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Kondo lattice behavior and magnetic field effects in Al20V2Eu
ARTICLE IN PRESS
Physica B 403 (2008) 1426–1427 www.elsevier.com/locate/physb
Kondo lattice behavior and magnetic field effects in Al20V2Eu Ji Chi, Yang Li, Weiping Gou, V. Goruganti, K.D.D. Rathnayaka, Joseph H. Ross Jr. Department of Physics, Texas A&M University, College Station, TX 77843-4242, USA
Abstract Al20V2Eu has a framework identical to the Al10V intermetallic, but with the large structural void filled by Eu. Magnetization results indicate a nearly 2þ Eu valence state, and a magnetic transition near 4 K. Specific heat results exhibit a gradual enhancement of the electronic term g, to 340 mJ/mol K at 8 T. In addition, the high-field 27Al NMR T 1 exhibits a crossover from local-moment behavior to Korringa-like behavior near 50 K. These results indicate that the low-temperature magnetic state is built from moments affected by Kondo screening, which is enhanced by application of a magnetic field. r 2007 Elsevier B.V. All rights reserved. PACS: 75.20.Hr; 71.27.+a; 75.20.Cg; 71.20.Eh Keywords: Kondo effects; Heavy-fermion; Paramagnetism; Rare earth alloys
1. Introduction Al20V2Eu is one of the recently discovered Al20T2R compounds, T ¼ transition metal and R ¼ rare earth [1]. The structure, identical to that of the Zn20Fe2R materials [2], has Eu nested in Al16 Frank–Kasper Friauf polyhedra, connected by a V–Al Kagome´ network [3]. The regularly spaced R atoms in highly symmetric cages and the quasicrystal-like framework makes these materials potentially of interest for their magnetic behavior. From magnetic, thermodynamic, and NMR studies of Al20V2Eu we show evidence for a transformation to a Fermi liquid state at low temperatures through Kondo screening, which is enhanced by the application of a magnetic field. 2. Experimental results and discussion Sample preparation of the polycrystalline ingot was by arc-melting (several times), allowing for Al and Eu losses, and a two-week 650 C anneal. X-ray diffraction confirmed the anticipated Fd3m (#227) structure, while electron microprobe [wavelength dispersive spectroscopy (WDS)] indicated a Al20V2Eu0.7 composition, with a minor Al Corresponding author.
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.308
metal second phase. Thus the Al16 polyhedra are filled only to 70% of the theoretical maximum in this case. Dc-susceptibility results, using a Quantum Design magnetometer, are shown in Fig. 1. High-temperature data were fit to a Curie law, ½N A cp2 =3kB ðT yÞ þ wd , with N A Avogadro’s number, c the moment concentration per f.u., and p the effective moment. We obtained y ¼ 4 K, and using c ¼ 0:7 (the composition from WDS), p ¼ 8:3 mB per Eu, slightly higher than the 7:9 mB Euð2þ Þ free-ion moment. This is similar to results for other EuT2 Al20 materials [1]. At lower temperatures there is a magnetic transition near 4 K, with a cusp as seen from the lowest-T points in Fig. 1. We are currently investigating this magnetic transition in more detail. Specific heat (C) was measured from 1.8 to 300 K in fields between 0 and 8 T. C=T is plotted vs. T 2 in Fig. 2. Below 10 K, a Schottky anomaly is observed, however, between 22 and 32 K a straight-line C=T ¼ g þ bT 2 fit was obtained, with electron and phonon contributions g and b, respectively [4]. g (inset, Fig. 2) increases steadily from 45 to 344 mJ/(mole K f.u.) with field up to 8 T (or up to 490 per Eu). This is unlikely to result from transition orbitals and thus appears to signal participation of Eu moments in the conduction band, with a moderately heavy Fermion state at the highest field available in our apparatus.
ARTICLE IN PRESS J. Chi et al. / Physica B 403 (2008) 1426–1427
Fig. 1. Dc susceptibility, wðTÞ, per mol Eu, with H ¼ 0:1 T. Inset: w1 . Dashed curves: Curie fit described in text.
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Fig. 3. 27Al NMR relaxation rates. Lines: fits to Korringa plus constant local moment terms. Inset: experimental/theoretical magnetization ratios, described in text, for the indicated fields.
Field-induced heavy Fermion behavior has been observed in Ce0.5La0.5B6 [7] and PrFe4P12[8], however, these cases are associated with antiferroquadrupolar interactions. It may be that the present enhancement is attributable to a field effect on the narrow electronic density of states features of the Al10V framework [3]. This may be akin to the behavior of SmB6 [9], a Kondo insulator, in which the Kondo gap can be closed and the electron mass increased by the presence of an applied magnetic field [10]. Experiments at higher magnetic fields may help to further elucidate the changes in Al20V2Eu. Fig. 2. C=T vs. T 2 up to ð32 KÞ2 in fields between 0 and 8 T. Dashed lines: g þ bT 2 fits for 0 and 8 T. Inset: g vs. field.
To further understand the properties, 27Al NMR experiments were performed at a field of 9 T. The single observed line contains signals from three inequivalent Al sites, broadened by the presence of magnetic moments. Fig. 3 shows T 1 1 for this line from 5 to 450 K. Above 50 K metallic-like Korringa behavior (T 1 1 / T) is observed plus a large temperature-independent term due to rapidly fluctuating local moments. Such behavior is typical of concentrated paramagnetic metals [5]. Near 50 K, T 1 1 ðTÞ exhibits a clear change, with a larger Korringa slope and little local moment behavior. The disappearance of the local-moment term is consistent with the enhanced g, and similar behavior is seen in dense Kondo systems such as CeNiAl4 [6]. M2H measurements show a small moment loss (Fig. 3 inset); here we used Brilliouin functions for MðH; TÞ based on high-T Curie fits to calculate the theoretical values. The upturn with decreasing temperature is due to the absence of y in the Brillouin function, however a divergence is seen near 50 K, the same as the NMR crossover temperature. Higher-field curves have reduced magnetization, equivalent to a moment loss corresponding to a valence enhancement by roughly 5%.
3. Summary Specific heat, magnetic, resistivity and NMR measurements indicate that Al20V2Eu0.7 contains Euð2þ Þ local moments which begin to participate in the conduction band at about 40 K, with a large enhancement as the applied magnetic field is increased. Acknowledgements This work is supported by the Robert A. Welch Foundation (grant A-1526), and the National Science Foundation (DMR-0315476). References [1] V.M.T. Thiede, et al., J. Alloys Compd. 267 (1998) 23. [2] S. Jia, S.L. Bud’ko, G.D. Samolyuk, P.C. Canfield, Nature Physics 3 (2007) 334. [3] M. Jahna´tek, et al., J. Phys.: Condens. Matter 15 (2003) 5675. [4] A. Cezairliyan, C.Y. Ho, Cindas Data Series on Material Properties 1–2 (1988). [5] J. Chi, et al., Phys. Rev. B 71 (2005) 24431. [6] K. Ghoshray, et al., Phys. Rev. B 65 (2002) 174412. [7] S. Nakamura, et al., Phys. Rev. B 68 (2003) 100402 (R). [8] Y. Aoki, et al., Phys. Rev. B 65 (2002) 064446. [9] S. Gaba´ni, et al., Phys. Rev. B 67 (2003) 172406. [10] T. Ohashi, et al., Phys. Rev. B 70 (2004) 245104.
Physica B 403 (2008) 1426–1427 www.elsevier.com/locate/physb
Kondo lattice behavior and magnetic field effects in Al20V2Eu Ji Chi, Yang Li, Weiping Gou, V. Goruganti, K.D.D. Rathnayaka, Joseph H. Ross Jr. Department of Physics, Texas A&M University, College Station, TX 77843-4242, USA
Abstract Al20V2Eu has a framework identical to the Al10V intermetallic, but with the large structural void filled by Eu. Magnetization results indicate a nearly 2þ Eu valence state, and a magnetic transition near 4 K. Specific heat results exhibit a gradual enhancement of the electronic term g, to 340 mJ/mol K at 8 T. In addition, the high-field 27Al NMR T 1 exhibits a crossover from local-moment behavior to Korringa-like behavior near 50 K. These results indicate that the low-temperature magnetic state is built from moments affected by Kondo screening, which is enhanced by application of a magnetic field. r 2007 Elsevier B.V. All rights reserved. PACS: 75.20.Hr; 71.27.+a; 75.20.Cg; 71.20.Eh Keywords: Kondo effects; Heavy-fermion; Paramagnetism; Rare earth alloys
1. Introduction Al20V2Eu is one of the recently discovered Al20T2R compounds, T ¼ transition metal and R ¼ rare earth [1]. The structure, identical to that of the Zn20Fe2R materials [2], has Eu nested in Al16 Frank–Kasper Friauf polyhedra, connected by a V–Al Kagome´ network [3]. The regularly spaced R atoms in highly symmetric cages and the quasicrystal-like framework makes these materials potentially of interest for their magnetic behavior. From magnetic, thermodynamic, and NMR studies of Al20V2Eu we show evidence for a transformation to a Fermi liquid state at low temperatures through Kondo screening, which is enhanced by the application of a magnetic field. 2. Experimental results and discussion Sample preparation of the polycrystalline ingot was by arc-melting (several times), allowing for Al and Eu losses, and a two-week 650 C anneal. X-ray diffraction confirmed the anticipated Fd3m (#227) structure, while electron microprobe [wavelength dispersive spectroscopy (WDS)] indicated a Al20V2Eu0.7 composition, with a minor Al Corresponding author.
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.308
metal second phase. Thus the Al16 polyhedra are filled only to 70% of the theoretical maximum in this case. Dc-susceptibility results, using a Quantum Design magnetometer, are shown in Fig. 1. High-temperature data were fit to a Curie law, ½N A cp2 =3kB ðT yÞ þ wd , with N A Avogadro’s number, c the moment concentration per f.u., and p the effective moment. We obtained y ¼ 4 K, and using c ¼ 0:7 (the composition from WDS), p ¼ 8:3 mB per Eu, slightly higher than the 7:9 mB Euð2þ Þ free-ion moment. This is similar to results for other EuT2 Al20 materials [1]. At lower temperatures there is a magnetic transition near 4 K, with a cusp as seen from the lowest-T points in Fig. 1. We are currently investigating this magnetic transition in more detail. Specific heat (C) was measured from 1.8 to 300 K in fields between 0 and 8 T. C=T is plotted vs. T 2 in Fig. 2. Below 10 K, a Schottky anomaly is observed, however, between 22 and 32 K a straight-line C=T ¼ g þ bT 2 fit was obtained, with electron and phonon contributions g and b, respectively [4]. g (inset, Fig. 2) increases steadily from 45 to 344 mJ/(mole K f.u.) with field up to 8 T (or up to 490 per Eu). This is unlikely to result from transition orbitals and thus appears to signal participation of Eu moments in the conduction band, with a moderately heavy Fermion state at the highest field available in our apparatus.
ARTICLE IN PRESS J. Chi et al. / Physica B 403 (2008) 1426–1427
Fig. 1. Dc susceptibility, wðTÞ, per mol Eu, with H ¼ 0:1 T. Inset: w1 . Dashed curves: Curie fit described in text.
1427
Fig. 3. 27Al NMR relaxation rates. Lines: fits to Korringa plus constant local moment terms. Inset: experimental/theoretical magnetization ratios, described in text, for the indicated fields.
Field-induced heavy Fermion behavior has been observed in Ce0.5La0.5B6 [7] and PrFe4P12[8], however, these cases are associated with antiferroquadrupolar interactions. It may be that the present enhancement is attributable to a field effect on the narrow electronic density of states features of the Al10V framework [3]. This may be akin to the behavior of SmB6 [9], a Kondo insulator, in which the Kondo gap can be closed and the electron mass increased by the presence of an applied magnetic field [10]. Experiments at higher magnetic fields may help to further elucidate the changes in Al20V2Eu. Fig. 2. C=T vs. T 2 up to ð32 KÞ2 in fields between 0 and 8 T. Dashed lines: g þ bT 2 fits for 0 and 8 T. Inset: g vs. field.
To further understand the properties, 27Al NMR experiments were performed at a field of 9 T. The single observed line contains signals from three inequivalent Al sites, broadened by the presence of magnetic moments. Fig. 3 shows T 1 1 for this line from 5 to 450 K. Above 50 K metallic-like Korringa behavior (T 1 1 / T) is observed plus a large temperature-independent term due to rapidly fluctuating local moments. Such behavior is typical of concentrated paramagnetic metals [5]. Near 50 K, T 1 1 ðTÞ exhibits a clear change, with a larger Korringa slope and little local moment behavior. The disappearance of the local-moment term is consistent with the enhanced g, and similar behavior is seen in dense Kondo systems such as CeNiAl4 [6]. M2H measurements show a small moment loss (Fig. 3 inset); here we used Brilliouin functions for MðH; TÞ based on high-T Curie fits to calculate the theoretical values. The upturn with decreasing temperature is due to the absence of y in the Brillouin function, however a divergence is seen near 50 K, the same as the NMR crossover temperature. Higher-field curves have reduced magnetization, equivalent to a moment loss corresponding to a valence enhancement by roughly 5%.
3. Summary Specific heat, magnetic, resistivity and NMR measurements indicate that Al20V2Eu0.7 contains Euð2þ Þ local moments which begin to participate in the conduction band at about 40 K, with a large enhancement as the applied magnetic field is increased. Acknowledgements This work is supported by the Robert A. Welch Foundation (grant A-1526), and the National Science Foundation (DMR-0315476). References [1] V.M.T. Thiede, et al., J. Alloys Compd. 267 (1998) 23. [2] S. Jia, S.L. Bud’ko, G.D. Samolyuk, P.C. Canfield, Nature Physics 3 (2007) 334. [3] M. Jahna´tek, et al., J. Phys.: Condens. Matter 15 (2003) 5675. [4] A. Cezairliyan, C.Y. Ho, Cindas Data Series on Material Properties 1–2 (1988). [5] J. Chi, et al., Phys. Rev. B 71 (2005) 24431. [6] K. Ghoshray, et al., Phys. Rev. B 65 (2002) 174412. [7] S. Nakamura, et al., Phys. Rev. B 68 (2003) 100402 (R). [8] Y. Aoki, et al., Phys. Rev. B 65 (2002) 064446. [9] S. Gaba´ni, et al., Phys. Rev. B 67 (2003) 172406. [10] T. Ohashi, et al., Phys. Rev. B 70 (2004) 245104.