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v h tran
Description
Synthesis and magnetic properties
of a new ferromagnetic Kondo-lattice system Np2PdGa3
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V. H. Tran,1 J. -C. Griveau,2 R. Eloirdi,2 W. Miiller, 1 E. Colineau,2
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1 Institute of Low Temperature and Structure Research,
Polish Academy of Sciences, P. O. Box 1410, 50-950 Wrocław, Poland
2European Commission, Joint Research Centre, Institute for Transuranium Elements,
Postfach 2340, D-76125 Karlsruhe, Germany
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A number of intermetallic of the stoichiometric compounds U2TM3, where T = 3d, 4d and 5d electron transition metals and M = Si or Ga have been discovered [1-3]. Generally, these compounds adopt two well known types of crystal structures, i.e., the hexagonal AlB2- or orthorhombic CeCu2-type, respectively. Most of the compounds which crystallize in the hexagonal AlB2-type are those containing M = Si and showing spin-glass or ferromagnetic cluster glass behavior [1,4]. On the other hand, the compounds with M = Ga favor the CeCu2-type and exhibit various types of magnetic ordering including the spin fluctuation, ferromagnetic and antiferromagnetic order at low temperatures [3,5]. Amongst U2TGa3 , the magnetism of the Pd- and Pt-based compounds appears to be an enormously complex subject owing to a competition between the Kondo effect and randomness for long range antiferromagnetism. Therefore, in order to made a systematic study, an investigation of systems being isostoichiometic and/or isostructural to U2(Pd,Pt)Ga3 would be useful. In this contribution, we present the synthesis and crystallographic characterization, and as well magnetic properties for a new Np-based Np2PdGa3 compound.
Fig. 1 X-ray powder diffraction pattern of Np2PdGa3. The observed (open circles), calculated (solid line), positions of Bragg reflections (vertical lines) and the difference between observed and calculated data (bottom). Inset: Crystal structure of Np2PdGa3. Large balls represent the Np atoms and small ones the Pd or Ga atoms. Note that the nearest Np neighbors form zigzag chains parallel to the b axis (thick line) and the next-nearest Np neighbors are connected by zigzag chains along the a axis (thin line).
The X-ray diffraction data collected in the range 20 – 100 deg, shown in Fig. 1, revealed that the majority phase (> 95 mass. %) has the orthorhombic CeCu2-type structure. We were able to identify NpO2, Np3Pd3Ga8 and NpC to be the main impurities. Traces of these impurities are denoted by arrows in the diffraction pattern. The observed Bragg reflections for Np2PdGa3 could be indexed with lattice parameters a = 0.4445(2) nm, b = 0.7089(3) nm and c = 0.7691(3) nm. Taking into account the size of An3+ and An4+ ions and comparing the lattice parameters of U2PdGa3 and Np2PdGa3 one suspects the 3+ valence of the magnetic Np ions.
The measurements of magnetization specific heat, electrical resistivity, magnetoresistance and Hall effect for Np2PdGa3 indicated a ferromagnetic ordering below 62.5(5) K. The analysis of magnetic susceptibility (Fig. 2) and specific heat (Fig. 3) consistently suggests a CEF splitting with doublet-doublet-doublet scheme and CEF ~ 60 K and 180 K.
Fig. 2 Temperature dependence of magnetic susceptibility of Np2PdGa3 and Lu2PdGa3. The solid line is the CEF fit
Fig. 3 Temperature dependence of the contribution of 5f-electron specific heat and 5f-electron entropy of Np2PdGa3.
The lines are calculated CEF, Kondo and magnon contributions to the specific heat.
The enhanced Sommerfeld ratio at low temperature (C5f/T = 120 mJ/K2mol.Np at 2 K) and lnT dependence of the resistivity can be interpreted due to the Kondo effect with TK ~ 35 K (see dash-dotted line). The Hall coefficient exhibits a behavior for localized moment ferromagnets with low carrier concentration (0.17 carrier/f.u) and with enhanced effective mass (~ 144 m0).
The presented data seem to be consistent with the underscreenced Kondo lattice model recently developed by Perkins et. al [6]. We argue thus that Np2PdGa3 is a new ferromagnetic Kondo lattice with TK < TRKKY ~ CEF. Among Np-based compounds, ferromagnetic Kondo behavior was previously found for NpNiSi2 [7].
References
[1] D. Kaczorowski and H. Noel, J. Phys.: Condens. Matter 5, 9185 (1993).
[2] B. Chevalier, R. Pottgen, B. Darriet, P. Gravereau, and J. Etourneau, J. Alloys Compd. 233 (1996) 150.
[3] V. H. Tran, J. Phys.: Condens. Matter 8, 6267 (1996).
[4] D. X. Li, S. Nimori, Y. Shiokawa, Y. Haga, E. Yamamoto, Y. Onuki, Phys. Rev. B 68, 172405 (2003).
[5] V. H. Tran, F. Steglich, G. Andre, Phys. Rev. B 65, 134401 (2002).
[6] N. B. Perkins, M. D. Nunez-Regueiro, B. Coqblin, and J. R. Iglesias, Phys. Rev. B 76, 125101 (2007).
[7] E. Colineau, F. Wastin, J. P. Sanchez, and J Rebizant, J. Phys.: Condens. Matter 20, 075207 (2008).
