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Transport properties under high pressure of antiferromagnet EuPt2Si2 with unstable Eu valence
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e62–e64 www.elsevier.com/locate/jmmm
Transport properties under high pressure of antiferromagnet EuPt2 Si2 with unstable Eu valence Y. Ikedaa, A. Mitsudab, N. Ietakaa, T. Mizushimaa, Y. Isikawaa, T. Kuwaia, a
b
Department of Physics, University of Toyama, Toyama 930-8555, Japan Department of Physics, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Available online 2 November 2006
Abstract Electrical resistivity rðTÞ of the antiferromagnet EuPt2 Si2 ðT N ¼ 16 KÞ between 2 and 300 K under hydrostatic pressure up to 2.6 GPa is presented. With applying pressure, T N monotonically shifts to the lower temperature range. Simultaneously, a characteristic upturn in rðTÞ, which lies between 150 K and T N at ambient pressure, moves to higher temperatures accompanying a broad peak on the lower temperature end. In addition, an extra logarithmic upturn around 20 K between 1.9 and 2.4 GPa was newly noticed. These results are brought by the competitive state between magnetic order, valence transformation and probably Kondo effect in this compound. r 2006 Elsevier B.V. All rights reserved. PACS: 75.20.Hr; 75.30.Mb; 75.50.Ee Keywords: EuPt2 Si2 ; Antiferromagnetism; Valence instability; Kondo effect
Magnetically unique phenomena are observed in some Eu-based intermetallic compounds. For example, EuPd2 Si2 exhibits a sharp transition of Eu valence between the magnetic Eu2þ (4f 7 ) state and the nonmagnetic Eu3þ (4f 6 ) [1]. Although EuPt2 Si2 had been thought to be an antiferromagnet (the antiferromagnetic transition temperature T N 15 K) with essentially divalent Eu ions [2], Mitsuda et al. recently reported the magnetic susceptibility and unusual behavior of the electrical resistivity [3], which is unexplained as a normal Eu2þ compound. The characteristic behavior of EuPt2 Si2 strongly depends on pressure. In particular, both the ln T behavior below 150 K down to T N and the hump structure below T N vanish at 2.5 GPa. The electrical resistivity under pressure below 2.5 GPa of this compound should be focused, because the process of vanishment in these anomalies has not been investigated closely and its mechanism hence not been understood. This paper reports the experimental results of the electrical resistivity rðTÞ with taking notice of the above-mentioned anomalies in EuPt2 Si2 under pressure Corresponding author. Tel.: +81 76 445 6586; fax: +81 76 445 6549.
E-mail address: [email protected] (T. Kuwai). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.211
up to 2.6 GPa in detail. In addition, we will discuss a novel anomaly from 1.9 to 2.4 GPa and briefly mention the results of a constant-volume Eu-diluted system ðEu1x ðLa0:606 Y0:394 Þx ÞPt2 Si2 . The polycrystalline samples of EuPt2 Si2 and ðEu1x ðLa0:606 Y0:394 Þx ÞPt2 Si2 ðx ¼ 0:7Þ were prepared by arc melting in an argon atmosphere, and then annealed at 900 C for one week. These samples were confirmed by X-ray powder diffraction analysis to be a single phase with CaBe2 Ge2 type structure. rðTÞ was measured by a standard four-probe DC technique from 2 to 300 K. Measurements at high pressures were performed in a piston cylinder-type pressure cell constructed from NiCrAl and BeCu alloys. The Daphne-7373 was used as pressure transmitting medium. Pressure applied to the sample was estimated from using pressure dependence of superconductive transition temperature of Sn with high purity [4]. The specific heat of the Eu-diluted system was measured by a thermal relaxation method using PPMS (Quantum Design Inc.). Fig. 1 shows the electrical resistivity rðTÞ of EuPt2 Si2 as a function of temperature in a logarithmic scale under various pressures. The inset provides the close-up between 4 and 30 K in a linear scale of temperature. At a glance, a
ARTICLE IN PRESS Y. Ikeda et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e62–e64
110
1.4 0.9 0.4
100
0.0
(μΩcm)
2.1
EuPt2Si2
1.9
90
2.4 2.6
98 1.4
80
1.9
96 1.4 2.1 94
70
2.4
0.4 0.9
60 1
92
4
10
10
20
100
30
700
T (K) Fig. 1. Temperature dependence of electrical resistivity in EuPt2 Si2 under various pressures. Its close-up in a linear scale of temperature between 4 and 30 K at 1.4–2.4 GPa is represented in the inset.
hump is recognized at low temperatures under ambient pressure. Its onset temperature T of about 16 K is equivalent to T N within the experimental accuracy. In addition, the characteristic upturn below 150 K down to T is in good agreement with the previous results [3]. T systematically shifts to lower temperatures with increasing pressure, and seems to attain about 10 K at 2.6 GPa. As long as T retains identical to T N even in the present high pressure region, this observation implies the long-range magnetic order is being repressed with increasing pressure as well as the susceptibility results of Ref. [3]. On the other hand, the upturn structure between T and 150 K at ambient pressure and the broad peak, which lies in the lower temperature end of the above upturn and becomes visible above 0.4 GPa, systematically shift to the higher temperatures with applying pressure. This broad peak finally becomes considerably stable for the present highest pressures above 1.9 GPa. Similar resistivity variation in pressure was observed around the valence transition in, EuNi2 ðSi1x Gex Þ2 , which was discussed by Wada et al. [5] in terms of the interconfigurational fluctuation (ICF) model [6]. According to their explanation, the gradual change of the extent in the admixture of Eu2þ and Eu3þ states which depends on temperature through valence fluctuations, is responsible for the peak. Thus, the broad peak corresponds to the state that the occupation probability of the two ionic states is just on the fifty-fifty ratio. Interestingly, a novel extra upturn, in addition to the higher temperature upturn, appears in rðTÞ between 1.9 and 2.4 GPa from about 30 K down to each T as seen in the inset of Fig. 1. As long as the hump is corresponding to the antiferromagnetic ordering, the extra upturn, which
disappears at 2.6 GPa, is possibly due to Kondo effect. The characteristic Kondo temperature T K is known to be generally enhanced by applying pressure. On the other hand, the application of pressure tends to stabilize the trivalent state in EuPt2 Si2 , and then represses the antiferromagnetic ordering. Hence, it is not surprising that the ln T behavior of Kondo effect appears in the narrow limited pressure and temperature ranges because of a sensitive energy balance between antiferromagnetic ordering, the valence transformation and the Kondo effect. The discovery of the extra upturn supports our idea that the upturn originally observed is not the indication of the Kondo effect but of the gradual valence transformation. To confirm the existence of the impurity Kondo effect, we tentatively measured specific heat CðTÞ and electrical resistivity rðTÞ of the Eu diluted system for x ¼ 0:7. Fig. 2 and its inset show the temperature dependence of DC=T, specific heat of electronic and magnetic contribution divided by T, and rðTÞ of the Eu-diluted system, respectively. We estimated the phonon part of Eu-diluted system as the one of the counterpart ðLa0:606 Y0:394 ÞPt2 Si2 ðx ¼ 1Þ. DC=T has a broad peak centering around 4 K. The dotted line represents the impurity Kondo model [7] with J ¼ 72 and T K ¼ 37:4 K and well reproduces the present experimental result below about 20 K. Correspondingly, a clear enhancement of the resistivity is also observed below 30 K as shown in the Fig. 2 inset. In order to obtain additional information in detail for the Kondo effect in this compound, we need to proceed the investigation for the diluted alloys on the Eu site, and it is now in progress.
0.5 50
48
0.4 (μΩcm)
2.1 2.4 1.9 2.6
0.0 GPa
ΔC/T (J/mol K2)
120
e63
0.3
46 x = 0.7
44
42
0.2
40
1
10
100
T (K)
0.1
Kondo model (TK = 37.4 K) (Eu0.3(La0.61Y0.39)0.7)Pt2Si2
0 1
10 T (K)
100
Fig. 2. Temperature dependence of DC=T of ðEu0:3 ðLa0:606 Y0:394 Þ0:7 ÞPt2 Si2 . The dotted line shows the Kondo model with 30% mole of Eu with T K ¼ 37:4 K. The inset shows electrical resistivity as a function of temperature for the same sample.
ARTICLE IN PRESS e64
Y. Ikeda et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e62–e64
References [1] E.V. Sampathkumaran, et al., in: M. Falicov, W. Hanke, M.B. Maple (Eds.), North-Holland, Amsterdam, 1981, p. 193. [2] R. Nagarajan, et al., Physica B 163 (1990) 591.
[3] [4] [5] [6] [7]
A. Mitsuda, et al., Acta Phys. Pol. B 34 (2003) 1149. A. Eiling, J.S. Silling, J. Phys. F 11 (1981) 623. H. Wada, et al., Phys. Rev. B 59 (1999) 1141. M. Croft, et al., Phys. Rev. Lett. 48 (1982) 826. V.T. Rajan, Phys. Rev. Lett. 51 (1983) 308.
Journal of Magnetism and Magnetic Materials 310 (2007) e62–e64 www.elsevier.com/locate/jmmm
Transport properties under high pressure of antiferromagnet EuPt2 Si2 with unstable Eu valence Y. Ikedaa, A. Mitsudab, N. Ietakaa, T. Mizushimaa, Y. Isikawaa, T. Kuwaia, a
b
Department of Physics, University of Toyama, Toyama 930-8555, Japan Department of Physics, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Available online 2 November 2006
Abstract Electrical resistivity rðTÞ of the antiferromagnet EuPt2 Si2 ðT N ¼ 16 KÞ between 2 and 300 K under hydrostatic pressure up to 2.6 GPa is presented. With applying pressure, T N monotonically shifts to the lower temperature range. Simultaneously, a characteristic upturn in rðTÞ, which lies between 150 K and T N at ambient pressure, moves to higher temperatures accompanying a broad peak on the lower temperature end. In addition, an extra logarithmic upturn around 20 K between 1.9 and 2.4 GPa was newly noticed. These results are brought by the competitive state between magnetic order, valence transformation and probably Kondo effect in this compound. r 2006 Elsevier B.V. All rights reserved. PACS: 75.20.Hr; 75.30.Mb; 75.50.Ee Keywords: EuPt2 Si2 ; Antiferromagnetism; Valence instability; Kondo effect
Magnetically unique phenomena are observed in some Eu-based intermetallic compounds. For example, EuPd2 Si2 exhibits a sharp transition of Eu valence between the magnetic Eu2þ (4f 7 ) state and the nonmagnetic Eu3þ (4f 6 ) [1]. Although EuPt2 Si2 had been thought to be an antiferromagnet (the antiferromagnetic transition temperature T N 15 K) with essentially divalent Eu ions [2], Mitsuda et al. recently reported the magnetic susceptibility and unusual behavior of the electrical resistivity [3], which is unexplained as a normal Eu2þ compound. The characteristic behavior of EuPt2 Si2 strongly depends on pressure. In particular, both the ln T behavior below 150 K down to T N and the hump structure below T N vanish at 2.5 GPa. The electrical resistivity under pressure below 2.5 GPa of this compound should be focused, because the process of vanishment in these anomalies has not been investigated closely and its mechanism hence not been understood. This paper reports the experimental results of the electrical resistivity rðTÞ with taking notice of the above-mentioned anomalies in EuPt2 Si2 under pressure Corresponding author. Tel.: +81 76 445 6586; fax: +81 76 445 6549.
E-mail address: [email protected] (T. Kuwai). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.211
up to 2.6 GPa in detail. In addition, we will discuss a novel anomaly from 1.9 to 2.4 GPa and briefly mention the results of a constant-volume Eu-diluted system ðEu1x ðLa0:606 Y0:394 Þx ÞPt2 Si2 . The polycrystalline samples of EuPt2 Si2 and ðEu1x ðLa0:606 Y0:394 Þx ÞPt2 Si2 ðx ¼ 0:7Þ were prepared by arc melting in an argon atmosphere, and then annealed at 900 C for one week. These samples were confirmed by X-ray powder diffraction analysis to be a single phase with CaBe2 Ge2 type structure. rðTÞ was measured by a standard four-probe DC technique from 2 to 300 K. Measurements at high pressures were performed in a piston cylinder-type pressure cell constructed from NiCrAl and BeCu alloys. The Daphne-7373 was used as pressure transmitting medium. Pressure applied to the sample was estimated from using pressure dependence of superconductive transition temperature of Sn with high purity [4]. The specific heat of the Eu-diluted system was measured by a thermal relaxation method using PPMS (Quantum Design Inc.). Fig. 1 shows the electrical resistivity rðTÞ of EuPt2 Si2 as a function of temperature in a logarithmic scale under various pressures. The inset provides the close-up between 4 and 30 K in a linear scale of temperature. At a glance, a
ARTICLE IN PRESS Y. Ikeda et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e62–e64
110
1.4 0.9 0.4
100
0.0
(μΩcm)
2.1
EuPt2Si2
1.9
90
2.4 2.6
98 1.4
80
1.9
96 1.4 2.1 94
70
2.4
0.4 0.9
60 1
92
4
10
10
20
100
30
700
T (K) Fig. 1. Temperature dependence of electrical resistivity in EuPt2 Si2 under various pressures. Its close-up in a linear scale of temperature between 4 and 30 K at 1.4–2.4 GPa is represented in the inset.
hump is recognized at low temperatures under ambient pressure. Its onset temperature T of about 16 K is equivalent to T N within the experimental accuracy. In addition, the characteristic upturn below 150 K down to T is in good agreement with the previous results [3]. T systematically shifts to lower temperatures with increasing pressure, and seems to attain about 10 K at 2.6 GPa. As long as T retains identical to T N even in the present high pressure region, this observation implies the long-range magnetic order is being repressed with increasing pressure as well as the susceptibility results of Ref. [3]. On the other hand, the upturn structure between T and 150 K at ambient pressure and the broad peak, which lies in the lower temperature end of the above upturn and becomes visible above 0.4 GPa, systematically shift to the higher temperatures with applying pressure. This broad peak finally becomes considerably stable for the present highest pressures above 1.9 GPa. Similar resistivity variation in pressure was observed around the valence transition in, EuNi2 ðSi1x Gex Þ2 , which was discussed by Wada et al. [5] in terms of the interconfigurational fluctuation (ICF) model [6]. According to their explanation, the gradual change of the extent in the admixture of Eu2þ and Eu3þ states which depends on temperature through valence fluctuations, is responsible for the peak. Thus, the broad peak corresponds to the state that the occupation probability of the two ionic states is just on the fifty-fifty ratio. Interestingly, a novel extra upturn, in addition to the higher temperature upturn, appears in rðTÞ between 1.9 and 2.4 GPa from about 30 K down to each T as seen in the inset of Fig. 1. As long as the hump is corresponding to the antiferromagnetic ordering, the extra upturn, which
disappears at 2.6 GPa, is possibly due to Kondo effect. The characteristic Kondo temperature T K is known to be generally enhanced by applying pressure. On the other hand, the application of pressure tends to stabilize the trivalent state in EuPt2 Si2 , and then represses the antiferromagnetic ordering. Hence, it is not surprising that the ln T behavior of Kondo effect appears in the narrow limited pressure and temperature ranges because of a sensitive energy balance between antiferromagnetic ordering, the valence transformation and the Kondo effect. The discovery of the extra upturn supports our idea that the upturn originally observed is not the indication of the Kondo effect but of the gradual valence transformation. To confirm the existence of the impurity Kondo effect, we tentatively measured specific heat CðTÞ and electrical resistivity rðTÞ of the Eu diluted system for x ¼ 0:7. Fig. 2 and its inset show the temperature dependence of DC=T, specific heat of electronic and magnetic contribution divided by T, and rðTÞ of the Eu-diluted system, respectively. We estimated the phonon part of Eu-diluted system as the one of the counterpart ðLa0:606 Y0:394 ÞPt2 Si2 ðx ¼ 1Þ. DC=T has a broad peak centering around 4 K. The dotted line represents the impurity Kondo model [7] with J ¼ 72 and T K ¼ 37:4 K and well reproduces the present experimental result below about 20 K. Correspondingly, a clear enhancement of the resistivity is also observed below 30 K as shown in the Fig. 2 inset. In order to obtain additional information in detail for the Kondo effect in this compound, we need to proceed the investigation for the diluted alloys on the Eu site, and it is now in progress.
0.5 50
48
0.4 (μΩcm)
2.1 2.4 1.9 2.6
0.0 GPa
ΔC/T (J/mol K2)
120
e63
0.3
46 x = 0.7
44
42
0.2
40
1
10
100
T (K)
0.1
Kondo model (TK = 37.4 K) (Eu0.3(La0.61Y0.39)0.7)Pt2Si2
0 1
10 T (K)
100
Fig. 2. Temperature dependence of DC=T of ðEu0:3 ðLa0:606 Y0:394 Þ0:7 ÞPt2 Si2 . The dotted line shows the Kondo model with 30% mole of Eu with T K ¼ 37:4 K. The inset shows electrical resistivity as a function of temperature for the same sample.
ARTICLE IN PRESS e64
Y. Ikeda et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e62–e64
References [1] E.V. Sampathkumaran, et al., in: M. Falicov, W. Hanke, M.B. Maple (Eds.), North-Holland, Amsterdam, 1981, p. 193. [2] R. Nagarajan, et al., Physica B 163 (1990) 591.
[3] [4] [5] [6] [7]
A. Mitsuda, et al., Acta Phys. Pol. B 34 (2003) 1149. A. Eiling, J.S. Silling, J. Phys. F 11 (1981) 623. H. Wada, et al., Phys. Rev. B 59 (1999) 1141. M. Croft, et al., Phys. Rev. Lett. 48 (1982) 826. V.T. Rajan, Phys. Rev. Lett. 51 (1983) 308.