- Email: [email protected]
Thermoelectric power on Ce1−xLaxPd2Si2
PllYSICA
Physica B 186-188 (1993) 525-527 North-Holland
Thermoelectric power on C e l _ x L a x P d 2 S i 2 Y. B a n d o a, J. S a k u r a i b a n d E.V. S a m p a t h k u m a r a n c
aFaculty of Integrated Arts and Sciences, Hiroshima University, Japan bDepartment of Physics, Toyama University, Japan ~Tata Institute of Fundamental Research, Bombay, India Thermopower of pseudo-binary compounds Cel_~LaxPd2Si2 was measured in the temperature range 2-300 K. A huge and flat maximum at around 100 K, a deep minimum at around 25 K and a second sharp maximum below 10 K were observed. These are discussed with regard to the Kondo and antiferromagnetic interactions.
CePd2Si2, which crystallizes in the tetragonal ThCr2Si2-type structure, is known to be an antiferromagnetic Kondo compound [1-7]. The N6el temperature TN of the compound is 10 K, and the Kondo temperature T K (determined from neutron scattering experiments [5]) is very close to T N. Thus, there is a competition between the Kondo effect and magnetic ordering in CePd2Si 2. For the pseudo-binary system Cel_xLaxPd2Si2, the magnetic susceptibility, the electrical resistivity and the specific heat have been reported [7,8]. It was found that both TN and T K are close to 10K for CePd2Si2, and that they decrease progressively with decreasing Ce concentration for Cel_xLaxPd2Si 2, while the CEF splitting scheme of the Ce (4f) electron stays nearly constant [7,8]. In the present study, we measured the thermopower S (a quantity that is highly sensitive to details of interactions among conduction electrons and their scattering mechanisms) of the system Ce I xLaxPd~Si2 in order to understand better the magnetic interactions. Sample preparation and characterization methods were the same as described in ref. [7]. The samples were cut from ingots using a diamond saw. Typical dimensions were 1 × 1 × 6 mm. The S measurements were carried out, from 2 to 300 K, by a differential method using a pair of thermocouples consisting of a fine chromel wire and a fine wire of Au + 0.07 at% Fe. The S curves for Cel_~LaxPdzSi 2 are shown in fig. 1 for the whole temperature range of our measurement
and in fig. 2 for the low-temperature range on an expanded scale. The S curve for CePd2Si 2 shows a huge and broad maximum attaining near to 20 IxV/K at around 100K, and a deep minimum down to -101xV/K at around 25K. Our curve for the compound is in fair agreement with that reported by Amato and Sierro [6]. Similar behaviour of S has been observed for many other antiferromagnetic Kondo compounds, such as CePb 3 [10] and CeTX ( T = transition metal, X - - S n , Ge or Ga) [11]. The maximum in S is usually attributed to the Kondo effect in the presence of the crystalline electric field (CEF) effect of a Ce (4f) electron. The temperature of this maximum measures the CEF separation of Ce (4f) i
20
Cel_xLaxPd2Si2
10
0
-10 0
100
200
300
T(K)
Correspondence to: Y. Bando, Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Senda-Machi, Nakaku, Hiroshima 730, Japan.
t
x = 0,0
Fig. 1. Thermoelectric power S for Cel_xLaxPdzSi2 plotted as a function of temperature T ~om 2 to 300 K.
0921-4526/93/$06.00 t~) 1993 - Elsevier Science Publishers B.V. All rights reserved
526
Y. Bando et al. / Thermoelectric power on (Ce, La)PdeSi 2
10
o O3
-5
-lO o
lO
20
30
40
50
-r(K) Fig. 2. Thermoelectric power S for Ce~ xLaxPd2Si~ in the
low-temperature region plotted as a function of temperature T. Arrows indicate the reported values of TN [7,8].
maximum, the minimum and the second maximum in S are plotted in figs. 3(a), (b) and (c), respectively. The temperature of the first maximum in S stays practically unchanged, as seen in fig. 3(a). This temperature, in principle, reflects the CEF splitting scheme as well as the value of T K [14]. However, in previous work [8], it was understood that T K varies with Ce concentration, while the CEF level scheme does not. Therefore, this temperature seems to reflect not the T K value, but mainly the CEF splitting in the present system. On the other hand, the maximum value of S decreases almost linearly with decreasing Ce concentration in the samples. In Mott's expression of S, the concentration of Ce ions as scatterers cancels, thus the behaviour of S for alloys dose not change as long as the value of relaxation time r remains unchanged. This was actually observed in both (Ce 1 ~La~)Cu 6 [15] and (Cel_xLax)In 3 [16], for example, as long as the Ce concentration is not too low. Our observation of the strong dependence of the maximum value of S for the present samples suggests that the Kondo hybridization strength decreases progressively with decreasing Ce concentration. We stress here again that for the Ce, xLaxPd2Si2 samples, the
(a)
electron energy levels from the ground level, and the maximum value is related to the Kondo interaction strength [12]. On the other hand, the minimum in S appears for almost all the antiferromagnetic Kondo compounds of Ce at temperatures several times higher than their N6el temperatures TN; this is considered as experimental evidence of the onset of antiferromagnetic correlation above T N [13]. The maximum in S with a positive sign and the minimum with a negative sign for Ce~_,LaxPd2Si 2 appear at temperatures very close to those for CePdzSi2, namely, at 100 and 25K, respectively. However, the absolute maximum and minimum values decrease with decreasing Ce concentration, as seen in fig. 1. At the same time, a second prominent and sharp maximum in S starts to appear at low temperature below 10 K. The existence of the peak is not clear for the samples with x = 0 and 0.9. The temperature of the second maximum decreases, while its height increases with decreasing Ce concentration. In LaPd2Si 2 S has a small value and is approximately linear without any structures in accordance with the Mott expression of S for normal metals. Therefore, the maxima and minima in figs. 1 and 2 in the S curves for Cel_xLa~Pd2Si 2 are understood to originate in anomalies associated with Ce. The temperature and the magnitude for the first
100
--
50
--
o
0
0
0
20
0
-
~'
z~
10
Ce l_xLaxPd2Si2 0
(b) ..,... v t-
3O
--
0 0
0
0
0
20 lO
09 -'~o~<< o
a,
t,
-lO
(c) 5
o
*
~
5
o
0
1.0 (La)
;
0
0.5
0,0 (Ce)
X
Fig. 3. Temperature and the value of (a) the positive peak at about 100 K, (b) the negative peak at about 25 K and (c) the second positive peak below 10 K plotted against Ce concentration. (3: temperature; A: peak value of S.
Y. Bando et al. / Thermoelectric p o w e r on (Ce, L a ) P d z S i 2
value of T K estimated from specific heat measurements [8], decreases almost linearly with decreasing Ce concentration. Therefore, the peak value of S is assumed to be directly associated with the value of T K. The temperature of the maximum in S in fig. 3(b) is seen to be unchanged for the alloys, while its absolute value decreases with Ce concentration, similar to the concentration dependences of the maximum in S. Therefore, both maximum and minimum seem to originate from the same physical mechanism, while a minimum in S with a negative sign is considered to be associated with an antiferromagnetic correlation, as mentioned before. A t the moment we do not have a simple explanation of why the magnetic correlation is associated with the minimum in S. On the other hand, the temperature of the second peak in S agrees with the Nrel temperature [7,8]. As for the resistivity, the - l n T behaviour associated with the C E F ground level of Ce was observed in the low-temperature range below 30 K. The samples with low Ce concentrations, and hence with low values of TN and TK, exhibit - l n T behaviour over a wide temperature range [7,8]. These samples were found to have prominent and sharp peaks in S, as shown in figs. 1 and 2. For CePd2Si 2, with a Kondo increase in p in onl~ a narrow temperature region, the second peak in S is hardly discernible. This can be understood if an onset of antiferromagnetic order appears prior to the Kondo increase in S. The second peak in S at T~ appears under certain conditions: small TN and T K values along with a large energy separation of the higher C E F levels from the ground state. Under these conditions. Kondo scattering due to the CEF ground state dominates in the wider temperature range. All aspects of the anomalous behaviour of S for the present samples in the paramagnetic state are noted to be associated with a single-ion Kondo interaction, which further supports the conclusion derived from the susceptibility and resistivity behaviour [7]. There-
527
fore, the anomalous magnetic behaviour for the present samples is typical of antiferromagnetic Kondo compounds.
References
[1] S.K. Dhar and E.V. Sampathkumaran, Phys. Lett. A 121 (1987) 454. [2] R.A. Steeman, E. Frikkee, R.B. Helmhoidt, A.A. Menovsky, J. van der Berg, G.J. Nieuwenhuys and J.A. Mydosh, Solid State Commun. 66 (1988) 103. [3] J.D. Tompson, R.D. Parks and H. Borges, J. Magn. Magn. Mater. 54-57 (1986) 377. [4] A. Severing, E. Holland-Moritz and B. Frick, Phys. Rev. B 39 (1989) 2557. [5] A. Severing, E. Holland-Moritz and B. Frick, Phys. Rev. B 39 (1989) 2557. [6] A. Amato and J. Sierro, J. Magn. Magn. Mater. 47-48 (1985) 526. [7] E.V. Sampathkumaran, Y. Nakazawa, M. Ishikawa and R. Vijayaraghavan, Phys. Rev. B 40 (1989) 11452. [8] M.J. Besnus, A. Braght and A. Meyer, Z. Phys. B 83 (1991) 207. [9] I. Das and K.V. Sampathkumaran, Solid State Commun. 81 (1992) 905; M.J. Besnus, A. Braghta, J.P. Kappler and A. Meyer, in: Theoretical and Experimental Aspects of Valence Fluctuations and Heavy Fermions, eds. L.C. Gupta and S.K. Malik (Plenum Press, New York, 1987) p. 417. [10] J. Sakurai, H. Kamimura and Y. Komura, J. Magn. Magn. Mater. 76-77 (1988) 287. [11] J. Sakurai, Y. Yamaguchi, S. Nishigori, T. Suzuki and T. Fujita, J. Magn. Magn. Mater. 90-91 (1991) 422. [12] A.K. Bhattacharjee and B. Coqblin, Phys. Rev. B 13 (1976) 3441. [13] K.H. Fischer, Z. Phys. B 76 (1989) 315. [14] S. Maekawa, S. Kashiba, M. Tachiki and S. Takahashi, J. Phys. Soc. Jpn. 55 (1986) 3194. [15] Y. Onuki and T. Komatsubara, J. Magn. Magn. Mater. 63-63 (1987) 281. [16] J. Sakurai, T. Ohyama and Y. Komura, J. Magn. Magn. Mater. 52 (1985) 320.
Physica B 186-188 (1993) 525-527 North-Holland
Thermoelectric power on C e l _ x L a x P d 2 S i 2 Y. B a n d o a, J. S a k u r a i b a n d E.V. S a m p a t h k u m a r a n c
aFaculty of Integrated Arts and Sciences, Hiroshima University, Japan bDepartment of Physics, Toyama University, Japan ~Tata Institute of Fundamental Research, Bombay, India Thermopower of pseudo-binary compounds Cel_~LaxPd2Si2 was measured in the temperature range 2-300 K. A huge and flat maximum at around 100 K, a deep minimum at around 25 K and a second sharp maximum below 10 K were observed. These are discussed with regard to the Kondo and antiferromagnetic interactions.
CePd2Si2, which crystallizes in the tetragonal ThCr2Si2-type structure, is known to be an antiferromagnetic Kondo compound [1-7]. The N6el temperature TN of the compound is 10 K, and the Kondo temperature T K (determined from neutron scattering experiments [5]) is very close to T N. Thus, there is a competition between the Kondo effect and magnetic ordering in CePd2Si 2. For the pseudo-binary system Cel_xLaxPd2Si2, the magnetic susceptibility, the electrical resistivity and the specific heat have been reported [7,8]. It was found that both TN and T K are close to 10K for CePd2Si2, and that they decrease progressively with decreasing Ce concentration for Cel_xLaxPd2Si 2, while the CEF splitting scheme of the Ce (4f) electron stays nearly constant [7,8]. In the present study, we measured the thermopower S (a quantity that is highly sensitive to details of interactions among conduction electrons and their scattering mechanisms) of the system Ce I xLaxPd~Si2 in order to understand better the magnetic interactions. Sample preparation and characterization methods were the same as described in ref. [7]. The samples were cut from ingots using a diamond saw. Typical dimensions were 1 × 1 × 6 mm. The S measurements were carried out, from 2 to 300 K, by a differential method using a pair of thermocouples consisting of a fine chromel wire and a fine wire of Au + 0.07 at% Fe. The S curves for Cel_~LaxPdzSi 2 are shown in fig. 1 for the whole temperature range of our measurement
and in fig. 2 for the low-temperature range on an expanded scale. The S curve for CePd2Si 2 shows a huge and broad maximum attaining near to 20 IxV/K at around 100K, and a deep minimum down to -101xV/K at around 25K. Our curve for the compound is in fair agreement with that reported by Amato and Sierro [6]. Similar behaviour of S has been observed for many other antiferromagnetic Kondo compounds, such as CePb 3 [10] and CeTX ( T = transition metal, X - - S n , Ge or Ga) [11]. The maximum in S is usually attributed to the Kondo effect in the presence of the crystalline electric field (CEF) effect of a Ce (4f) electron. The temperature of this maximum measures the CEF separation of Ce (4f) i
20
Cel_xLaxPd2Si2
10
0
-10 0
100
200
300
T(K)
Correspondence to: Y. Bando, Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Senda-Machi, Nakaku, Hiroshima 730, Japan.
t
x = 0,0
Fig. 1. Thermoelectric power S for Cel_xLaxPdzSi2 plotted as a function of temperature T ~om 2 to 300 K.
0921-4526/93/$06.00 t~) 1993 - Elsevier Science Publishers B.V. All rights reserved
526
Y. Bando et al. / Thermoelectric power on (Ce, La)PdeSi 2
10
o O3
-5
-lO o
lO
20
30
40
50
-r(K) Fig. 2. Thermoelectric power S for Ce~ xLaxPd2Si~ in the
low-temperature region plotted as a function of temperature T. Arrows indicate the reported values of TN [7,8].
maximum, the minimum and the second maximum in S are plotted in figs. 3(a), (b) and (c), respectively. The temperature of the first maximum in S stays practically unchanged, as seen in fig. 3(a). This temperature, in principle, reflects the CEF splitting scheme as well as the value of T K [14]. However, in previous work [8], it was understood that T K varies with Ce concentration, while the CEF level scheme does not. Therefore, this temperature seems to reflect not the T K value, but mainly the CEF splitting in the present system. On the other hand, the maximum value of S decreases almost linearly with decreasing Ce concentration in the samples. In Mott's expression of S, the concentration of Ce ions as scatterers cancels, thus the behaviour of S for alloys dose not change as long as the value of relaxation time r remains unchanged. This was actually observed in both (Ce 1 ~La~)Cu 6 [15] and (Cel_xLax)In 3 [16], for example, as long as the Ce concentration is not too low. Our observation of the strong dependence of the maximum value of S for the present samples suggests that the Kondo hybridization strength decreases progressively with decreasing Ce concentration. We stress here again that for the Ce, xLaxPd2Si2 samples, the
(a)
electron energy levels from the ground level, and the maximum value is related to the Kondo interaction strength [12]. On the other hand, the minimum in S appears for almost all the antiferromagnetic Kondo compounds of Ce at temperatures several times higher than their N6el temperatures TN; this is considered as experimental evidence of the onset of antiferromagnetic correlation above T N [13]. The maximum in S with a positive sign and the minimum with a negative sign for Ce~_,LaxPd2Si 2 appear at temperatures very close to those for CePdzSi2, namely, at 100 and 25K, respectively. However, the absolute maximum and minimum values decrease with decreasing Ce concentration, as seen in fig. 1. At the same time, a second prominent and sharp maximum in S starts to appear at low temperature below 10 K. The existence of the peak is not clear for the samples with x = 0 and 0.9. The temperature of the second maximum decreases, while its height increases with decreasing Ce concentration. In LaPd2Si 2 S has a small value and is approximately linear without any structures in accordance with the Mott expression of S for normal metals. Therefore, the maxima and minima in figs. 1 and 2 in the S curves for Cel_xLa~Pd2Si 2 are understood to originate in anomalies associated with Ce. The temperature and the magnitude for the first
100
--
50
--
o
0
0
0
20
0
-
~'
z~
10
Ce l_xLaxPd2Si2 0
(b) ..,... v t-
3O
--
0 0
0
0
0
20 lO
09 -'~o~<< o
a,
t,
-lO
(c) 5
o
*
~
5
o
0
1.0 (La)
;
0
0.5
0,0 (Ce)
X
Fig. 3. Temperature and the value of (a) the positive peak at about 100 K, (b) the negative peak at about 25 K and (c) the second positive peak below 10 K plotted against Ce concentration. (3: temperature; A: peak value of S.
Y. Bando et al. / Thermoelectric p o w e r on (Ce, L a ) P d z S i 2
value of T K estimated from specific heat measurements [8], decreases almost linearly with decreasing Ce concentration. Therefore, the peak value of S is assumed to be directly associated with the value of T K. The temperature of the maximum in S in fig. 3(b) is seen to be unchanged for the alloys, while its absolute value decreases with Ce concentration, similar to the concentration dependences of the maximum in S. Therefore, both maximum and minimum seem to originate from the same physical mechanism, while a minimum in S with a negative sign is considered to be associated with an antiferromagnetic correlation, as mentioned before. A t the moment we do not have a simple explanation of why the magnetic correlation is associated with the minimum in S. On the other hand, the temperature of the second peak in S agrees with the Nrel temperature [7,8]. As for the resistivity, the - l n T behaviour associated with the C E F ground level of Ce was observed in the low-temperature range below 30 K. The samples with low Ce concentrations, and hence with low values of TN and TK, exhibit - l n T behaviour over a wide temperature range [7,8]. These samples were found to have prominent and sharp peaks in S, as shown in figs. 1 and 2. For CePd2Si 2, with a Kondo increase in p in onl~ a narrow temperature region, the second peak in S is hardly discernible. This can be understood if an onset of antiferromagnetic order appears prior to the Kondo increase in S. The second peak in S at T~ appears under certain conditions: small TN and T K values along with a large energy separation of the higher C E F levels from the ground state. Under these conditions. Kondo scattering due to the CEF ground state dominates in the wider temperature range. All aspects of the anomalous behaviour of S for the present samples in the paramagnetic state are noted to be associated with a single-ion Kondo interaction, which further supports the conclusion derived from the susceptibility and resistivity behaviour [7]. There-
527
fore, the anomalous magnetic behaviour for the present samples is typical of antiferromagnetic Kondo compounds.
References
[1] S.K. Dhar and E.V. Sampathkumaran, Phys. Lett. A 121 (1987) 454. [2] R.A. Steeman, E. Frikkee, R.B. Helmhoidt, A.A. Menovsky, J. van der Berg, G.J. Nieuwenhuys and J.A. Mydosh, Solid State Commun. 66 (1988) 103. [3] J.D. Tompson, R.D. Parks and H. Borges, J. Magn. Magn. Mater. 54-57 (1986) 377. [4] A. Severing, E. Holland-Moritz and B. Frick, Phys. Rev. B 39 (1989) 2557. [5] A. Severing, E. Holland-Moritz and B. Frick, Phys. Rev. B 39 (1989) 2557. [6] A. Amato and J. Sierro, J. Magn. Magn. Mater. 47-48 (1985) 526. [7] E.V. Sampathkumaran, Y. Nakazawa, M. Ishikawa and R. Vijayaraghavan, Phys. Rev. B 40 (1989) 11452. [8] M.J. Besnus, A. Braght and A. Meyer, Z. Phys. B 83 (1991) 207. [9] I. Das and K.V. Sampathkumaran, Solid State Commun. 81 (1992) 905; M.J. Besnus, A. Braghta, J.P. Kappler and A. Meyer, in: Theoretical and Experimental Aspects of Valence Fluctuations and Heavy Fermions, eds. L.C. Gupta and S.K. Malik (Plenum Press, New York, 1987) p. 417. [10] J. Sakurai, H. Kamimura and Y. Komura, J. Magn. Magn. Mater. 76-77 (1988) 287. [11] J. Sakurai, Y. Yamaguchi, S. Nishigori, T. Suzuki and T. Fujita, J. Magn. Magn. Mater. 90-91 (1991) 422. [12] A.K. Bhattacharjee and B. Coqblin, Phys. Rev. B 13 (1976) 3441. [13] K.H. Fischer, Z. Phys. B 76 (1989) 315. [14] S. Maekawa, S. Kashiba, M. Tachiki and S. Takahashi, J. Phys. Soc. Jpn. 55 (1986) 3194. [15] Y. Onuki and T. Komatsubara, J. Magn. Magn. Mater. 63-63 (1987) 281. [16] J. Sakurai, T. Ohyama and Y. Komura, J. Magn. Magn. Mater. 52 (1985) 320.