- Email: [email protected]
Transport properties of the filled skutterudite PrxFe4Sb12 synthesized under high pressure
ARTICLE IN PRESS
Physica B 403 (2008) 866–868 www.elsevier.com/locate/physb
Transport properties of the filled skutterudite PrxFe4Sb12 synthesized under high pressure Kenya Tanakaa,, Daisuke Kikuchia, Yusuke Kawahitoa, Makoto Uedaa, Hidekazu Aokia, Keitaro Kuwaharaa, Hitoshi Sugawarab, Yuji Aokia, Hideyuki Satoa a Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Faculty of Integrated Arts and Science, The University of Tokushima, Tokushima 770-8502, Japan
b
Abstract We have investigated the thermoelectric power (S), Hall coefficient (RH ), and magnetoresistance (MR) of Prx Fe4 Sb12 (x1) synthesized under high pressure. The dependence of S and RH on the Pr-site filling fraction x become apparent at low temperatures. In MR, we confirmed that the electrical resistivity is enhanced by the 4f-electron crystalline electric field effect below 10 K and suppressed by the 3d-electron spin fluctuation effect above 10 K. The change of magnetic state in Prx Fe4 Sb12 with x, realized as a combined effect of 4f-electrons and the high electronic density of state at Fermi level, is well reflected in the transport properties. r 2007 Published by Elsevier B.V. PACS: 71.27.+a; 71.20.Eh; 75.50.Bb Keywords: Skutterudite; PrFe4 Sb12 ; High-pressure synthesis; Thermoelectric power; Hall effect; Magnetoresistance
The filled skutterudite compounds, especially Pr-based ones, have attracted considerable attention because of their various novel features, i.e., unconventional superconductivity, metal–insulator transition and heavy fermion behavior [1]. In Prx Fe4 Sb12 , Pr0:73 Fe4 Sb12 and Pr0:87 Fe4 Sb12 made under ambient pressure were reported to show a magnetic ordering at 5 K [2,3] and their 4felectron crystalline electric field (CEF) ground state was estimated to be the triplet. Recently, we have succeeded in synthesizing fully filled Prx Fe4 Sb12 (x1) under high pressure [4] that is denoted hereafter as PrFe4 Sb12 . On PrFe4 Sb12 , we confirmed an absence of such magnetic ordering down to 0.15 K and estimated that CEF ground state is a singlet with the triplet first excited state at 20 K [5]. These results indicate that the Pr-site filling fraction x plays an important role in the physical properties. Among the filled skutterudite compounds, the uniqueness specific to the [Fe4 Sb12 ]-sublattice system is the coexistence of the localized Pr–4f electrons and the itinerant Fe-3d Corresponding author. Tel.: +81 426 77 2487; fax: +81 426 77 2483.
E-mail address: [email protected] (K. Tanaka). 0921-4526/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.physb.2007.10.050
electrons with the high density of states (DOS) at Fermi energy (E F )[6]. Such sensitivity of magnetic state with x has been reported in neither the other Pr-based skutterudite compounds nor Lax Fe4 Sb12 without 4f-electron, suggesting that the sensitivity might be understood as a combined effect of Pr–4f electrons and the high DOS near E F mainly ascribed to the Fe-3d electrons. Thus PrFe4 Sb12 is an important material to investigate the magnetism caused by 4f-electrons coexisting with the high DOS at E F . In this paper, we report the thermoelectric power (S), Hall coefficient (RH ), and magnetoresistance (MR) of the PrFe4 Sb12 . In order to investigate the x dependence of these transport properties, we also measured RH for a single crystal sample grown by conventional flux method at ambient pressure. We confirmed that the sample made at ambient pressure shows a magnetic ordering at 4 K, in agreement with the previous reports [2,3]. In addition, x of one sample taken out from the same crucible was estimated to be 0.85(0:05) by EPMA. S measurement was made by the differential method. RH and MR were measured by the ordinary dc four probe method.
ARTICLE IN PRESS K. Tanaka et al. / Physica B 403 (2008) 866–868
E. Bauer et al. [2] Pr0.73Fe4Sb12
0
La0.83Fe4Sb12 -40 PrFe4Sb12
RH (10-9 m3/C)
4 2 0
PrxFe4Sb12 (Amb.-press)
-2 0
100
1.5 T 200
300
T (K) Fig. 1. (a) Temperature dependence of thermoelectric power in PrFe4 Sb12 and LaFe4 Sb12 . The solid and broken lines are the results on Pr0:73 Fe4 Sb12 and La0:83 Fe4 Sb12 reported by Bauer et al. [2] (b) Temperature dependence of Hall coefficient in Prx Fe4 Sb12 at 1.5 T.
Fig. 1(a) shows the temperature dependence of S in PrFe4 Sb12 and LaFe4 Sb12 . SðTÞ for both compounds show a negative minimum at 30 K and increase almost linearly above 100 K. The absolute values are 95 mV=K and 70 mV=K at room temperature (RT) for PrFe4 Sb12 and LaFe4 Sb12 , respectively, which are larger than the other filled skutterudites such as RFe4 P12 and ROs4 Sb12 (R ¼ La, Pr) [1]. The large positive diffusion thermoelectric power component suggests that E F is located around the high-energy side of the sharp slope of a DOS peak, consistent with the band structure calculation [6]. We also show the results on Pr0:73 Fe4 Sb12 and La0:83 Fe4 Sb12 in Fig. 1(a) reported by Bauer et al. [2] for comparison. Overall features are not much different from their results, except the existence of a shoulder structure in the Bauer’s data near 120 K. Both similar shoulder structure and negative minimum in SðTÞ have been reported for the nearly ferromagnetic compound of CaFe4 Sb12 with the divalent Ca2þ filler [7], where these behaviors have been ascribed to the spin fluctuation (SF). From the result of CaFe4 Sb12 , the present result is also expected to be caused by the SF effect. Fig. 1(b) shows the temperature dependence of RH in Prx Fe4 Sb12 at 1.5 T. If we assume a single spherical Fermi surface, the carrier density n can be estimated from the normal part of Hall coefficient R0 ¼ 1=en. Assuming negligible contribution from the anomalous Hall effect at RT, the estimated carrier number per formula unit are 0.6 in PrFe4 Sb12 and 0:8 in Prx Fe4 Sb12 , consistent with the band structure calculation [8]. At low temperatures, they show apparently different behaviors. RH for Prx Fe4 Sb12 shows a rapid decrease with decreasing
We thank Prof. Harima for helpful discussions. This work was supported by a Grant-in-Aid for Scientific Research Priority Area ‘‘Skutterudite’’ (No.15072206) of the Minis-
150 PrFe4Sb12
I//H
0T 10 T
100
0T
8 dρ/dT
S (µV/K)
LaFe4Sb12
40
temperature below 100 K and has a negative minimum at 10 K. In contrast, RH for PrFe4 Sb12 decreases monotonously below 100 K with decreasing temperature. Such difference in RH between the two samples may be ascribed to the change in the anomalous Hall component reflecting the change in the magnetic states with x. In fact, the magnetic susceptibility of Prx Fe4 Sb12 deviates upward from that of PrFe4 Sb12 below 100 K [2–5]. Fig. 2 shows the temperature dependence of longitudinal MR in PrFe4 Sb12 . The shoulder-like structure near 10 K for 0 T resembles that reported in PrOs4 Sb12 [9], where it was ascribed to the conduction electron scattering accompanied by CEF excitation from the ground state to the first excited state. The shoulder-like structure shifts to lower temperatures with increasing magnetic field. The arrows show the temperature of dr=dT maximum shown in inset of Fig. 2. The shift can be naturally understood as the magnetic field effect on CEF level splitting: one out of the degenerate first CEF excited triplet comes down to the ground state with increasing magnetic field. This scenario is consistent with the estimated CEF level scheme which is the singlet ground state with the triplet first excited state at 20 K [5]. Another noticeable result is the negative MR above 10 K. The negative MR was also reported in Pr0:73 Fe4 Sb12 and Pr0:87 Fe4 Sb12 [2,3], and it was ascribed to the suppression of electron scattering by 3d-electron SF under magnetic field. In MR measurement, we confirmed the existence of two energy scales in PrFe4 Sb12 . CEF effect dominates below 10 K, which is not clear in present results of S and RH . SF effect appears above 10 K, which is consistent with the SF contribution to S.
ρ (μΩcm)
Present work PrFe4Sb12
80
867
50
4
0 0
10
20
30
T (K)
0 0
10
20
30
T (K) Fig. 2. Temperature dependence of longitudinal magnetoresistvity in PrFe4 Sb12 . Inset shows dr=dT vs. T plot at 0 T.
ARTICLE IN PRESS 868
K. Tanaka et al. / Physica B 403 (2008) 866–868
try of Education, Culture, Sports, Science and Technology, Japan. References [1] Y. Aoki, et al., J. Phys. Soc. Japan 74 (2005) 209. [2] E. Bauer, et al., Phys. Rev. B 66 (2002) 214421.
[3] [4] [5] [6] [7] [8] [9]
N.P. Butch, et al., Phys. Rev. B 71 (2005) 214417. K. Tanaka, et al., Physica B 378–380 (2006) 213. K. Tanaka, et al., J. Phys. Soc. Japan 76 (2007) 103704. K. Takegahara, H. Harima, J. Phys. Soc. Japan 71 (2002) Suppl. 240. T. Takabatake, et al., Physica B 383 (2006) 93. H. Harima, unpublished. N.A. Frederick, M.B. Maple, J. Phys. Condens Matter 15 (2003) 4789.
Physica B 403 (2008) 866–868 www.elsevier.com/locate/physb
Transport properties of the filled skutterudite PrxFe4Sb12 synthesized under high pressure Kenya Tanakaa,, Daisuke Kikuchia, Yusuke Kawahitoa, Makoto Uedaa, Hidekazu Aokia, Keitaro Kuwaharaa, Hitoshi Sugawarab, Yuji Aokia, Hideyuki Satoa a Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Faculty of Integrated Arts and Science, The University of Tokushima, Tokushima 770-8502, Japan
b
Abstract We have investigated the thermoelectric power (S), Hall coefficient (RH ), and magnetoresistance (MR) of Prx Fe4 Sb12 (x1) synthesized under high pressure. The dependence of S and RH on the Pr-site filling fraction x become apparent at low temperatures. In MR, we confirmed that the electrical resistivity is enhanced by the 4f-electron crystalline electric field effect below 10 K and suppressed by the 3d-electron spin fluctuation effect above 10 K. The change of magnetic state in Prx Fe4 Sb12 with x, realized as a combined effect of 4f-electrons and the high electronic density of state at Fermi level, is well reflected in the transport properties. r 2007 Published by Elsevier B.V. PACS: 71.27.+a; 71.20.Eh; 75.50.Bb Keywords: Skutterudite; PrFe4 Sb12 ; High-pressure synthesis; Thermoelectric power; Hall effect; Magnetoresistance
The filled skutterudite compounds, especially Pr-based ones, have attracted considerable attention because of their various novel features, i.e., unconventional superconductivity, metal–insulator transition and heavy fermion behavior [1]. In Prx Fe4 Sb12 , Pr0:73 Fe4 Sb12 and Pr0:87 Fe4 Sb12 made under ambient pressure were reported to show a magnetic ordering at 5 K [2,3] and their 4felectron crystalline electric field (CEF) ground state was estimated to be the triplet. Recently, we have succeeded in synthesizing fully filled Prx Fe4 Sb12 (x1) under high pressure [4] that is denoted hereafter as PrFe4 Sb12 . On PrFe4 Sb12 , we confirmed an absence of such magnetic ordering down to 0.15 K and estimated that CEF ground state is a singlet with the triplet first excited state at 20 K [5]. These results indicate that the Pr-site filling fraction x plays an important role in the physical properties. Among the filled skutterudite compounds, the uniqueness specific to the [Fe4 Sb12 ]-sublattice system is the coexistence of the localized Pr–4f electrons and the itinerant Fe-3d Corresponding author. Tel.: +81 426 77 2487; fax: +81 426 77 2483.
E-mail address: [email protected] (K. Tanaka). 0921-4526/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.physb.2007.10.050
electrons with the high density of states (DOS) at Fermi energy (E F )[6]. Such sensitivity of magnetic state with x has been reported in neither the other Pr-based skutterudite compounds nor Lax Fe4 Sb12 without 4f-electron, suggesting that the sensitivity might be understood as a combined effect of Pr–4f electrons and the high DOS near E F mainly ascribed to the Fe-3d electrons. Thus PrFe4 Sb12 is an important material to investigate the magnetism caused by 4f-electrons coexisting with the high DOS at E F . In this paper, we report the thermoelectric power (S), Hall coefficient (RH ), and magnetoresistance (MR) of the PrFe4 Sb12 . In order to investigate the x dependence of these transport properties, we also measured RH for a single crystal sample grown by conventional flux method at ambient pressure. We confirmed that the sample made at ambient pressure shows a magnetic ordering at 4 K, in agreement with the previous reports [2,3]. In addition, x of one sample taken out from the same crucible was estimated to be 0.85(0:05) by EPMA. S measurement was made by the differential method. RH and MR were measured by the ordinary dc four probe method.
ARTICLE IN PRESS K. Tanaka et al. / Physica B 403 (2008) 866–868
E. Bauer et al. [2] Pr0.73Fe4Sb12
0
La0.83Fe4Sb12 -40 PrFe4Sb12
RH (10-9 m3/C)
4 2 0
PrxFe4Sb12 (Amb.-press)
-2 0
100
1.5 T 200
300
T (K) Fig. 1. (a) Temperature dependence of thermoelectric power in PrFe4 Sb12 and LaFe4 Sb12 . The solid and broken lines are the results on Pr0:73 Fe4 Sb12 and La0:83 Fe4 Sb12 reported by Bauer et al. [2] (b) Temperature dependence of Hall coefficient in Prx Fe4 Sb12 at 1.5 T.
Fig. 1(a) shows the temperature dependence of S in PrFe4 Sb12 and LaFe4 Sb12 . SðTÞ for both compounds show a negative minimum at 30 K and increase almost linearly above 100 K. The absolute values are 95 mV=K and 70 mV=K at room temperature (RT) for PrFe4 Sb12 and LaFe4 Sb12 , respectively, which are larger than the other filled skutterudites such as RFe4 P12 and ROs4 Sb12 (R ¼ La, Pr) [1]. The large positive diffusion thermoelectric power component suggests that E F is located around the high-energy side of the sharp slope of a DOS peak, consistent with the band structure calculation [6]. We also show the results on Pr0:73 Fe4 Sb12 and La0:83 Fe4 Sb12 in Fig. 1(a) reported by Bauer et al. [2] for comparison. Overall features are not much different from their results, except the existence of a shoulder structure in the Bauer’s data near 120 K. Both similar shoulder structure and negative minimum in SðTÞ have been reported for the nearly ferromagnetic compound of CaFe4 Sb12 with the divalent Ca2þ filler [7], where these behaviors have been ascribed to the spin fluctuation (SF). From the result of CaFe4 Sb12 , the present result is also expected to be caused by the SF effect. Fig. 1(b) shows the temperature dependence of RH in Prx Fe4 Sb12 at 1.5 T. If we assume a single spherical Fermi surface, the carrier density n can be estimated from the normal part of Hall coefficient R0 ¼ 1=en. Assuming negligible contribution from the anomalous Hall effect at RT, the estimated carrier number per formula unit are 0.6 in PrFe4 Sb12 and 0:8 in Prx Fe4 Sb12 , consistent with the band structure calculation [8]. At low temperatures, they show apparently different behaviors. RH for Prx Fe4 Sb12 shows a rapid decrease with decreasing
We thank Prof. Harima for helpful discussions. This work was supported by a Grant-in-Aid for Scientific Research Priority Area ‘‘Skutterudite’’ (No.15072206) of the Minis-
150 PrFe4Sb12
I//H
0T 10 T
100
0T
8 dρ/dT
S (µV/K)
LaFe4Sb12
40
temperature below 100 K and has a negative minimum at 10 K. In contrast, RH for PrFe4 Sb12 decreases monotonously below 100 K with decreasing temperature. Such difference in RH between the two samples may be ascribed to the change in the anomalous Hall component reflecting the change in the magnetic states with x. In fact, the magnetic susceptibility of Prx Fe4 Sb12 deviates upward from that of PrFe4 Sb12 below 100 K [2–5]. Fig. 2 shows the temperature dependence of longitudinal MR in PrFe4 Sb12 . The shoulder-like structure near 10 K for 0 T resembles that reported in PrOs4 Sb12 [9], where it was ascribed to the conduction electron scattering accompanied by CEF excitation from the ground state to the first excited state. The shoulder-like structure shifts to lower temperatures with increasing magnetic field. The arrows show the temperature of dr=dT maximum shown in inset of Fig. 2. The shift can be naturally understood as the magnetic field effect on CEF level splitting: one out of the degenerate first CEF excited triplet comes down to the ground state with increasing magnetic field. This scenario is consistent with the estimated CEF level scheme which is the singlet ground state with the triplet first excited state at 20 K [5]. Another noticeable result is the negative MR above 10 K. The negative MR was also reported in Pr0:73 Fe4 Sb12 and Pr0:87 Fe4 Sb12 [2,3], and it was ascribed to the suppression of electron scattering by 3d-electron SF under magnetic field. In MR measurement, we confirmed the existence of two energy scales in PrFe4 Sb12 . CEF effect dominates below 10 K, which is not clear in present results of S and RH . SF effect appears above 10 K, which is consistent with the SF contribution to S.
ρ (μΩcm)
Present work PrFe4Sb12
80
867
50
4
0 0
10
20
30
T (K)
0 0
10
20
30
T (K) Fig. 2. Temperature dependence of longitudinal magnetoresistvity in PrFe4 Sb12 . Inset shows dr=dT vs. T plot at 0 T.
ARTICLE IN PRESS 868
K. Tanaka et al. / Physica B 403 (2008) 866–868
try of Education, Culture, Sports, Science and Technology, Japan. References [1] Y. Aoki, et al., J. Phys. Soc. Japan 74 (2005) 209. [2] E. Bauer, et al., Phys. Rev. B 66 (2002) 214421.
[3] [4] [5] [6] [7] [8] [9]
N.P. Butch, et al., Phys. Rev. B 71 (2005) 214417. K. Tanaka, et al., Physica B 378–380 (2006) 213. K. Tanaka, et al., J. Phys. Soc. Japan 76 (2007) 103704. K. Takegahara, H. Harima, J. Phys. Soc. Japan 71 (2002) Suppl. 240. T. Takabatake, et al., Physica B 383 (2006) 93. H. Harima, unpublished. N.A. Frederick, M.B. Maple, J. Phys. Condens Matter 15 (2003) 4789.