Thermal properties of UPdSn and UCuSn

ELSEVIER

Physica B 237 238 (1997) 226 228

Thermal properties of UPdSn and UCuSn H. Kawanaka a'*, H. Nakotte a'b, E. Briick c, K. Proke~ a, N.H. Kim-Ngan c' 1, T. Takabatake d, H. Fujii d, J. S a k u r a i e a Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba 305, Japan b Los Alamos National Laboratory, Los Alamos N M 87545, USA c University o f Amsterdam, 1018 XE Amsterdam, The Netherlands d Hiroshima University, Higahashi-Hiroshima 739, Japan e Toyama University, Toyama 930, Japan

Abstract We report on the specific heat and the thermopower of UPdSn and UCuSn, both of which order antiferromagnetically at low temperatures. The compounds show similar behaviour in the specific heat, and the large magnetic-entropy changes around TN are evidence for a large degree of 5f-electron localization. For both compounds, we find that thermopower results are consistent with the findings for the electrical resistance. While for UCuSn very abrupt changes at 25 and 60 K are observed for both quantities, more continuous changes at the magnetic transitions (25 and 40 K) are found for UPdSn.

Keywords: Specific heat; Thermoelectric power; UPdSn; UCuSn; 5f-electron

1. Introduction 5f-ligand hybridization is known to dominate the magnetism within the large group of UTX compounds (T = transition metal, X = p-electron metal), where the full range of properties from localized to itinerant magnetism can be found [1]. Here, we concentrate on two UTX compounds with more localized 5f-electrons, namely U P d S n and UCuSn. UPdSn has been studied extensively on single crystals by bulk 1-2] and neutron-diffraction [3] experiments, and two antiferromagnetic transitions around 25 and 40 K have been identified unambiguously. Likewise, the bulk results for UCuSn, indicate two transitions around 25 and 60 K 1-4, 5]. Neutron diffraction [5] showed that * Corresponding author. Present address: Pedagogical University, SW. Filipa 5/5, 31150 Krakow, Poland. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 1 1 I - 7

antiferromagnetic order sets in at 60 K, but the nature of the 25 K transition has not been resolved yet. In this paper, we present the specific heat and thermopower data for U P d S n and UCuSn and the analysis.

2. Experimental results and discussion The specific heat was measured over the temperature range between 1.5 and 250 K. In Fig. 1, the compounds show a similar behavior in temperature dependence of the specific heat. While we find pronounced maxima at the Neel temperatures (40 K in UPdSn and 60 K in UCuSn), only very weak shoulders are seen at the low-temperature transitions (around 25 K). The low-temperature extrapolations of both specific heats to T = 0 K yields electronic contributions ~, of about 5 and 64 mJ m o l - 1 K - 2 for UPdSn and UCuSn, respectively. In order to

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Fig. 1. Temperature dependence of the specific heat of (a) UPdSn and (b) UCuSn. The solid lines represent the Debye functions.

Fig. 2. Temperature dependence of the thermopower of (a) UPdSn and (b) UCuSn.For comparison,we havealso included the temperaturedependencesof the electricalresistivityas solid lines.

get the estimates of the lattice contribution to the specific heat we have measured the specific heat up to 250 K, where phonons dominate. We have then fitted the Debye function to the high-temperature part of the specific heats, and the resulting Debye functions f are shown in Fig. 1 by solid lines. We obtain similar Debye temperatures for both compounds yielding values of about 185-190 K. Subtraction of such lattice contributions yield magnetic-entropy changes to about 1.1R ln2 (at 45 K) and R In2 (at 65 K) for UPdSn and UCuSn, respectively. Such values for the magnetic entropy are among the highest in the UTX series (see e.g. Ref. [7]) and indicate the large degree of 5f-electron localization in both compound.' For this compound, crystal fields have been indeed observed by inelastic-neutron-scattering experiments [8] and further analysis of the specific heat taking into account the observed level splitting is underway.

The temperature dependence of the thermopower were measured between 4.2 and 300 K using the standard stationary technique [9] are shown in Fig. 2. For comparison, we have included the temperature dependences of the electrical resistivities [2, 4] in the figure. The thermopower of UPdSn shows large positive values at high temperatures with a broad maximum around 150K, a temperature where also a local minimum in the electrical resistivity is found. Below TN ( = 40 K), the thermopower drops to negative values, a behavior often found for antiferromagnets. Finally, there is a minimum in the thermopower, which occur around the lower magnetic transition (--25 K). This transition is hardly visible in the electrical resistivity and shows up only as a slight change of d p / d T . For UCuSn, on the other hand, very pronounced anomalies are found in the temperature dependences of the electrical resistivity and the

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H. Kawanaka et aL / Physica B 237-238 (1997) 226 228

thermopower around 25 and 60 K. Similar to UPdSn, the thermopower of UCuSn becomes negative around TN ( = 60 K) and it goes positive again at 25 K. In conclusion, our specific-heat results provide some evidence for a rather localized character to the magnetism in both compounds, UPdSn and UCuSn. For the thermopower, very abrupt changes at 25 and 60 K are observed in UCuSn, while more continuous changes at the magnetic transitions (25 and 40 K) are seen in UPdSn. This is consistent with the findings for the electrical resistivity. In uranium compounds, the thermopower reflects mainly the Fermi-surface effects, it should be noted, however, that magnetic transitions are often accompanied by changes in the Fermi surface. Future

studies are needed to show whether such a picture is indeed correct.

References [1] V. Sechovsky et al., Ferromagnetic Materials (NorthHolland, Amsterdam, 1988) Vol. 4. [2] F.R. de Boer et al., Physica B 176 (1992) 275. [3] R.A. Robinson et al., J. Appl. Phys. 75 (1994) 6589. [4] H. Fujii et al., J. Magn. Magn. Mater. 87 (1990) 237. [5] H Nakotte et al., J. Appl. Phys. 79 (1996) 6408. [6] N.H. Kim-Ngan, PhD Thesis, University of Amsterdam (1993), unpublished. I-7] F.R. de Boer et al., J. Appl. Phys. 69 (1991) 4702. [8] H. Nakotte et al., unpublished results. 1-9] Y. Yamaguchi et al., J. Phys.: Condens. Matter 2 (1990) 5715.