Magnetic and electronic properties of the ferromagnetic Kondo-lattice systemNp2PdGa3

Abstract

Intermetallic compounds Lu 2 PdGa 3 and Np 2 PdGa 3 have been found to crystallize in an orthorhombic CeCu 2 -type structure with the Imma space group. The lattice parameters at room temperature are: a = 0.4382͑2͒, b = 0.6870͑3͒, and c = 0.7593͑3͒ nm for Lu 2 PdGa 3 and a = 0.4445͑2͒, b = 0.7089͑3͒, and c = 0.7691͑3͒ nm for Np 2 PdGa 3 . The electronic ground-state properties of these compounds were established by magnetic, calorimetric, electrical resistivity, magnetoresistance, and Hall coefficient measurements. The experimental data reveal a metallic behavior for Pauli paramagnetic Lu 2 PdGa 3 , characterized by Sommerfeld ratio ␥ = 2.5 mJ/ K 2 mol Lu and temperature-independent susceptibility 0 ϳ 1 ϫ 10 −4 emu/ mol. In contrast to the nonmagnetic Lu 2 PdGa 3 reference, Np 2 PdGa 3 behaves as a local-moment ferromagnet with the Curie temperature T C = 62.5͑2͒ K. Low-temperature properties of Np 2 PdGa 3 are characterized by a large Sommerfeld ratio ␥ = 120͑2͒ mJ/ K 2 mol Np and a small Fermi momentum k F = 0.44 Å −1 . The observed features in Np 2 PdGa 3 are well interpreted by assuming competition of three different interactions: Rudermann-Kittel-Kasuya-Yosida ͑RKKY͒, crystal electric field ͑CEF͒ and Kondo effect, which are represented by respective energies k B T RKKY , ⌬ CEF , and k B T K . We argue that Np 2 PdGa 3 is the Np-based ferromagnetic Kondo-lattice system with T K Ͻ T RKKY Ϸ ⌬ CEF .

The Kondo Lattice Models and Magnetism for Cerium and Uranium Systems

“…A similar behavior has been recently observed in neptunium compounds NpNiSi 2 [27] and Np 2 PdGa 3 [28].…”

Kondo Screening Effect and Ferromagnetic Order in UCu2Si2

“…This new scenario presented here for UCu 2 Si 2 enlarges the list of the growing family of the UKL ferromagnetic compounds, with already known some uranium compounds such as UTe [9] or recently neptunium compounds like NpNiSi 2 [10] or Np 2 PdGa 3 [11]. This peculiar UKL behaviour arises from the localized spins of the actinide atoms which are larger than the s = 1/2 conduction electron spins and consequently cannot be completely screened at very low temperatures.…”

Coexistence of Kondo effect and ferromagnetism in the Underscreened Kondo Lattice model

“…In the case of actinide compounds with 5 f electrons, it is known that some of them exhibit a coexistence of Kondo effect and ferromagnetism. Some uranium and neptunium compounds such as UTe [2,3,4], UCu 0.9 Sb 2 [5], UCo 0.5 Sb 2 [6], NpNiSi 2 [7] and Np 2 PdGa 3 [8] present this behavior, with Curie temperatures of order T C ∼ 50 − 100K, which are higher than the magnetic transition temperatures observed in the rare-earth compounds. Another interesting behavior is the variation of T C as a function of applied pressure in UTe [9,10], where T C increases firstly with pressure, goes then through a maximum and finally decreases at very high pressures.…”