Magnetic properties of a new Kondo lattice compound: CePt2Si2

Volume 117, number 3

PHYSICS LETTERS A

4 August 1986

MAGNETIC PROPERTIES OF A NEW K O N D O L A T r l C E C O M P O U N D : CePtzSi 2

D. G I G N O U X , D. SCHMITT, M. Z E R G U I N E Laboratoire Louis Nbel, CNRS, 166X, 38042 Grenoble Cedex, France

C. A Y A C H E and E. B O N J O U R Service des Basses Tempbratures, DRFG/CENG, 85X, 38041 Grenoble Cedex, France Received 29 April 1986; accepted for publication 3 June 1986

Measurements of magnetic susceptibility, electrical resistivity and specific heat on CePt 2Si 2 and LaPt 2Si2 compounds are reported. Several anomalous properties are observed in CePt/Si2, characteristic of Kondo lattice and Fermi liquid systems with indication of coherence effects below 2 K. Since a few years a growing experimental and theoretical interest has been focused on new cerium systems which present exciting anomalous lowtemperature properties [1,2]. They constitute the class of heavy fermion, Kondo lattice or mixed valence systems, according to the value of the characteristic temperature T K. We present here the magnetic properties of the CePtzSi 2 compound, which appears to belong to the Kondo lattice group, with indications of coherence effects at low temperature. The compound LaPtzSi2 has also been investigated, solely in order to determine the lattice behaviour. The RPt2Si 2 compounds (R = rare earth) crystallize in a tetragonal structure close to the bodycentered ThCrzSia-type structure, the exact crystal structure being not precisely known until now. Early studies revealed a random occupation of both the 4d and 4f sites by Pt and Si atoms [3]. More recently the presence of additional lines in X-ray diffraction patterns suggested that these compounds could adopt a primitive tetragonal CaBe2Gez-type structure [4]. Our X-ray diffraction patterns seem to confirm this latter assumption. Polycrystalline samples of GePt2Si 2 and LaPt 2Si2 were prepared by melting a stoichiometric amount of pure elements in a cold-crucible induction furnace. The lattice parameters were found to be a = 4 . 2 6 ,~ and c = 9.80 A for

CePt2Si 2, and a = 4 . 2 8 A and c = 9.87 A for LaPt2Si2, in good agreement with literature. Annealing the samples at 800°C for 3 days did not seem to appreciably modify these values. Magnetic measurements have been performed by the extraction method in a superconducting coil between 1.5 and 300 K in fields up to 8 T. The temperature dependence of the low field reciprocal susceptibility of CePt 2Si 2 is shown in fig. 1. Above 150 K, it follows a Curie-Weiss law, leading to an effective moment of 2.57/~B close to that of the Ce3+ion (2.54t~B) and to a large negative paramagnetic Curie temperature 0p = - 8 6 K. At lower temperature the susceptibility exhibits a wide maximum of 4.5 × 10 -3 e m u / m o l e around 60 K. Below 20 K, a strong increase of the susceptibility occurs together with a more and more pronounced negative curvature of the field dependence of the magnetization. These effects could be interpreted as the contribution of about 1% of Ce3+-like paramagnetic impurities carrying full moments. Subtracting such a contribution leads to the intrinsic susceptibility of CePtzSi 2 shown by the dashed line in fig. 1. This susceptibility is almost constant below 20 K reaching 3.6 × 10 -3 e m u / m o l e at 1.5 K. Similar temperature variations of the magnetic susceptibility have been o b s e r v e d in several c o m p o u n d s such as Celn3_xSn x [5], CeRu2Si 2 [6] or CeA13 [7]. How-

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V o l u m e 117, n u m b e r 3

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4 A u g u s t 1986

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Fig. 1. Temperature dependence of the low field reciprocal susceptibility in CePtzSi 2. Dashed line corresponds to values corrected for impurity effects (see text),

ever, in these latter two c o m p o u n d s the susceptibility maximum occurs at a much lower temperature (10 and 0.7 K respectively). Contrary to recent magnetic measurements indicating a N4el temperature of 6 K in CePtzSi 2 [4], no magnetic ordering has been detected in our sample down to 1.5 K. The electrical resistivity of CePt2Si 2 as well as of the non-magnetic c o m p o u n d LaPt2Si 2 have been measured between 1.5 and 300 K using an ac four-probe method on parallelepipedic samples spark-cut from the original ingot. Both curves are shown in fig. 2. The main feature is the large m a x i m u m around 76 K in CePtzSi 2, associated with a steep decrease below this temperature and a smoother one above. This variation is very similar to that observed in CeA13 [8] and CeCu 6 [9], where the m a x i m u m occurs however at a lower temperature. After correction for the p h o n o n contribution, taken as the LaPt2Si 2 curve (in the 146

normal state, see below, and shifted so that p(LaPt2Si2) ~ 0 when T ~ 0), the same pronounced m a x i m u m occurs at 72 K, followed by a logarithmic decrease over a wide temperature range (inset of fig. 2). This high temperature behaviour is characteristic of a K o n d o effect in CePt 2Si2. On the other hand, the low temperature variation of the resistivity follows a quadratic ( A T 2) law up to 12 K with a coefficient A = 0.31 ~£ c m / K 2. This variation is typical of Fermi-liquid systems and has been observed in other compounds such as CeA13 [7] or CeCu2Si 2 [10] where this behaviour has been attributed to coherence effects developing at low temperature. However, in CePt2Si 2 the T 2 variation range is much wider while the coefficient A is noticeably smaller than in CeA13 and CeCuzSi 2. It is worth noting that the residual resistivity of both CePt2Si 2 and LaPt2Si 2 are rather large (105 and 390 ~2 cm respectively), that could be due to

Volume 117, number 3

PHYSICS LETTERS A I

f

4 August 1986

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Fig. 2. Temperature dependence of the resistivity in LaPt2Si 2 and CePt2Si 2. Note the superconducting transition at 2.2 K in LaPtzSi 2. Inset: magnetic contribution in CePt zSi z, after correction for the lattice.

metallurgical defects in our polycrystalline samples. At last, a superconducting transition occurs in LaPt2Si 2 at T~= 2.2 K (see fig. 2); this has been confirmed by the diamagnetic behaviour of the magnetization curves below T~ (critical field H~ = 3 kOe at 1.8 K). Specific heat measurements were performed by the adiabatic dynamic method [11] between 1.5 and 130 K on CePt2Si 2 and LaPt2Si 2. The variations of C/T as a function of T 2 in the low temperature range are shown in fig. 3. For LaPt2Si 2 the variation is linear between 2 and 10 K; that allows us to deduce the electronic and lattice contributions to the specific heat (Yea = 4 m J / m o l e K 2 and /3ca = 0.9 m J / m o l e K 4 respectively). The anomaly around 2 K corresponds to tile superconducting transition in agreement with resistivity and magnetic measurements. For CePt2Si 2 a linear variation with the same slope as in LaPtzSi 2 is observed between 5 and 10 K, indicating an identical p h o n o n contribution in this temperature range, as expected for compounds with the same structure and molar masses very close to each other; also this/3c~ value agrees well with that of CeCu2Si2 [12] after correction

for the different atomic masses. On the other hand, extrapolating the linear behaviour of C/T down to 0 K leads to a relatively large electronic contribution ¥ce = 80 m J / m o l e K 2. Below 5 K, C/T strongly increases, reaching a m a x i m u m close to 120 m J / m o l e K 2 around 2 K. Such a m a x i m u m has been also observed in other non-magnetic K o n d o lattice systems such as CeA13 and CeCu2Si 2 [13] (Tin, X= 0.5 K), o r C e C u 6 [14] (Tma × = 0.3 K), where this characteristic temperature is however clearly lower. In these c o m p o u n d s the m a x i m u m in C/T has been attributed to coherence effects developing between Ce ions in a K o n d o lattice at low temperature. Furthermore, the magnetic specific heat of CePt2Si 2, obtained after subtraction of the LaPt2Si 2 curve, evidences a Schottky-like anomaly near 70 K, with a magnitude of about 5 J / m o l e K. That could be associated with the presence of a crystal field doublet lying around 180 K above the ground state doublet, this behaviour being very similar to that observed in CeRuzSi 2 [15]. F r o m the experimental results reported above for CePtzSi 2, it appears that this c o m p o u n d presents anomalous properties at low temperature, 147

Volume 117, number 3 0.2

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PHYSICS LETTERS A

I

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Fig. 3. C/T versus T 2 in LaPt2Si 2 and CePt2Si 2 (C is the specific heat). For CePt2Si 2, the two symbols correspond to two successive measurements. The anomaly at 2.2 K on the LaPtzSi 2 curve is associated with the superconducting transition.

which are reminiscent from those found in intermediate valence compounds [16] or Kondo lattice systems [1,2]. Although most of the experimental results are in favour of a Kondo lattice compound, the existence of mixed valence effects cannot be ruled out in CePt2Si 2, at least at low temperature, for several reasons. First a preliminary thermal expansion experiment shows no anomaly between 4.2 and 150 K, but rather a continuous and rather strong increase of the relative change of length (6l/l-3 × 10 3 between 4.2 and 150 K) [17]. This variation, intermediate between normal and mixed valence behaviour, is very similar to the one observed in CeA13 [18], except that the overall change of length is three times larger in CePt2Si 2. If the valence of cerium is actually 3 between 150 and 300 K from susceptibility measurements, a valence slightly higher than 3 does not seem to be impossible at low temperature in CePt2Si 2. Ad148

4 August 1986

ditional experiments such as inelastic neutron scattering should clarify this point. Furthermore, it is worth noting that the A T 2 law observed on the resistivity below 12 K might be also attributed to an intermediate valence behaviour. Indeed, as observed in CeCu2Si 2 under pressure [19], the range of validity of such a law increases while the A value decreases, when going from Kondo lattice to mixed valence regime. The situation of CePtzSi 2 again is intermediate between these two limits, with respect to both the quadratic T-dependence range and the value of the coefficient A. An investigation of CePt2Si 2 under pressure should therefore be very fruitful. Another problem consists to evaluate the Kondo temperature T K in CePtzSi 2. Considering singleion theory of independent Kondo impurities, the relations T K = O . 6 8 R / Y m a × and T K = 0 . 1 5 / X ( 0 ) can be used [2,20], which give T K = 47 K and 42 K respectively in CePt 2Si2, values much larger than in CeAI 3 (3 K) and CeCu2Si 2 (4.5 K) [13] or CeCu 6 (3.9 K) [14]. Another estimation of T K comes from high temperature susceptibility, i.e. T K = - m o p , the constant m being of the order of unity depending on the model (m = 0.25 [2], 0.5 [21], - 1 [201). That leads to T K values ranging from 21 to 86 K. These three results seem to situate CePt2Si 2 in the Kondo lattice regime, between the heavy fermion behaviour (T K < 14 K) and the mixed valence regime ( T K > 100 K) [22]. The last interesting characteristic of CePtaSi 2 is the possible onset of coherence effects developing below 2 K. This coherence temperature is larger than in other compounds such as CeCu zSi2, CeA13 or CeCu 6 and should allow one, if confirmed, to study these effects over a wide range of temperature• As a conclusion, CePt2Si 2 appears to be a very interesting system, where the energy scales for coherence, Kondo lattice and crystal field effects seem to be well separated• Further experiments have to be performed in order to state these properties more precisely. References [1] G.R. Stewart, Rev. Mod. Phys. 56 (1984) 755. [2] N.B. Brandt and V.V. Mosbchalkov, Adv. Phys. 33 (1984) 373.

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[3] I. Mayer and P.D. Yetor, J. Less Common Met. 55 (1977) 171; D. Rossi, R. Marezza and R. Ferro, J. Less Common Met. 66 (1979) P17. [4] T.T.M. Palstra, A.A. Menovsky, G.J. Nieuwenhuys and J.A. Mydosh, J. Magn. Magn. Mater. 54-57 (1986) 435. [5] J. Lawrence, Phys. Rev. B 20 (1979) 3770. [6] L.C. Gupta, D.E. MacLaughlin, Cheng Tien, C. Godart, M.A. Edwards and R.D. Parks, Phys. Rev. B 28 (1983) 3673. [7] K. Andres, J.E. Graebner and H.R. Ott, Phys. Rev. Lett. 35 (1975) 1779. [8] K.H.J. Buschow and H.J. van Daal, Solid State Commun. 8 (1970) 363. [9] Y. Onuki, Y. Shimizu and T. Komatsubara, J. Phys. Soc. Japan 53 (1984) 1210. [10] F. Steglich, K.H. Wienand, W. Kl~imke, S. Horn and W. Lieke, in: Crystalline electric field effects in f-electrons magnetism, eds. R.P. Guertin, W. Suski and Z. Zo/nierek (Plenum, New York, 1982) p. 341. [11] R. Lagnier, J. Pierre and M.J. Mortimer, Cryogenics 17 (1977) 349.

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[12] G.R. Stewart, Z. Fisk and J.O. Willis, Phys. Rev. B 28 (1983) 172. [13] C.D. Bredl, S. Horn, F. Steglich, B. Lilhti and R.M. Martin, Phys. Rev. Lett. 52 (1984) 1982. [14] T. Fujita, K. Satoh, Y. Onuki and T. Komatsubara, J. Magn. Magn. Mater. 47/48 (1985) 66. [15] M.J. Besnus, J.P. Kappler, P. Lehmann and A. Meyer, Solid State Commun. 55 (1985) 779. [16] J.M. Lawrence, P.S. Riseborough and R.D. Parks, Rep. Prog. Phys. 44 (1981) 1. [17] G. Creuzet, D. Gignoux, D. Schmitt and M. Zerguine, to be published. [18] E. Umlauf and E. Hess, Solid State Commun. 44 (1982) 311. [19] B. Bellarbi, A. Beno]t, D. Jaccard, J.M. Mignot and H.F. Braun, Phys. Rev. B 30 (1984) 1182. [20] N. Andrei, K. Furuya and J.H. Lowenstein, Rev. Mod. Phys. 55 (1983) 331. [21] H.R. Krishna-Murthy, K.G. Wilson and J.W. Wilkins, Phys. Rev. Lett. 35 (1975) 110l. [22] R. Selim and T. Mihalisin, J. Magn. Magn. Mater. 54-57 (1986) 407.

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