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Low-energy spin fluctuations in heavy-Fermion filled-skutterudite compounds YbFe4P12 and YbFe4Sb12 investigated by 31P-NMR and 121Sb-NQR
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 835–837 www.elsevier.com/locate/jmmm
Low-energy spin fluctuations in heavy-Fermion filled-skutterudite compounds YbFe4P12 and YbFe4 Sb12 investigated by 31P-NMR and 121Sb-NQR A. Yamamotoa,, S. Wadaa, I. Shirotanib, C. Sekineb a
Department of Material Science, Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan b Faculty of Engineering, Muroran Institute of Technology, Mizumoto, Muroran 050-8585, Japan Available online 15 November 2006
Abstract To microscopically elucidate the magnetic quantum criticality of Yb-based filled skutterudites YbFe4 Sb12 and YbFe4 P12 , each of which has an intermediate valence and transforms into the heavy Fermi-liquid (HFL) state at low temperatures, we have carried out 121 Sb-NQR below 30 K and 31P-NMR below 200 K. The nuclear spin–lattice relaxation rates divided by temperature 1=T 1 T in both compounds are dominated by the generalized susceptibility wðqÞ with a finite wave vector q far from q ¼ 0. 1=T 1 T of 121Sb in YbFe4 Sb12 in zero field exhibits a near T-independent behavior with the small broad maximum around 4 K, indicating that the HFL state with rather small antiferromagnetic (AFM) spin fluctuations with T N 4 K takes place below at least 20 K. 1=T 1 T of 31P in YbFe4 P12 at high field H ¼ 7 T also exhibits the near T-independent behavior, indicating that the HFL state takes place below at least 200 K. The most important feature is that 1=T 1 T of 31P at low-H strongly depends on both temperature and field and follows the power-law relation 1=T 1 T ¼ AT a over one decade with a ’ 0:7 at 0.2 T. This result leads to the conclusion that YbFe4 Sb12 at low-H region is located very close to the non-Fermi-liquid state than YbFe4 Sb12 is. r 2006 Published by Elsevier B.V. Keywords: Filled skutterudite; Heavy fermion; Spin fluctuations; NMR/NQR
The wide variety of physical properties associated with RX 12 cages in filled skutterudite compounds RT 4 X 12 (R ¼ rare earth, T ¼ transition metal, X ¼ punictogen) has been currently of intense interest. Recently, new filled skutterudites RFe4 X12 with heavy lanthanide atoms have been successfully synthesized [1]. Among these compounds, YbFe4 X12 (X ¼ P and Sb) are in an intermediate valence state between Yb2þ and Yb3þ . The magnetic susceptibility w at high temperatures exhibits the Curie–Weiss (CW)-type behavior with the effective moments meff ’ 3:6 and 3:0 mB for X ¼ P and Sb, respectively [1,2]. The specific heat measurements indicate that the heavy Fermi-liquid (HFL) state takes place at low temperatures with the Sommerfeld coefficient gð0Þ0:3 and ’ 0:14 J=mol K2 for X ¼ P and Sb, respectively [2,3], without any magnetic ordering. In this paper, we have elucidated characteristic of low-energy spin Corresponding author.
E-mail address: [email protected] (A. Yamamoto). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.10.712
fluctuations in these filled skutterudites from a microscopic point of view by using the 31P-NMR and 121Sb-NQR measurements. The results are also compared with those previously reported for the La-based skutterudites LaFe4 X12 (X ¼ P and Sb) [8,9] to clarify a role of YbX12 cages carrying out in their physical properties. The polycrystalline samples used in this study were synthesized at high pressures in order to obtain completely Yb-filled compounds. YbFe4 Sb12 : The magnetic susceptibility w at low temperature has a plateau or a shoulder around 50 K followed by a rapid increase. The latter Curie-tail behavior strongly depends on the sample preparation procedures [2,4–7]. Fig. 1 shows the NQR spectrum of 121,123Sb in YbFe4 Sb12 observed at 4.2 K. The resonance frequency and rather narrow width of each resonance line are nearly independent of temperature, indicating that w below 50 K should have the near temperature-independent behavior, and the Curie-tail in w at low temperatures could be ascribed to
ARTICLE IN PRESS A. Yamamoto et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 835–837
836
YbFe4Sb12
1
121
Spin-echo Intensity (arb.unit)
Sb
123
0.11T
0.13T
Sb
0.8
1/T1T (sK)−1
0.2T 0.6
0.38T 0.4 1T
30
40
50
60
70
80
0.2
f (MHz) Fig. 1. NQR spectrum of
121,123
Sb in YbFe4 Sb12 observed at 4.2 K.
7T 0 1
10
100
1000
T (K)
10
Fig. 3. 1=T 1 T of 31P in YbFe4 P12 plotted against temperature, measured at various fixed fields.
1/T1T (sK)−1
8
6
4
2
0 0
10
20
30
T (K) Fig. 2. 1=T 1 T of 121Sb in YbFe4 Sb12 plotted against temperature measured in zero field. The solid curve is the best fit of the Curie–Weiss law with the data at temperatures above 4 K.
either the incomplete filling of Yb atoms in the cages of punictogen atoms or spurious magnetic impurity phases, or both. The measurements of spin–lattice relaxation rate divided by temperature 1=T 1 T reveal the spin-fluctuation character from the wave vector q averaged dynamical spin susceptibility wðqÞ. 1=T 1 T of 121Sb in YbFe4 Sb12 in zero field is plotted in Fig. 2 against temperature, which exhibits a near temperature-independent behavior with the small broad maximum around 4 K. This is an indication that the HFL state with rather small antiferromagnetic (AFM) spin fluctuations ðT N 4 KÞ takes place below at least 20 K. In contrast, the 1=T 1 T data of LaFe4 Sb12 reported by Magishi et al. [8] is scaled to the CW-type w, suggesting
dominant ferromagnetic (FM) spin fluctuations without taking the Fermi-liquid state at low temperatures. YbFe4 P12 : The magnetic susceptibility w below 100 K exhibits a rapid increase suggesting the development of FM correlations. On the other hand, 1=T 1 T of 31P in YbFe4 P12 observed at high field of H ¼ 7 T exhibits the near temperature-independent behavior as shown in Fig. 3, indicating that it is dominated by wðqÞ with a finite wave vector q (most probably q ¼ QAF ) far from q ¼ 0. It is worth noting that in the paramagnetic LaFe4 P12 , Nakai et al. [9] reported that 1=T 1 T is dominated by the spin fluctuations that is characteristic of itinerant weak antiferromagnets. Then the contribution to 1=T 1 T from underlying conduction bands is small enough, and the near temperature-independent 1=T 1 T in YbFe4 P12 is ascribed to the HFL state that takes place below at least 200 K. The most important feature is that, as can be seen in Fig. 3, 1=T 1 T at low field strongly depends on both temperature and field, and follows the power-law relation 1=T 1 T ¼ AT a over one decade. The power a ’ 0:1 at 7.0 T increases with decreasing field and takes the maximum value of 0:7 at 0.2 T, which is close to the value 34 expected for the non-Fermi-liquid (NFL) state associated with AFM spin fluctuations. In conclusion, we suggest that YbFe4 P12 at low field is located very close to the NFL state, and a magnetic field of about 0.2 T is sufficient to bring it to its quantum critical point. In contrast, YbFe4 Sb12 is located somewhat far from the NFL state. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, Grant no. 16340105, and
ARTICLE IN PRESS A. Yamamoto et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 835–837
also by a Grant-in-Aid for Scientific Research Priority Area, Skutterudite (no. 15072202). One of the authors (A.Y.) has been supported by the Japan Society for the Promotion of Science for Young Scientists. References [1] I. Shirotani, et al., J. Phys. Condens. Matter 17 (2005) 4383. [2] N.R. Dilley, et al., Phys. Rev. B 58 (1998) 6287.
837
[3] M. Wakeshima, et al., Proceedings of Joint Workshop on NQPskutterudites and NPM in Multi-approach, Hachioji, Tokyo, 2005, PB28. [4] A. Leithe-Jasper, et al., J. Solid State Chem. 109 (1999) 395. [5] E. Bauer, et al., Eur. Phys. J. 14 (2000) 483. [6] T. Ikeno, et al., 59th Autumn Meeting of PSJ, Aomori, 2004. [7] I. Tamura, et al., J. Phys. Soc. Japan 75 (2006) 014707. [8] K. Magishi, et al., J. Phys. Soc. Japan 75 (2006) 014707. [9] Y. Nakai, et al., J. Phys. Soc. Japan 74 (2005) 3370.
Journal of Magnetism and Magnetic Materials 310 (2007) 835–837 www.elsevier.com/locate/jmmm
Low-energy spin fluctuations in heavy-Fermion filled-skutterudite compounds YbFe4P12 and YbFe4 Sb12 investigated by 31P-NMR and 121Sb-NQR A. Yamamotoa,, S. Wadaa, I. Shirotanib, C. Sekineb a
Department of Material Science, Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan b Faculty of Engineering, Muroran Institute of Technology, Mizumoto, Muroran 050-8585, Japan Available online 15 November 2006
Abstract To microscopically elucidate the magnetic quantum criticality of Yb-based filled skutterudites YbFe4 Sb12 and YbFe4 P12 , each of which has an intermediate valence and transforms into the heavy Fermi-liquid (HFL) state at low temperatures, we have carried out 121 Sb-NQR below 30 K and 31P-NMR below 200 K. The nuclear spin–lattice relaxation rates divided by temperature 1=T 1 T in both compounds are dominated by the generalized susceptibility wðqÞ with a finite wave vector q far from q ¼ 0. 1=T 1 T of 121Sb in YbFe4 Sb12 in zero field exhibits a near T-independent behavior with the small broad maximum around 4 K, indicating that the HFL state with rather small antiferromagnetic (AFM) spin fluctuations with T N 4 K takes place below at least 20 K. 1=T 1 T of 31P in YbFe4 P12 at high field H ¼ 7 T also exhibits the near T-independent behavior, indicating that the HFL state takes place below at least 200 K. The most important feature is that 1=T 1 T of 31P at low-H strongly depends on both temperature and field and follows the power-law relation 1=T 1 T ¼ AT a over one decade with a ’ 0:7 at 0.2 T. This result leads to the conclusion that YbFe4 Sb12 at low-H region is located very close to the non-Fermi-liquid state than YbFe4 Sb12 is. r 2006 Published by Elsevier B.V. Keywords: Filled skutterudite; Heavy fermion; Spin fluctuations; NMR/NQR
The wide variety of physical properties associated with RX 12 cages in filled skutterudite compounds RT 4 X 12 (R ¼ rare earth, T ¼ transition metal, X ¼ punictogen) has been currently of intense interest. Recently, new filled skutterudites RFe4 X12 with heavy lanthanide atoms have been successfully synthesized [1]. Among these compounds, YbFe4 X12 (X ¼ P and Sb) are in an intermediate valence state between Yb2þ and Yb3þ . The magnetic susceptibility w at high temperatures exhibits the Curie–Weiss (CW)-type behavior with the effective moments meff ’ 3:6 and 3:0 mB for X ¼ P and Sb, respectively [1,2]. The specific heat measurements indicate that the heavy Fermi-liquid (HFL) state takes place at low temperatures with the Sommerfeld coefficient gð0Þ0:3 and ’ 0:14 J=mol K2 for X ¼ P and Sb, respectively [2,3], without any magnetic ordering. In this paper, we have elucidated characteristic of low-energy spin Corresponding author.
E-mail address: [email protected] (A. Yamamoto). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.10.712
fluctuations in these filled skutterudites from a microscopic point of view by using the 31P-NMR and 121Sb-NQR measurements. The results are also compared with those previously reported for the La-based skutterudites LaFe4 X12 (X ¼ P and Sb) [8,9] to clarify a role of YbX12 cages carrying out in their physical properties. The polycrystalline samples used in this study were synthesized at high pressures in order to obtain completely Yb-filled compounds. YbFe4 Sb12 : The magnetic susceptibility w at low temperature has a plateau or a shoulder around 50 K followed by a rapid increase. The latter Curie-tail behavior strongly depends on the sample preparation procedures [2,4–7]. Fig. 1 shows the NQR spectrum of 121,123Sb in YbFe4 Sb12 observed at 4.2 K. The resonance frequency and rather narrow width of each resonance line are nearly independent of temperature, indicating that w below 50 K should have the near temperature-independent behavior, and the Curie-tail in w at low temperatures could be ascribed to
ARTICLE IN PRESS A. Yamamoto et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 835–837
836
YbFe4Sb12
1
121
Spin-echo Intensity (arb.unit)
Sb
123
0.11T
0.13T
Sb
0.8
1/T1T (sK)−1
0.2T 0.6
0.38T 0.4 1T
30
40
50
60
70
80
0.2
f (MHz) Fig. 1. NQR spectrum of
121,123
Sb in YbFe4 Sb12 observed at 4.2 K.
7T 0 1
10
100
1000
T (K)
10
Fig. 3. 1=T 1 T of 31P in YbFe4 P12 plotted against temperature, measured at various fixed fields.
1/T1T (sK)−1
8
6
4
2
0 0
10
20
30
T (K) Fig. 2. 1=T 1 T of 121Sb in YbFe4 Sb12 plotted against temperature measured in zero field. The solid curve is the best fit of the Curie–Weiss law with the data at temperatures above 4 K.
either the incomplete filling of Yb atoms in the cages of punictogen atoms or spurious magnetic impurity phases, or both. The measurements of spin–lattice relaxation rate divided by temperature 1=T 1 T reveal the spin-fluctuation character from the wave vector q averaged dynamical spin susceptibility wðqÞ. 1=T 1 T of 121Sb in YbFe4 Sb12 in zero field is plotted in Fig. 2 against temperature, which exhibits a near temperature-independent behavior with the small broad maximum around 4 K. This is an indication that the HFL state with rather small antiferromagnetic (AFM) spin fluctuations ðT N 4 KÞ takes place below at least 20 K. In contrast, the 1=T 1 T data of LaFe4 Sb12 reported by Magishi et al. [8] is scaled to the CW-type w, suggesting
dominant ferromagnetic (FM) spin fluctuations without taking the Fermi-liquid state at low temperatures. YbFe4 P12 : The magnetic susceptibility w below 100 K exhibits a rapid increase suggesting the development of FM correlations. On the other hand, 1=T 1 T of 31P in YbFe4 P12 observed at high field of H ¼ 7 T exhibits the near temperature-independent behavior as shown in Fig. 3, indicating that it is dominated by wðqÞ with a finite wave vector q (most probably q ¼ QAF ) far from q ¼ 0. It is worth noting that in the paramagnetic LaFe4 P12 , Nakai et al. [9] reported that 1=T 1 T is dominated by the spin fluctuations that is characteristic of itinerant weak antiferromagnets. Then the contribution to 1=T 1 T from underlying conduction bands is small enough, and the near temperature-independent 1=T 1 T in YbFe4 P12 is ascribed to the HFL state that takes place below at least 200 K. The most important feature is that, as can be seen in Fig. 3, 1=T 1 T at low field strongly depends on both temperature and field, and follows the power-law relation 1=T 1 T ¼ AT a over one decade. The power a ’ 0:1 at 7.0 T increases with decreasing field and takes the maximum value of 0:7 at 0.2 T, which is close to the value 34 expected for the non-Fermi-liquid (NFL) state associated with AFM spin fluctuations. In conclusion, we suggest that YbFe4 P12 at low field is located very close to the NFL state, and a magnetic field of about 0.2 T is sufficient to bring it to its quantum critical point. In contrast, YbFe4 Sb12 is located somewhat far from the NFL state. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, Grant no. 16340105, and
ARTICLE IN PRESS A. Yamamoto et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 835–837
also by a Grant-in-Aid for Scientific Research Priority Area, Skutterudite (no. 15072202). One of the authors (A.Y.) has been supported by the Japan Society for the Promotion of Science for Young Scientists. References [1] I. Shirotani, et al., J. Phys. Condens. Matter 17 (2005) 4383. [2] N.R. Dilley, et al., Phys. Rev. B 58 (1998) 6287.
837
[3] M. Wakeshima, et al., Proceedings of Joint Workshop on NQPskutterudites and NPM in Multi-approach, Hachioji, Tokyo, 2005, PB28. [4] A. Leithe-Jasper, et al., J. Solid State Chem. 109 (1999) 395. [5] E. Bauer, et al., Eur. Phys. J. 14 (2000) 483. [6] T. Ikeno, et al., 59th Autumn Meeting of PSJ, Aomori, 2004. [7] I. Tamura, et al., J. Phys. Soc. Japan 75 (2006) 014707. [8] K. Magishi, et al., J. Phys. Soc. Japan 75 (2006) 014707. [9] Y. Nakai, et al., J. Phys. Soc. Japan 74 (2005) 3370.