J.-C. Griveau

Magnetic Memory Effect in a Transuranic Mononuclear Complex

Molecular nanomagnets that display magnetic bistability are the subject of intensive investigation due to their unique potential in ultrahigh-density memory components and spintronic devices. So far, the best practical realization of such single-molecule magnets (SMMs) are polymetallic transition-metal complexes with strong intramolecular exchange coupling, giving rise to a high-spin ground state and negligible intercluster interactions. However, 3d metals are restricted by their comparatively low anisotropy, and SMMs with better performance could be produced by exploiting the higher single-ion anisotropy typical of felectron ions. This possibility has been practically demonstrated by Ishikawa et al., who discovered that mononuclear rare earth metal bis-phthalocyanine compounds (Pc2RE) display magnetic hysteresis under favorable conditions. On these grounds, the use of actinides in molecular magnetism appears timely, and indeed slow relaxation effects have recently been reported in a mononuclear uranium-based molecule. Future SMMs displaying magnetic hysteresis could benefit from the fact that, whilst the 5f electron shell can remain relatively well localized, its larger radial extension with respect to the 4f shell can result both in an increased ligand-field potential (and therefore a higher anisotropy energy barrier) and in the possibility to trigger a sizeable exchange coupling in polynuclear complexes, usually precluded to trivalent rare earth metal ions. Moreover, discrete molecules based on 5f ions should allow much greater understanding of the peculiar behavior observed in actinide materials, including multipolar superexchange coupling. Recently, we obtained evidence that a neptunium trimetallic compound displays slow magnetic relaxation and superexchange interaction; nevertheless, we were unable to find any signs of hysteresis in the measured magnetization curves. Here we report the first observation of such low-temperature magnetic memory effects in another transuranic molecular complex, namely, bis(cyclooctatetraenyl)neptunium(IV), commonly known as neptunocene [Np(COT)2] (COT= C8H8 2 ), which was first described in 1970 and belongs to the whole actinocene row. The molecule has a single Np ion between two planar COT rings in a sandwich structure (Figure 1) with D8h symmetry. [9] The degeneracy of the lowest

Superconductivity in the Heusler Family of Intermetallics

Przeprowadzono badania szeregu związkow nadprzewodzących z klasy związkow Heuslera, w szczegolności rodziny (Sc, Y, Lu)Pd2Sn i APd2M (A= Hf, Zr, i M = In, Al). Zwrocono uwage na istotny wplyw sprzezenia elektron - fonon na obserwowane nadprzewodnictwo.

Localized 5f electrons in superconducting PuCoIn5: consequences for superconductivity in PuCoGa5

The physical properties of the first In analog of the PuMGa(5) (M = Co, Rh) family of superconductors, PuCoIn(5), are reported. With its unit cell volume being 28% larger than that of PuCoGa(5), the characteristic spin-fluctuation energy scale of PuCoIn(5) is three to four times smaller than that of PuCoGa(5), which suggests that the Pu 5f electrons are in a more localized state relative to PuCoGa(5). This raises the possibility that the high superconducting transition temperature T(c) = 18.5 K of PuCoGa(5) stems from the proximity to a valence instability, while the superconductivity at T(c) = 2.5 K of PuCoIn(5) is mediated by antiferromagnetic spin fluctuations associated with a quantum critical point.

Synthesis of bimetallic uranium and neptunium complexes of a binucleating macrocycle and determination of the solid-state structure by magnetic analysis.

Syntheses of the bimetallic uranium(III) and neptunium(III) complexes [(UI)(2)(L)], [(NpI)(2)(L)], and [{U(BH(4))}(2)(L)] of the Schiff-base pyrrole macrocycles L are described. In the absence of single-crystal structural data, fitting of the variable-temperature solid-state magnetic data allows the prediction of polymeric structures for these compounds in the solid state.

Thorium/uranium mixed oxide nanocrystals: Synthesis, structural characterization and magnetic properties

One of the primary aims of the actinide community within nanoscience is to develop a good understanding similar to what is currently the case for stable elements. As a consequence, efficient, reliable and versatile synthesis techniques dedicated to the formation of new actinide-based nano-objects (e.g. nanocrystals) are necessary. Hence, a “library” dedicated to the preparation of various actinidebased nanoscale building blocks is currently being developed. Nanoscale building blocks with tunable sizes, shapes and compositions are of prime importance. So far, the non-aqueous synthesis method in highly coordinating organic media is the only approach which has demonstrated the capability to provide size and shape control of actinide-based nanocrystals (both for thorium and uranium, and recently extended to neptunium and plutonium). In this paper, we demonstrate that the non-aqueous approach is also well adapted to control the chemical composition of the nanocrystals obtained when mixing two different actinides. Indeed, the controlled hot co-injection of thorium acetylacetonate and uranyl acetate (together with additional capping agents) into benzyl ether can be used to synthesize thorium/uranium mixed oxide nanocrystals covering the full compositional spectrum. Additionally, we found that both size and shape are modified as a function of the thorium:uranium ratio. Finally, the magnetic properties of the different thorium/uranium mixed oxide nanocrystals were investigated. Contrary to several reports, we did not observe any ferromagnetic behavior. As a consequence, ferromagnetism cannot be described as a universal feature of nanocrystals of non-magnetic oxides as recently claimed in the literature.

Strong-coupling d-wave superconductivity in PuCoGa5 probed by point-contact spectroscopy

Superconductivity is due to an attractive interaction between electrons that, below a critical temperature, drives them to form Cooper pairs and to condense into a ground state separated by an energy gap from the unpaired states. In the simplest cases, the pairing is mediated by lattice vibrations and the wavefunction of the pairs is isotropic. Less conventional pairing mechanisms can favour more exotic symmetries of the Cooper pairs. Here, we report on point-contact spectroscopy measurements in PuCoGa5, a moderate heavy-fermion superconductor with a record high critical temperature Tc=18.5 K. The results prove that the wavefunction of the paired electrons has a d-wave symmetry, with four lobes and nodes, and show that the pairing is likely to be mediated by spin fluctuations. Electronic structure calculations, which take into account the full structure of the f-orbital multiplets of Pu, provide a hint of the possible origin of these fluctuations.

High-resolution solid-state oxygen-17 NMR of actinide-bearing compounds: an insight into the 5f chemistry.

A massive interest has been generated lately by the improvement of solid-state magic-angle spinning (MAS) NMR methods for the study of a broad range of paramagnetic organic and inorganic materials. The open-shell cations at the origin of this paramagnetism can be metals, transition metals, or rare-earth elements. Actinide-bearing compounds and their 5f unpaired electrons remain elusive in this intensive research area due to their well-known high radiotoxicity. A dedicated effort enabling the handling of these highly radioactive materials now allows their analysis using high-resolution MAS NMR (>55 kHz). Here, the study of the local structure of a series of actinide dioxides, namely, ThO2, UO2, NpO2, PuO2, and AmO2, using solid-state (17)O MAS NMR is reported. An important increase of the spectral resolution is found due to the removal of the dipolar broadening proving the efficiency of this technique for structural analysis. The NMR parameters in these systems with numerous and unpaired 5f electrons were interpreted using an empirical approach. Single-ion model calculations were performed for the first time to determine the z component of electron spin on each of the actinide atoms, which is proportional to the shifts. A similar variation thereof was observed only for the heavier actinides of this study.

Negative Thermal Expansion and Antiferromagnetism in the Actinide Oxypnictide NpFeAsO

A neptunium analog of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature

Ultra-Small Plutonium Oxide Nanocrystals: An Innovative Material in Plutonium Science

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