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High pressure electrical resistivity study on itinerant ferromagnetic β -UB2C
ARTICLE IN PRESS
Physica B 403 (2008) 1375–1377 www.elsevier.com/locate/physb
High pressure electrical resistivity study on itinerant ferromagnetic b-UB2 C V.H. Trana,, R.T. Khanb, E. Bauerb, P. Roglc a
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroc!aw, Poland b Institute of Solid State Physics, Vienna University of Technology, A-1040, Wien, Austria c Institute of Physical Chemistry, Faculty of Chemistry, University of Vienna A-1090 Wien, Austria
Abstract We report the effect of pressure P on the electrical resistivity rðTÞ of a polycrystalline sample of an itinerant ferromagnet b-UB2 C with the Curie temperature T C ¼ 74:5 K and characteristic temperature T ¼ 35 K. It is observed that both T C and T decrease with increasing pressure, and the critical pressure, where the long-range ferromagnetic order becomes completely quenched, is estimated as large as Pcr 18 kbar. The metallic character of the rðTÞ curves of b-UB2 C is changed into a Kondo-like one around 12 kbar. We suppose that the suppression of the magnetic order in b-UB2 C by applying pressure is related with the development of Kondo effect. r 2007 Published by Elsevier B.V. PACS: 71.10.Hf; 71.20.Lp; 71.27.+a; 75.30.Mb Keywords: b-UB2 C; Ferromagnetism; Resistivity measurement; Pressure effect
1. Introduction Some uranium ferromagnets like UGe2 and UIr exhibit unconventional superconductivity under pressure [1,2]. In such compounds, the superconducting state appears in the pressure range where the ferromagnetic ordering starts to change into a paramagnetic state. Recently, the rhombo¯ hedral b-UB2 C (space group R3m) has reported to be an itinerant electron ferromagnet with T C ¼ 74:5 K [3]. It was shown that both magnetic specific heat divided by temperature and the temperature derivative of electrical resistivity show a hump at a characteristic temperature T , resembling the behavior of the compounds mentioned above at ambient pressure. Therefore, it is of interest to investigate the effect of pressure P on physical properties of b-UB2 C. In this contribution, we report on measurements of the temperature dependence of electrical resistivity rðTÞ
Corresponding author. Tel.: +48 71 3435021; fax: +48 71 3441029.
E-mail address: [email protected] (V.H. Tran). 0921-4526/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.physb.2007.10.361
of a polycrystalline sample of b-UB2 C under pressures up to 16 kbar. 2. Experimental details and results The polycrystalline sample of b-UB2 C was synthesized and its quality was examined by methods described elsewhere [3]. The measurements of the electrical resistivity were performed in the temperature range 1.5–300 K by means of the standard four-probe technique. Hydrostatic pressure up to 16 kbar was generated by a piston– cylinder cell using silicon oil as the pressure-transmitting medium. Fig. 1a shows the temperature dependencies of normalized electrical resistivity rðTÞ=rð300 KÞ of b-UB2 C at pressures up to 9 kbar while Fig. 1b shows the data collected between 11.5 and 16 kbar. In the first pressure range, the resistivity represents the behaviour of typical metallic ferromagnets, for which the resistivity consists mainly of spin-disorder and electron–phonon scattering contributions in the paramagnetic state and electron–magnon
ARTICLE IN PRESS V.H. Tran et al. / Physica B 403 (2008) 1375–1377
1.0
a T*
3
TC
0.6
1 kbar -UB2C
0.4 0.2
0
50
b
1.2
d/dT (arb.units)
0.8 /300K
9 kbar
2 1 0 0
100
20
40 60 T (K)
150 T (K)
200
80
250
100
300
11.5 kbar
1.2
0.8
/300K
/300K
1.0
0.6
0.4 0.0
0.4 0.2
0.8
10
16 kbar 0
50
100
150 T (K)
T (K)
200
100
250
pressure influences also the magnitude of resistivity at low temperatures. The increase in the ratio rð2 KÞ= rð300 KÞ with increasing pressure implies that the Kondo interactions increase and become more important till the Kondo temperature is of the order of room temperature. At pressures above 9 kbar, there is evidence of a change in the shape of the rðTÞ curves. At high temperatures, the resistivity (Fig. 1b) exhibits a broad maximum at T max , which usually occurs in Kondo systems. With pressure further increasing, the position of T max , indicated by a vertical line, shifts to higher values and can be recognized as a result of Kondo effect. Below 75 K the resistivity starts to decrease. However, at present we are not able to determine the mechanism behind this phenomenon. Because of the lack of any anomaly in the drðTÞ=dT-curves, one may suspect that the long-range magnetic order no longer exists or at least the contribution of scattering on magnetic moments to the total resistivity is negligible. Alternatively, the resistivity drop may be ascribed to the coherence as in the case of nonmagnetic Kondo lattices. The low-temperature part of the resistivity is presented in the inset of Fig. 1b. There appears a shallow minimum at about 10 K and followed by a small upturn when temperature approaches 1.5 K.
300
Fig. 1. (a) Temperature dependence of normalized resistivity rðTÞ=rð300 KÞ of b-UB2 C under pressures up to 9 kbar. The inset shows the temperature derivative of the resistivity. (b) The normalized resistivity measured between 11.5 and 16 kbar. The inset shows the low-temperature part of the resistivity.
scattering contribution in the ordered state. Therefore, at high temperatures the temperature derivative of the resistivity has a positive value, typical of ordinary metals. For b-UB2 C, below Curie temperature, in addition to the drop related to the magnetic ordering the resistivity shows a hump at T in its temperature derivative (see inset of Fig. 1). The mechanism of this anomaly was proposed as a quantum spin fluctuation [3]. In spite of the fact that the shape of the rðTÞ-curves collected at pressures up to 9 kbar is similar to that observed at ambient pressure previously reported [3], the applied pressure has several effects on the resistivity behavior of b-UB2 C. First of all P significantly reduces both T C and T . We can recognize also that the energy gap deduced from the relation predicted for electron–magnon scattering [4] : rðTÞ ¼ r0 þ AT 2 þ bð1 þ 2kB T=DÞ expðD=kB TÞ, where r0 is the residual resistivity, A is the Fermi-liquid T 2 coefficient of resistivity and D is an energy gap present in the magnon spectrum, decreases with increasing pressure. The decrease in T C and D is easily understood, since the pressure destroys the magnetic order. The applied
3. Concluding remarks The main results of our study are summarized in the magnetic phase diagram shown in Fig. 2, where one may distinguish two magnetic ranges, denoted as long-range ferromagnetic order (LFO) and dominating Kondo-like (K) behaviour. The fact that the long-range ferromagnetic order seems to vanish before the system reaches Pcr of
80 TC 60
TC, T* (K)
1376
β-UB2C Pcr ~ 18 kbar
40
T*
20
0
0
4
8
12 16 P (kbar)
20
24
Fig. 2. Tentative phase diagram of b-UB2 C under pressures up to 16 kbar. Extrapolation of the T C ðPÞ-curve yields the critical pressure of about 18 kbar.
ARTICLE IN PRESS V.H. Tran et al. / Physica B 403 (2008) 1375–1377
about 18 kbar (see Fig. 2), is probably related with the development of Kondo interaction, which origins from the increasing mixing between localized 5f- and conduction electron states. Acknowledgements One of us (TVH) wants to thank the Ministry of Science and Higher Education in Poland for financial support within Grant no. N202 082 31/0449. The measurement of resistivity under pressure was made possible due to support
1377
from COST Action P16 Emergent Behaviour in Correlated Matter (ECOM). T.V.H. and P.R. are grateful to the OEAD grant P10/2006.
References [1] [2] [3] [4]
S.S. Saxena, et al., Nature 406 (2000) 587. T. Akazawa, et al., J. Phys.: Condens. Matter 16 (2004) L29. V.H. Tran, et al., J. Phys.: Condens. Matter 18 (2006) 703. H.H. Andersen, H. Smith, Phys. Rev. B 19 (1979) 384.
Physica B 403 (2008) 1375–1377 www.elsevier.com/locate/physb
High pressure electrical resistivity study on itinerant ferromagnetic b-UB2 C V.H. Trana,, R.T. Khanb, E. Bauerb, P. Roglc a
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroc!aw, Poland b Institute of Solid State Physics, Vienna University of Technology, A-1040, Wien, Austria c Institute of Physical Chemistry, Faculty of Chemistry, University of Vienna A-1090 Wien, Austria
Abstract We report the effect of pressure P on the electrical resistivity rðTÞ of a polycrystalline sample of an itinerant ferromagnet b-UB2 C with the Curie temperature T C ¼ 74:5 K and characteristic temperature T ¼ 35 K. It is observed that both T C and T decrease with increasing pressure, and the critical pressure, where the long-range ferromagnetic order becomes completely quenched, is estimated as large as Pcr 18 kbar. The metallic character of the rðTÞ curves of b-UB2 C is changed into a Kondo-like one around 12 kbar. We suppose that the suppression of the magnetic order in b-UB2 C by applying pressure is related with the development of Kondo effect. r 2007 Published by Elsevier B.V. PACS: 71.10.Hf; 71.20.Lp; 71.27.+a; 75.30.Mb Keywords: b-UB2 C; Ferromagnetism; Resistivity measurement; Pressure effect
1. Introduction Some uranium ferromagnets like UGe2 and UIr exhibit unconventional superconductivity under pressure [1,2]. In such compounds, the superconducting state appears in the pressure range where the ferromagnetic ordering starts to change into a paramagnetic state. Recently, the rhombo¯ hedral b-UB2 C (space group R3m) has reported to be an itinerant electron ferromagnet with T C ¼ 74:5 K [3]. It was shown that both magnetic specific heat divided by temperature and the temperature derivative of electrical resistivity show a hump at a characteristic temperature T , resembling the behavior of the compounds mentioned above at ambient pressure. Therefore, it is of interest to investigate the effect of pressure P on physical properties of b-UB2 C. In this contribution, we report on measurements of the temperature dependence of electrical resistivity rðTÞ
Corresponding author. Tel.: +48 71 3435021; fax: +48 71 3441029.
E-mail address: [email protected] (V.H. Tran). 0921-4526/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.physb.2007.10.361
of a polycrystalline sample of b-UB2 C under pressures up to 16 kbar. 2. Experimental details and results The polycrystalline sample of b-UB2 C was synthesized and its quality was examined by methods described elsewhere [3]. The measurements of the electrical resistivity were performed in the temperature range 1.5–300 K by means of the standard four-probe technique. Hydrostatic pressure up to 16 kbar was generated by a piston– cylinder cell using silicon oil as the pressure-transmitting medium. Fig. 1a shows the temperature dependencies of normalized electrical resistivity rðTÞ=rð300 KÞ of b-UB2 C at pressures up to 9 kbar while Fig. 1b shows the data collected between 11.5 and 16 kbar. In the first pressure range, the resistivity represents the behaviour of typical metallic ferromagnets, for which the resistivity consists mainly of spin-disorder and electron–phonon scattering contributions in the paramagnetic state and electron–magnon
ARTICLE IN PRESS V.H. Tran et al. / Physica B 403 (2008) 1375–1377
1.0
a T*
3
TC
0.6
1 kbar -UB2C
0.4 0.2
0
50
b
1.2
d/dT (arb.units)
0.8 /300K
9 kbar
2 1 0 0
100
20
40 60 T (K)
150 T (K)
200
80
250
100
300
11.5 kbar
1.2
0.8
/300K
/300K
1.0
0.6
0.4 0.0
0.4 0.2
0.8
10
16 kbar 0
50
100
150 T (K)
T (K)
200
100
250
pressure influences also the magnitude of resistivity at low temperatures. The increase in the ratio rð2 KÞ= rð300 KÞ with increasing pressure implies that the Kondo interactions increase and become more important till the Kondo temperature is of the order of room temperature. At pressures above 9 kbar, there is evidence of a change in the shape of the rðTÞ curves. At high temperatures, the resistivity (Fig. 1b) exhibits a broad maximum at T max , which usually occurs in Kondo systems. With pressure further increasing, the position of T max , indicated by a vertical line, shifts to higher values and can be recognized as a result of Kondo effect. Below 75 K the resistivity starts to decrease. However, at present we are not able to determine the mechanism behind this phenomenon. Because of the lack of any anomaly in the drðTÞ=dT-curves, one may suspect that the long-range magnetic order no longer exists or at least the contribution of scattering on magnetic moments to the total resistivity is negligible. Alternatively, the resistivity drop may be ascribed to the coherence as in the case of nonmagnetic Kondo lattices. The low-temperature part of the resistivity is presented in the inset of Fig. 1b. There appears a shallow minimum at about 10 K and followed by a small upturn when temperature approaches 1.5 K.
300
Fig. 1. (a) Temperature dependence of normalized resistivity rðTÞ=rð300 KÞ of b-UB2 C under pressures up to 9 kbar. The inset shows the temperature derivative of the resistivity. (b) The normalized resistivity measured between 11.5 and 16 kbar. The inset shows the low-temperature part of the resistivity.
scattering contribution in the ordered state. Therefore, at high temperatures the temperature derivative of the resistivity has a positive value, typical of ordinary metals. For b-UB2 C, below Curie temperature, in addition to the drop related to the magnetic ordering the resistivity shows a hump at T in its temperature derivative (see inset of Fig. 1). The mechanism of this anomaly was proposed as a quantum spin fluctuation [3]. In spite of the fact that the shape of the rðTÞ-curves collected at pressures up to 9 kbar is similar to that observed at ambient pressure previously reported [3], the applied pressure has several effects on the resistivity behavior of b-UB2 C. First of all P significantly reduces both T C and T . We can recognize also that the energy gap deduced from the relation predicted for electron–magnon scattering [4] : rðTÞ ¼ r0 þ AT 2 þ bð1 þ 2kB T=DÞ expðD=kB TÞ, where r0 is the residual resistivity, A is the Fermi-liquid T 2 coefficient of resistivity and D is an energy gap present in the magnon spectrum, decreases with increasing pressure. The decrease in T C and D is easily understood, since the pressure destroys the magnetic order. The applied
3. Concluding remarks The main results of our study are summarized in the magnetic phase diagram shown in Fig. 2, where one may distinguish two magnetic ranges, denoted as long-range ferromagnetic order (LFO) and dominating Kondo-like (K) behaviour. The fact that the long-range ferromagnetic order seems to vanish before the system reaches Pcr of
80 TC 60
TC, T* (K)
1376
β-UB2C Pcr ~ 18 kbar
40
T*
20
0
0
4
8
12 16 P (kbar)
20
24
Fig. 2. Tentative phase diagram of b-UB2 C under pressures up to 16 kbar. Extrapolation of the T C ðPÞ-curve yields the critical pressure of about 18 kbar.
ARTICLE IN PRESS V.H. Tran et al. / Physica B 403 (2008) 1375–1377
about 18 kbar (see Fig. 2), is probably related with the development of Kondo interaction, which origins from the increasing mixing between localized 5f- and conduction electron states. Acknowledgements One of us (TVH) wants to thank the Ministry of Science and Higher Education in Poland for financial support within Grant no. N202 082 31/0449. The measurement of resistivity under pressure was made possible due to support
1377
from COST Action P16 Emergent Behaviour in Correlated Matter (ECOM). T.V.H. and P.R. are grateful to the OEAD grant P10/2006.
References [1] [2] [3] [4]
S.S. Saxena, et al., Nature 406 (2000) 587. T. Akazawa, et al., J. Phys.: Condens. Matter 16 (2004) L29. V.H. Tran, et al., J. Phys.: Condens. Matter 18 (2006) 703. H.H. Andersen, H. Smith, Phys. Rev. B 19 (1979) 384.