493 [9] B Cornut and B Coqbhn, Phys Rev B 5 (1972) 4541 [10] D Wohlleben and B W~ttershagen, Advan Phys 34 (1985) 403 [11] U Rauchschwalbe, U Gottwlck, U Ahlhelm, H M Mayer and F Steghch, J Less-Common Metals 111 (1985) 265 [12] E Bauer, E Gratz, E Mlkovlts, H Sasslk and H Kirchmayer, J Magn Magn Mat 29 (1982)192 [13] N B Brandt and V V Moshchalkov, Advan Phys 33 (1984) 373 [14] R Schefzyk, M Peschke, F Steghch, K Wmzer and W Assmus, Z Phys B 60 (1985) 373