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Study of the strongly correlated electron system UCu5Sn
Physica B 259—261 (1999) 263—264
Study of the strongly correlated electron system UCu Sn V.H. Tran*, R. Troc´, T. Cichorek W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland
Abstract We report on measurements of magnetisation and magnetoresistance of the Kondo lattice compound UCu Sn, being magnetically ordered at ¹ +54 K and showing an enhanced value of the linear term in the specific heat at low ! temperatures. The magnetic ordered state is characterised by a large ferromagnetic component of the magnetic structure and is related to a distinct irreversibility of the ZFC and FC-magnetisation. The magnetoresistance, measured at ¹)3 K, changes its sign from positive to negative, while above this temperature it is always negative. This behaviour is supposed to be attributed to the presence of a transition from a coherent to an incoherent scattering state at very low temperatures. 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetisation; Magnetoresistance; Heavy-fermion magnetically ordered material
The discovery of heavy-fermion properties in the orthorhombic CeCu system [1] and the non-Fermi-liquid behaviour in its solid solutions CeCu Au around \V V x+0.1 [2] has turned our attention to uranium-based compounds with similar stoichiometry and crystal structure. In the case of these latter intermetallics, we have recently identified and studied the magnetic and transport properties of the novel compound UCu Sn [3]. Preliminary studies have demonstrated that this compound possesses a relatively large electronic heat coefficient (c"C/¹(at 0.4 K)"330 mJ/mol K [3]) and hence it can be classified as a moderate heavy-fermion system. Furthermore, a dense Kondo-type behaviour with a Kondo temperature ¹ "16 K was also deduced from ) magnetic susceptibility, electrical resistivity and specific heat data. In the present work, we performed detailed magnetisation (M) and magnetoresistance (MR) measurements on UCu Sn in order to gain further insight into the heavy-fermion properties of this compound. The studied compound crystallises in a hexagonal structure
* Corresponding author. Tel.: #48-71-3435021; fax: #4871-441029; e-mail: [email protected].
with room-temperature lattice parameters: a"4.983(2) and c"20.295(3) As . In Fig. 1, we have plotted the magnetisation data collected in selected strengths of magnetic fields up to 5 T. In all these fields, the magnetisation shows a sharp rise below about 60 K indicating a ferromagnetic-like phase transition. The ordering temperature ¹ , deter! mined as an inflection point of the magnetisation taken at the lowest magnetic fields, is equal to 53.9 K. It should be noted that below ¹ the magnetisation versus ! ¹ curves depend on the applied measurement conditions, i.e. the zero-field cooled (ZFC) or in field cooled (FC) ones. Such a behaviour is due to the large magnetocrystalline anisotropy existing in the sample in the magnetically ordered state. As one expects, the thermomagnetic irreversibility point ¹ , marked by a dashed arrow in Fig. 1, shifts towards lower temperatures as the strength of the magnetic field increases. This phenomenon is still observable at B"5 T. In Fig. 2 we display the temperature dependence of the electrical resistivity, o(¹), obtained in zero field and at 8 T. Below ¹ "54 K a distinct drop in the resistivity ! appears. At 25 K it passes through a distinct minimum and then steeply increases as (!ln ¹) with further lowering temperature. At a field of 8 T, o(¹) is dramatically
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 9 7 4 - 0
264
V.H. Tran et al. / Physica B 259—261 (1999) 263—264
Fig. 1. Magnetisation of UCu Sn vs. temperature measured in selected magnetic field strengths.
Fig. 2. Electrical resistivity of UCu Sn vs. temperature mea sured in zero fields and in a field of 8 T. The inset shows the temperature dependence of the magnetoresistance *o/o(¹). The solid lines illustrate the Kondo-like behaviour.
reduced in the ordered state. Thus, the magnetoresistance *o/o"[o(¹, B)!o(¹)]/o(¹) taken at B"8 T is negative. As shown in the inset of Fig. 2, the magnitude of *o/o of UCu Sn grows with increasing temperature showing no distinct anomaly at ¹ . Hence, we suspect ! that at low temperatures the importance of the Kondo effect [4,5] dominates the other possible effects [6,7]. In Fig. 3, we show the field dependencies of MR of UCu Sn. For the ¹"50 and 250 mK isotherms, *o/o exhibits an anomalous positive contribution below about 1 T, but at higher fields it becomes negative. At 3 K, *o/o in weak fields is close to zero and decreases with increasing field as B, while *o/o for ¹'4 K is negative in the entire range of the applied field. At the maximum applied magnetic field of 14 T, the magnitude of *o/o is quite large, and at 50 mK it amounts to about !15%. The observed magnetoresistance behaviour might be discussed in terms of a coherent-incoherent crossover in the presence of the magnetic field, in accordance with the common manner exhibited by other well-known heavy fermion materials, like CeCu Si , CeAl [8,12] or CeCu [9]. For example, Zlatic´ [10] as well as Kawakami and Okiji [11] have predicted for a Kondo lattice system the
Fig. 3. Field dependence of the magnetoresistance for UCu Sn at very low temperatures. The inset shows its low-field part.
occurrence of a positive MR in the coherent state. The above authors have also predicted that the magnetoresistance should change its sign at ¹ +¹ /2p. An applica ) tion of the latter relation to the Kondo temperature of ¹ "16 K, determined for UCu Sn [3], leads to ) ¹ +2.6 K. This value is close to that found in our experiment (3 K). In the incoherent state of a Kondo lattice, the magnetoresistance is always negative. We think this happens also in the case of our compound at temperatures ¹'4 K. In conclusion, we have reported on the magnetisation and magnetoresistance for the strongly correlated electron system UCu Sn. Based on the magnetoresistance data, we have concluded that UCu Sn is passing from the incoherent to coherent Kondo state at around 3 K, though such a behaviour has not been detected in the temperature dependence of the electrical resistivity [3], measured down to 20 mK. This work was supported by the “State Committee for Scientific Research” in Poland within the Grant No. 2 P03B 14710.
References [1] A. Amato et al., J. Low Temp. Phys. 68 (1987) 371. [2] H. von Lo¨hneysen, J. Phys.: Condens. Matter 8 (1996) 9689. [3] V.H. Tran et al., An abstract booklet of the 28´ Journe´es des Actinides, May 14—16, 1998, Uppsala, Sweden. [4] M.T. Be´al-Monod, R.A. Weiner, Phys. Rev. 170 (1968) 552. [5] P. Schlottman, Z. Phys. B Condens. Matter 51 (1983) 223. [6] H. Yamada, S. Takada, Progr. Theoret. Phys. 49 (1973) 1401. [7] K. Ueda, Solid State Commun. 19 (1976) 965. [8] U. Rauchschwalbe, Physica B 147 (1987) 1. [9] P.T. Coleridge, J. Phys. F 17 (1987) L79. [10] V. Zlatic, J. Phys. F 11 (1981) 2147. [11] N. Kawakami, A. Okiji, J. Phys. Soc. Japan 55 (1986) 2114. [12] U. Rauchschwalbe et al., Physica B 148 (1987) 33.
Study of the strongly correlated electron system UCu Sn V.H. Tran*, R. Troc´, T. Cichorek W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland
Abstract We report on measurements of magnetisation and magnetoresistance of the Kondo lattice compound UCu Sn, being magnetically ordered at ¹ +54 K and showing an enhanced value of the linear term in the specific heat at low ! temperatures. The magnetic ordered state is characterised by a large ferromagnetic component of the magnetic structure and is related to a distinct irreversibility of the ZFC and FC-magnetisation. The magnetoresistance, measured at ¹)3 K, changes its sign from positive to negative, while above this temperature it is always negative. This behaviour is supposed to be attributed to the presence of a transition from a coherent to an incoherent scattering state at very low temperatures. 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetisation; Magnetoresistance; Heavy-fermion magnetically ordered material
The discovery of heavy-fermion properties in the orthorhombic CeCu system [1] and the non-Fermi-liquid behaviour in its solid solutions CeCu Au around \V V x+0.1 [2] has turned our attention to uranium-based compounds with similar stoichiometry and crystal structure. In the case of these latter intermetallics, we have recently identified and studied the magnetic and transport properties of the novel compound UCu Sn [3]. Preliminary studies have demonstrated that this compound possesses a relatively large electronic heat coefficient (c"C/¹(at 0.4 K)"330 mJ/mol K [3]) and hence it can be classified as a moderate heavy-fermion system. Furthermore, a dense Kondo-type behaviour with a Kondo temperature ¹ "16 K was also deduced from ) magnetic susceptibility, electrical resistivity and specific heat data. In the present work, we performed detailed magnetisation (M) and magnetoresistance (MR) measurements on UCu Sn in order to gain further insight into the heavy-fermion properties of this compound. The studied compound crystallises in a hexagonal structure
* Corresponding author. Tel.: #48-71-3435021; fax: #4871-441029; e-mail: [email protected].
with room-temperature lattice parameters: a"4.983(2) and c"20.295(3) As . In Fig. 1, we have plotted the magnetisation data collected in selected strengths of magnetic fields up to 5 T. In all these fields, the magnetisation shows a sharp rise below about 60 K indicating a ferromagnetic-like phase transition. The ordering temperature ¹ , deter! mined as an inflection point of the magnetisation taken at the lowest magnetic fields, is equal to 53.9 K. It should be noted that below ¹ the magnetisation versus ! ¹ curves depend on the applied measurement conditions, i.e. the zero-field cooled (ZFC) or in field cooled (FC) ones. Such a behaviour is due to the large magnetocrystalline anisotropy existing in the sample in the magnetically ordered state. As one expects, the thermomagnetic irreversibility point ¹ , marked by a dashed arrow in Fig. 1, shifts towards lower temperatures as the strength of the magnetic field increases. This phenomenon is still observable at B"5 T. In Fig. 2 we display the temperature dependence of the electrical resistivity, o(¹), obtained in zero field and at 8 T. Below ¹ "54 K a distinct drop in the resistivity ! appears. At 25 K it passes through a distinct minimum and then steeply increases as (!ln ¹) with further lowering temperature. At a field of 8 T, o(¹) is dramatically
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 9 7 4 - 0
264
V.H. Tran et al. / Physica B 259—261 (1999) 263—264
Fig. 1. Magnetisation of UCu Sn vs. temperature measured in selected magnetic field strengths.
Fig. 2. Electrical resistivity of UCu Sn vs. temperature mea sured in zero fields and in a field of 8 T. The inset shows the temperature dependence of the magnetoresistance *o/o(¹). The solid lines illustrate the Kondo-like behaviour.
reduced in the ordered state. Thus, the magnetoresistance *o/o"[o(¹, B)!o(¹)]/o(¹) taken at B"8 T is negative. As shown in the inset of Fig. 2, the magnitude of *o/o of UCu Sn grows with increasing temperature showing no distinct anomaly at ¹ . Hence, we suspect ! that at low temperatures the importance of the Kondo effect [4,5] dominates the other possible effects [6,7]. In Fig. 3, we show the field dependencies of MR of UCu Sn. For the ¹"50 and 250 mK isotherms, *o/o exhibits an anomalous positive contribution below about 1 T, but at higher fields it becomes negative. At 3 K, *o/o in weak fields is close to zero and decreases with increasing field as B, while *o/o for ¹'4 K is negative in the entire range of the applied field. At the maximum applied magnetic field of 14 T, the magnitude of *o/o is quite large, and at 50 mK it amounts to about !15%. The observed magnetoresistance behaviour might be discussed in terms of a coherent-incoherent crossover in the presence of the magnetic field, in accordance with the common manner exhibited by other well-known heavy fermion materials, like CeCu Si , CeAl [8,12] or CeCu [9]. For example, Zlatic´ [10] as well as Kawakami and Okiji [11] have predicted for a Kondo lattice system the
Fig. 3. Field dependence of the magnetoresistance for UCu Sn at very low temperatures. The inset shows its low-field part.
occurrence of a positive MR in the coherent state. The above authors have also predicted that the magnetoresistance should change its sign at ¹ +¹ /2p. An applica ) tion of the latter relation to the Kondo temperature of ¹ "16 K, determined for UCu Sn [3], leads to ) ¹ +2.6 K. This value is close to that found in our experiment (3 K). In the incoherent state of a Kondo lattice, the magnetoresistance is always negative. We think this happens also in the case of our compound at temperatures ¹'4 K. In conclusion, we have reported on the magnetisation and magnetoresistance for the strongly correlated electron system UCu Sn. Based on the magnetoresistance data, we have concluded that UCu Sn is passing from the incoherent to coherent Kondo state at around 3 K, though such a behaviour has not been detected in the temperature dependence of the electrical resistivity [3], measured down to 20 mK. This work was supported by the “State Committee for Scientific Research” in Poland within the Grant No. 2 P03B 14710.
References [1] A. Amato et al., J. Low Temp. Phys. 68 (1987) 371. [2] H. von Lo¨hneysen, J. Phys.: Condens. Matter 8 (1996) 9689. [3] V.H. Tran et al., An abstract booklet of the 28´ Journe´es des Actinides, May 14—16, 1998, Uppsala, Sweden. [4] M.T. Be´al-Monod, R.A. Weiner, Phys. Rev. 170 (1968) 552. [5] P. Schlottman, Z. Phys. B Condens. Matter 51 (1983) 223. [6] H. Yamada, S. Takada, Progr. Theoret. Phys. 49 (1973) 1401. [7] K. Ueda, Solid State Commun. 19 (1976) 965. [8] U. Rauchschwalbe, Physica B 147 (1987) 1. [9] P.T. Coleridge, J. Phys. F 17 (1987) L79. [10] V. Zlatic, J. Phys. F 11 (1981) 2147. [11] N. Kawakami, A. Okiji, J. Phys. Soc. Japan 55 (1986) 2114. [12] U. Rauchschwalbe et al., Physica B 148 (1987) 33.