- Email: [email protected]
Suppression of the energy gap in CeRbSb by partial substitution of Pd for Rh
ELSEVIER
Physica B 206 & 207 (1995) 822-824
Suppression of the energy gap in CeRhSb by partial substitution of Pd for Rh Y. Bando a, T. Takabatake a, H. Fujii a'*, G.
Kido b
aFaculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 724, Japan blnstitute for Materials Research, Tohoku University, Sendai 980, Japan
Abstract Result of transport and magnetic measurements are reported on CeRh I xPdxSb (0~ 0.1, the valence-fluctuating state is gradually transformed into the Kondo regime.
1. Introduction The valence-fluctuating (VF) compound CeRhSb is characterized by an energy scale of about 120 K. At around this temperature, the magnetic susceptibility X, electrical resistivity p and thermopower S have a local maximum. As the temperature is decreased, a coherent renormalized band is formed and a pseudogap opens therein below 8 K [1,2]. For the isostructural Kondo semiconductor CeNiSn, the effect of substitution on the energy gap has been studied by replacing Ni with Co, Cu and Pt [3,4]. All these substitutions result in strong suppression of the energy gap, although the variations in the number of conduction electrons and in the lattice parameters are considerably different. Therefore, it has been inferred that the loss of coherence in the Kondo lattice is very destructive for the gap formation [4]. In the present work, we have studied the effect of substitution of Pd for Rh in CeRhSb. By this substitution, the number of 4d electrons may increase so that the hybridization of the 4f electron states with the 4d states may be weakened. In fact, a gradual transition
* Corresponding author.
from a VF state to a magnetic Kondo state has been reported in the system CeRhl_xPdxln with increasing x [5]. The results of transport and magnetic measurements on CeRhl_xPdxSb ( 0 ~ x ~ < 0 . 4 ) are presented here. The results are compared with those reported for CeNi~_xCuxSn, where the number of 3d electrons increases with x [3].
2. Experimental Polycrystalline samples of CeRhl_xPdxSb in the range 0~
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0921-4526(94)00596-6
823
Y. Bando et al. / Physica B 206 & 207 (1995) 822-824
Measurements of X were carried out by the Faraday method for 4.2 K ~< T ~<300 K. Magnetization M was measured by using a vibrating sample magnetometer in fields up to 15 T which were produced by a watercooled magnet.
0.7
|
0.6
i
CeRh1.xPdxSb T=4.2K ~
0.5 "-1
0.40
0.20 _.
/
..~ 0.4
.
In
0.3 3. Results and discussion
0.2 The results of X are shown in Fig. 1 in a plot of 1/X versus T. For x = 0, the weak minimum at around 120K is typical of a VF system. For x ~ 0 . 1 , x(T) shows Curie-Weiss behavior and for x = 0.4 the effective magnetic moment attains a value of 2.47/xs/f.u., which is close to 2.54gs/f.u. expected for trivalent Ce ions. The concomitant increase in the paramagnetic Curie temperature from - 2 7 6 K for x = 0 to - 5 8 K for x = 0.4 indicates that the system is gradually converted from the VF regime to the Kondo regime. Fig. 2 represents the magnetization curve M(H) at 4 . 2 K for powdered samples which are free to be oriented by the applied field. The data on the increasing and decreasing field were almost identical, suggesting weak magnetic anisotropy of this system. The transition from the VF regime to the Kondo regime is reflected in the remarkable enhancement of M with increasing x. The M(H) for x = 0 linearly increases as the field is increased up to 15T. Measurements extended up to 35 T using a pulsed magnet have revealed a weak metamagnetic increase above 30T [6]. This behavior suggests that the energy gap for x = 0 is closed near 40 T. The M(H) for x = 0.4 has a downward curvature and reaches a value of 0.62/xB/ f.u. at 15T. This value is comparable with that
500~
CeRhl.x'PdxS b'
'
•
0.%
5
2ooV// / /
/ i
o
I!/L,f'-
reported for a heavy-fermion compound CePdln under similar conditions [5]. Figs. 3 and 4 show temperature dependences of p and S, respectively. Since the samples had many cracks, the p data were normalized to the room temperature value. However, S is known to be insensitive to the cracks. For the host with x -= 0, p(T) and S(T) have a maximum at around 120 K, which is a characteristic temperature for single-site spin-fluctuations. As mentioned previously, the decrease in p(T) between 100 and 10 K is regarded as the manifestation of the coherence in the Kondo lattice, and the maximum at 20 K in S(T) as a result of the formation of renormalized band. The gap formation in the renormalized band gives rise to a rapid increase in p(T) below 8 K and a drop in S(T) below 10 K [2].
. . .CeRhl . . . . . ".xPdxSb ..
-(3
1.1[
/0.10
' ~
10t ~
0"4"0"i'"';
0.10
0.20
"
IV
15
~- 1.0 &
•
00~/~ ~I
x:o 0.03
10
Fig. 2. Magnetization curves of CeRhl_xPdxSb at 4.2 K.
1,5 ~ II / f
=._.__._~0 X
Magnetic Field ( T )
2.0
_j~
400
1
0.1
[]
0.5
0.40
I
I
I
I
I
50
100
150
200
250
300
T(K) Fig. 1. Temperature dependence of the inverse magnetic susceptibility of CeRh]_xPdxSb.
.
.
.
.
.
.
.
.
,
.
.
10 T(K)
.
.
.
.
.
.
,
100
Fig. 3. Temperature dependence of the electrical resistivity of CeRhl xPd~Sb.
Y. Bando et al. / Physica B 206 & 207 (1995) 822-824
824 .
.
.
.
.
.
.
.
|
.
.
.
.
.
.
.
.
i
0
60
0.03
f.
'..'~
40
o9 2O 0
1
10
100
T(K) Fig. 4. Temperature dependence of the thermopower of CeRh, ~PdxSb. All the above features in p(T) for CeRhSb are strongly suppressed on substituting by Pd even at x = 0.03. However, the maximum in S(T) shifts from 20 to 9 K and becomes sharper. Such changes are expected if the width of the renormalized band and thus the pseudogap become narrower. This idea is consistent with the strong decrease in the Kondo temperature with x, which was indicated by the magnetic measurements. With the further increase of x to 0.06, the characteristic temperature variations in both p(T) and S(T) almost disappear. It is noteworthy that the high temperature maximum in S(T) at 120 K is hardly affected by the substitution. For x/>0.2, a weak maximum appears in p(T), as can be seen in the inset of Fig. 3. The inflection at 2.5 K for x = 0.03 and 0.4 may originate in magnetic ordering. It was noted that the more apparent cusp in p(T) was observed in CeNi~ xCuxSn for x/> 0.2 when a long-range magnetic order sets in [3]. The results of resistivity and thermopower for CeRhl_xPd~Sb showed that the partial substitution by Pd strongly disturb the formation of a coherence band. The sharp structure in S(T) for x = 0.03 suggested that the width of both the coherence band and the pseudogap becomes narrower at the initial stage of substitution. On further substitution up to x = 0.1, both the coherence band and the gap are completely destroyed. On the other hand, magnetic measurements indicated that the substitution induces a transition from the VF
state into the Kondo regime. By this substitution, 4d electrons are doped into the system and the Fermi level is raised from the energy level of the 4f states, accordingly. Therefore, the hybridization of the 4f states with the 4d states should be weakened. In addition, the hybridization of the 4f states with the Sb 5p states may be weakened by the increase in the unit-cell volume with the substitution. These facts may be responsible for the gap suppression and also for the transition toward the Kondo regime. However, as was indicated by the study of CeNit_xTxSn with T = Co, Cu and Pt [4], the disturbance of coherence should be also responsible for the gap suppression. In order to distinguish the effect of disturbance of coherence from the doping effect, further systematic study of CeRh l_xTxSb by using T = Ru and Ir is underway.
Acknowledgements This work was supported in part by Toray Science Foundation and a grant-in-aid for scientific research from the Ministry of Education, Science and Culture of Japan.
References [1] S.K. Malik and D.T. Adroja, Phys. Rev. B 43 (1991) 6277. [2] T. Takabatake, G. Natkamoto, H. Tanaka, Y. Bando, H. Fujii, S. Nishigiri, H. Goshima, T. Suzuki, T. Fujita, I. Oguro, T. Hiraoka and S.K. Malik, Physica B 199 & 200 (1994) 457. [3] T. Takabatake, Y. Nakazawa, M. Ishikawa, T. Sakakibara, K. Koga and I. Oguro, J. Magn. Magn. Mater. 76 & 77 (1988) 87. [4] T. Takabatake, G. Nakamoto, H. Tanaka, H. Fujii, S. Nishigori, T. Suzuki, T. Fujita, M. Ishikawa, I. Oguro, M. Kurisu and A.A. Menovsky, in: Transport and Thermal Properties of f-Electron Systems, eds. G. Oomi et al, (Plenum, New York, 1993) p. 1. [5] E. Briick, H. Nakotte, K. Bakker, F.R. de Boer, P.F. de Ch~tel, J.-Y. Li, J.P. Kuang and F.-M. Yang, J. Alloys Comp. 200 (1993) 79. [6] K. Sugiyama, K. Oda, M. Date, T. Takabatake, T. Tanaka and H. Fujii, unpublished.
Physica B 206 & 207 (1995) 822-824
Suppression of the energy gap in CeRhSb by partial substitution of Pd for Rh Y. Bando a, T. Takabatake a, H. Fujii a'*, G.
Kido b
aFaculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 724, Japan blnstitute for Materials Research, Tohoku University, Sendai 980, Japan
Abstract Result of transport and magnetic measurements are reported on CeRh I xPdxSb (0~
1. Introduction The valence-fluctuating (VF) compound CeRhSb is characterized by an energy scale of about 120 K. At around this temperature, the magnetic susceptibility X, electrical resistivity p and thermopower S have a local maximum. As the temperature is decreased, a coherent renormalized band is formed and a pseudogap opens therein below 8 K [1,2]. For the isostructural Kondo semiconductor CeNiSn, the effect of substitution on the energy gap has been studied by replacing Ni with Co, Cu and Pt [3,4]. All these substitutions result in strong suppression of the energy gap, although the variations in the number of conduction electrons and in the lattice parameters are considerably different. Therefore, it has been inferred that the loss of coherence in the Kondo lattice is very destructive for the gap formation [4]. In the present work, we have studied the effect of substitution of Pd for Rh in CeRhSb. By this substitution, the number of 4d electrons may increase so that the hybridization of the 4f electron states with the 4d states may be weakened. In fact, a gradual transition
* Corresponding author.
from a VF state to a magnetic Kondo state has been reported in the system CeRhl_xPdxln with increasing x [5]. The results of transport and magnetic measurements on CeRhl_xPdxSb ( 0 ~ x ~ < 0 . 4 ) are presented here. The results are compared with those reported for CeNi~_xCuxSn, where the number of 3d electrons increases with x [3].
2. Experimental Polycrystalline samples of CeRhl_xPdxSb in the range 0~
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0921-4526(94)00596-6
823
Y. Bando et al. / Physica B 206 & 207 (1995) 822-824
Measurements of X were carried out by the Faraday method for 4.2 K ~< T ~<300 K. Magnetization M was measured by using a vibrating sample magnetometer in fields up to 15 T which were produced by a watercooled magnet.
0.7
|
0.6
i
CeRh1.xPdxSb T=4.2K ~
0.5 "-1
0.40
0.20 _.
/
..~ 0.4
.
In
0.3 3. Results and discussion
0.2 The results of X are shown in Fig. 1 in a plot of 1/X versus T. For x = 0, the weak minimum at around 120K is typical of a VF system. For x ~ 0 . 1 , x(T) shows Curie-Weiss behavior and for x = 0.4 the effective magnetic moment attains a value of 2.47/xs/f.u., which is close to 2.54gs/f.u. expected for trivalent Ce ions. The concomitant increase in the paramagnetic Curie temperature from - 2 7 6 K for x = 0 to - 5 8 K for x = 0.4 indicates that the system is gradually converted from the VF regime to the Kondo regime. Fig. 2 represents the magnetization curve M(H) at 4 . 2 K for powdered samples which are free to be oriented by the applied field. The data on the increasing and decreasing field were almost identical, suggesting weak magnetic anisotropy of this system. The transition from the VF regime to the Kondo regime is reflected in the remarkable enhancement of M with increasing x. The M(H) for x = 0 linearly increases as the field is increased up to 15T. Measurements extended up to 35 T using a pulsed magnet have revealed a weak metamagnetic increase above 30T [6]. This behavior suggests that the energy gap for x = 0 is closed near 40 T. The M(H) for x = 0.4 has a downward curvature and reaches a value of 0.62/xB/ f.u. at 15T. This value is comparable with that
500~
CeRhl.x'PdxS b'
'
•
0.%
5
2ooV// / /
/ i
o
I!/L,f'-
reported for a heavy-fermion compound CePdln under similar conditions [5]. Figs. 3 and 4 show temperature dependences of p and S, respectively. Since the samples had many cracks, the p data were normalized to the room temperature value. However, S is known to be insensitive to the cracks. For the host with x -= 0, p(T) and S(T) have a maximum at around 120 K, which is a characteristic temperature for single-site spin-fluctuations. As mentioned previously, the decrease in p(T) between 100 and 10 K is regarded as the manifestation of the coherence in the Kondo lattice, and the maximum at 20 K in S(T) as a result of the formation of renormalized band. The gap formation in the renormalized band gives rise to a rapid increase in p(T) below 8 K and a drop in S(T) below 10 K [2].
. . .CeRhl . . . . . ".xPdxSb ..
-(3
1.1[
/0.10
' ~
10t ~
0"4"0"i'"';
0.10
0.20
"
IV
15
~- 1.0 &
•
00~/~ ~I
x:o 0.03
10
Fig. 2. Magnetization curves of CeRhl_xPdxSb at 4.2 K.
1,5 ~ II / f
=._.__._~0 X
Magnetic Field ( T )
2.0
_j~
400
1
0.1
[]
0.5
0.40
I
I
I
I
I
50
100
150
200
250
300
T(K) Fig. 1. Temperature dependence of the inverse magnetic susceptibility of CeRh]_xPdxSb.
.
.
.
.
.
.
.
.
,
.
.
10 T(K)
.
.
.
.
.
.
,
100
Fig. 3. Temperature dependence of the electrical resistivity of CeRhl xPd~Sb.
Y. Bando et al. / Physica B 206 & 207 (1995) 822-824
824 .
.
.
.
.
.
.
.
|
.
.
.
.
.
.
.
.
i
0
60
0.03
f.
'..'~
40
o9 2O 0
1
10
100
T(K) Fig. 4. Temperature dependence of the thermopower of CeRh, ~PdxSb. All the above features in p(T) for CeRhSb are strongly suppressed on substituting by Pd even at x = 0.03. However, the maximum in S(T) shifts from 20 to 9 K and becomes sharper. Such changes are expected if the width of the renormalized band and thus the pseudogap become narrower. This idea is consistent with the strong decrease in the Kondo temperature with x, which was indicated by the magnetic measurements. With the further increase of x to 0.06, the characteristic temperature variations in both p(T) and S(T) almost disappear. It is noteworthy that the high temperature maximum in S(T) at 120 K is hardly affected by the substitution. For x/>0.2, a weak maximum appears in p(T), as can be seen in the inset of Fig. 3. The inflection at 2.5 K for x = 0.03 and 0.4 may originate in magnetic ordering. It was noted that the more apparent cusp in p(T) was observed in CeNi~ xCuxSn for x/> 0.2 when a long-range magnetic order sets in [3]. The results of resistivity and thermopower for CeRhl_xPd~Sb showed that the partial substitution by Pd strongly disturb the formation of a coherence band. The sharp structure in S(T) for x = 0.03 suggested that the width of both the coherence band and the pseudogap becomes narrower at the initial stage of substitution. On further substitution up to x = 0.1, both the coherence band and the gap are completely destroyed. On the other hand, magnetic measurements indicated that the substitution induces a transition from the VF
state into the Kondo regime. By this substitution, 4d electrons are doped into the system and the Fermi level is raised from the energy level of the 4f states, accordingly. Therefore, the hybridization of the 4f states with the 4d states should be weakened. In addition, the hybridization of the 4f states with the Sb 5p states may be weakened by the increase in the unit-cell volume with the substitution. These facts may be responsible for the gap suppression and also for the transition toward the Kondo regime. However, as was indicated by the study of CeNit_xTxSn with T = Co, Cu and Pt [4], the disturbance of coherence should be also responsible for the gap suppression. In order to distinguish the effect of disturbance of coherence from the doping effect, further systematic study of CeRh l_xTxSb by using T = Ru and Ir is underway.
Acknowledgements This work was supported in part by Toray Science Foundation and a grant-in-aid for scientific research from the Ministry of Education, Science and Culture of Japan.
References [1] S.K. Malik and D.T. Adroja, Phys. Rev. B 43 (1991) 6277. [2] T. Takabatake, G. Natkamoto, H. Tanaka, Y. Bando, H. Fujii, S. Nishigiri, H. Goshima, T. Suzuki, T. Fujita, I. Oguro, T. Hiraoka and S.K. Malik, Physica B 199 & 200 (1994) 457. [3] T. Takabatake, Y. Nakazawa, M. Ishikawa, T. Sakakibara, K. Koga and I. Oguro, J. Magn. Magn. Mater. 76 & 77 (1988) 87. [4] T. Takabatake, G. Nakamoto, H. Tanaka, H. Fujii, S. Nishigori, T. Suzuki, T. Fujita, M. Ishikawa, I. Oguro, M. Kurisu and A.A. Menovsky, in: Transport and Thermal Properties of f-Electron Systems, eds. G. Oomi et al, (Plenum, New York, 1993) p. 1. [5] E. Briick, H. Nakotte, K. Bakker, F.R. de Boer, P.F. de Ch~tel, J.-Y. Li, J.P. Kuang and F.-M. Yang, J. Alloys Comp. 200 (1993) 79. [6] K. Sugiyama, K. Oda, M. Date, T. Takabatake, T. Tanaka and H. Fujii, unpublished.