27 March 2010 to 1 April 2010
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Specific heat of the antiferro / ferro-magnet NpGa3

29 Mar 2010, 09:40
20m
503/1-001 - Council Chamber (CERN)

503/1-001 - Council Chamber

CERN

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Speaker

Dr eric colineau (European Commission)

Description

The actinide compounds AnX3, where X is an element from Group IIIA or IVA, crystallize in the cubic AuCu3 structure. They are characterized by an actinide-actinide interatomic distance far above the Hill limit, therefore 5f–ligand hybridization is the main mechanism responsible for the delocalization of the 5f electrons. The systematic of this hybridization has been well demonstrated for the UX3 compounds, which either do not order magnetically (X=Al, Si, Ge, Sn) or exhibit antiferromagnetism (X=Ga, In, Tl, Pb). It was concluded that the 5f–ligand hybridization increases as one moves up a column of the Periodic Table or moves from a Group IIIA element to a Group IVA element [1,2]. Although data on corresponding Np intermetallics are much less documented, a similar trend was noticed. However, it is clear that the Np-based compounds are more “magnetic” than their uranium analogues, which is consistent with the general picture that the hybridization decreases as one substitutes a heavier actinide. Indeed, all NpX3 compounds (X=Al, Ga, In, Sn) order magnetically at the exception of NpGe3 and NpSi3 [3,4,5]. Several UX3 systems (X=Al, Ga, In, Sn, Pb) present enhanced specific heat at low temperature, in particular USn3 ( = 170 mJ mol-1 K-2) and UPb3 ( = 110 mJ mol-1 K-2). In the neptunium analogues, only NpSn3 ( = 88 mJ mol-1 K-2), NpIn3 ( = 72 mJ mol-1 K-2) and NpGe3 ( = 34 mJ mol-1 K-2) have been investigated [6]. In the present study, we focus on the specific heat properties of NpGa3. It is worth noticing that UGa3 is the only UX3 compound considered to be itinerant that displays magnetic order [2]. NpGa3 exhibits antiferromagnetic ordering (k = (½ ½ ½)) below TN65K, but ferromagnetic order with an ordered moment Np1.5B is stabilized below TC50K [7]. The high pressure behavior of NpGa3 indicates a weak delocalization of the 5f electrons [8]. Figure 1 shows the temperature dependence of the specific heat in NpGa3. The room-temperature value corresponds to the Dulong-Petit limit (100 J mol-1 K-2). Going down in temperature, we observe a lambda-type anomaly at 66K that corresponds to the onset of antiferromagnetic ordering. At 51K, an intense and narrow peak emerges from the curve, indicating the magnetic phase transition from antiferromagnetism to ferromagnetism. This sharp peak reveals the first-order nature of the transition, as previously pointed out by the thermal variation of the magnetic moment measured by Mössbauer spectroscopy and neutron diffraction [7]. At low-temperature, the extraction of the electronic specific heat is delicate, due to the magnetic contribution and the presence of an upturn, but the Sommerfeld coefficient can be roughly estimated to ~ 40 mJ mol-1 K-2, which is smaller than the  values reported for the other ordered NpX3 compounds and for UGa3 . This observation is consistent with the narrow-band picture suggested by high-pressure experiments [8]. Experiments in magnetic fields up to 14T have also been performed and will be presented. The antiferromagnetic phase is destroyed above 4T. The magnetic phase diagram was precisely rebuilt and essentially confirms the previously established one. Acknowledgements The high-purity Np metal required for the fabrication of NpGa3 was made available in the framework of a collaboration with the Lawrence Livermore and Los Alamos National Laboratories and the U.S. Department of Energy. References [1] D.D. Koelling, B.D. Dunlap, G.W. Crabtree, Phys. Rev. B 31, 4966 (1985). [2] A. L. Cornelius, A. J. Arko, J. L. Sarrao, J. D. Thompson, M. F. Hundley, C. H. Booth, N. Harrison and P. M. Oppeneer, Phys. Rev. B 59, 14473 (1999). [3] J. Gal, I. Yaar, S. Fredo, I. Halevy, W. Potzel, S. Zwirner and G.M. Kalvius, Phys. Rev. B 46, 5351 (1992). [4] J.P. Sanchez, M.N. Bouillet, E. Colineau, A. Blaise, M. Amanowicz,, P. Burlet, J.M. Fournier, Physica B 186–188, 675 (1993). [5] J.P. Sanchez , E. Colineau, P. Vulliet, K. Tomala, J. Alloys and Comp. 275–277, 154 (1998). [6] D. Aoki, Y. Homma, H. Sakai, S. Ikeda, Y. Shiokawa, E. Yamamoto, A. Nakamura, Y. Haga, R. Settai and Y. Onuki, J. Phys. Soc. Japan 75, 084710 (2006) and references therein. [7] E. Colineau, F. Bourdarot, P. Burlet, J.P. Sanchez, J. Larroque, Physica B 230–232, 773 (1997). [8] S. Zwirner,V. Ichas, D. Braithwaite, J.C.Warenborgh, S. Heathman, W. Potzel, G.M. Kalvius, J.C. Spirlet, J. Rebizant, Phys. Rev. B 54, 12283 (1996).

Primary author

Dr eric colineau (European Commission)

Presentation materials