Magnetic properties and chiral states of a trimetallic uranium complex

The magnetic properties of the triangular molecular nanomagnet [UO2L]3 (L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate) have been investigated through electron paramagnetic resonance spectroscopy, high-field magnetization and susceptibility measurements. The experimental findings are well reproduced by a microscopic model including exchange interactions and local crystal fields. These results show that [UO2L]3 is characterized by a non-magnetic ground doublet corresponding to two oppositely twisted chiral arrangements of the uranium moments. The non-axial character of single-ion crystal fields leads to quantum tunneling of the noncollinear magnetization in the presence of a magnetic field applied perpendicularly to the triangle plane.

Oxo-functionalization and reduction of the uranyl ion through lanthanide-element bond homolysis: synthetic, structural, and bonding analysis of a series of singly reduced uranyl-rare earth 5f1-4f(n) complexes

The heterobimetallic complexes [{UO2Ln(py)2(L)}2], combining a singly reduced uranyl cation and a rare-earth trication in a binucleating polypyrrole Schiff-base macrocycle (Pacman) and bridged through a uranyl oxo-group, have been prepared for Ln = Sc, Y, Ce, Sm, Eu, Gd, Dy, Er, Yb, and Lu. These compounds are formed by the single-electron reduction of the Pacman uranyl complex [UO2(py)(H2L)] by the rare-earth complexes Ln(III)(A)3 (A = N(SiMe3)2, OC6H3Bu(t)2-2,6) via homolysis of a Ln-A bond. The complexes are dimeric through mutual uranyl exo-oxo coordination but can be cleaved to form the trimetallic, monouranyl "ate" complexes [(py)3LiOUO(μ-X)Ln(py)(L)] by the addition of lithium halides. X-ray crystallographic structural characterization of many examples reveals very similar features for monomeric and dimeric series, the dimers containing an asymmetric U2O2 diamond core with shorter uranyl U═O distances than in the monomeric complexes. The synthesis by Ln(III)-A homolysis allows [5f(1)-4f(n)]2 and Li[5f(1)-4f(n)] complexes with oxo-bridged metal cations to be made for all possible 4f(n) configurations. Variable-temperature SQUID magnetometry and IR, NIR, and EPR spectroscopies on the complexes are utilized to provide a basis for the better understanding of the electronic structure of f-block complexes and their f-electron exchange interactions. Furthermore, the structures, calculated by restricted-core or all-electron methods, are compared along with the proposed mechanism of formation of the complexes. A strong antiferromagnetic coupling between the metal centers, mediated by the oxo groups, exists in the U(V)Sm(III) monomer, whereas the dimeric U(V)Dy(III) complex was found to show magnetic bistability at 3 K, a property required for the development of single-molecule magnets.

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.

Magnetic and electronic properties of NpFeGa5

NpFeGa(5) single crystals prepared by the Ga-flux method have been investigated by magnetisation, specific heat and (237)Np Mössbauer spectroscopy measurements. An antiferromagnetic (AF) transition has been observed at T(N) ~114 K. The second AF transition which takes place at T* ~74 K has only been detected in the magnetisation data. A saturated ordered Np moment of 1.00(5) μ(B) and the occurrence of an Np(3 + ) charge state were inferred from the Mössbauer data. The angle between the Np ordered moment and the basal plane was shown to be ~17°, in good agreement with the value observed by neutron diffraction in the low temperature AF phase. The absence of change of the quadrupole interaction parameters at T* did not allow us to gain information on the Np moment reorientation at T*. The trend of the isomer shift along the NpTGa(5) series and the discrepancy between the Np moments determined by neutron diffraction and Mössbauer spectroscopy indicated a partial spatial delocalisation of the 5f electrons in NpFeGa(5).

Quadrupolar waves in uranium dioxide

In presence of active orbital degrees of freedom, elementary excitations around a broken-symmetry state may include multipolar waves, but none of these exotic dispersive excitation branches has ever been identified. We show that quadrupolar waves constitute a major component of the dynamics of uranium dioxide in its magnetoquadrupolar ordered phase, and that many unexplained features in existing inelastic neutron scattering data, including a whole excitation branch, are associated with these propagating quadrupolar fluctuations. Our model permits us to separate the role of Jahn-Teller and superexchange mechanisms as sources of quadrupolar interactions.

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.

Superexchange coupling and slow magnetic relaxation in a transuranium polymetallic complex

{Np(VI)O2Cl2}{Np(V)O2Cl(thf)3}2 is the first studied example of a polymetallic transuranic complex displaying both slow relaxation of the magnetization and effective superexchange interactions between 5f centers. The coupling constant for Np(V)-Np(VI) pairs is 10.8 K, more than 1 order of magnitude larger than the common values found for rare-earth ions in similar environments. The dynamic magnetic behavior displays slow relaxation of magnetization of molecular origin with an energy barrier of 140 K, which is nearly twice the size of the highest barrier found in polymetallic clusters of the d block. Our observations also suggest that future actinide-based molecular magnets will have very different behavior to lanthanide-based clusters.

Polarized-neutron-scattering study of the spin-wave excitations in the 3-k ordered phase of uranium antimonide

The anisotropy of magnetic fluctuations propagating along the [1 1 0] direction in the ordered phase of uranium antimonide has been studied using polarized inelastic neutron scattering. The observed polarization behavior of the spin waves is a natural consequence of the longitudinal 3-k magnetic structure; together with recent results on the 3-k-transverse uranium dioxide, these findings establish this technique as an important tool to study complex magnetic arrangements. Selected details of the magnon excitation spectra of USb have also been reinvestigated, indicating the need to revise the currently accepted theoretical picture for this material.

Electronic state of PuCoGa5 and NpCoGa5 as probed by polarized neutrons

By using single crystals and polarized neutrons, we have measured the orbital and spin components of the microscopic magnetization in the paramagnetic state of NpCoGa(5) and PuCoGa(5). The microscopic magnetization of NpCoGa(5) agrees with that observed in bulk susceptibility measurements and the magnetic moment has spin and orbital contributions as expected for intermediate coupling. In contrast, for PuCoGa(5), which is a superconductor with a high transition temperature, the microscopic magnetization in the paramagnetic state is small, temperature-independent, and significantly below the value found with bulk techniques at low temperatures. The orbital moment dominates the magnetization.

Quantum oscillations of the total spin in a heterometallic antiferromagnetic ring: evidence from neutron spectroscopy

Using inelastic neutron scattering and applied fields up to 11.4 T, we have studied the spin dynamics of the Cr7Ni antiferromagnetic ring in the energy window 0.05-1.6 meV. We demonstrate that the external magnetic field induces an avoided crossing (anticrossing) between energy levels with different total-spin quantum numbers. This corresponds to quantum oscillations of the total spin of each molecule. The inelastic character of the observed excitation and the field dependence of its linewidth indicate that molecular spins oscillate coherently for a significant number of cycles. Precise signatures of the anticrossing are also found at higher energy, where measured and calculated spectra match very well.

Hidden order and low-energy excitations in NpO2

We investigate the nature of the hidden order parameter in the ordered phase of NpO2, which had been identified with a staggered arrangement of Gamma5 magnetic multipoles. By analyzing the existing experimental data, we show that the most likely driving order parameter is not provided by octupoles, as usually assumed, but rather by the rank-5 triakontadipoles. Calculations of the coupled dynamics of spins, Gamma5 quadrupoles, and Gamma5 triakontadipoles in the ordered phase enable us to analyze the resulting structure of low-energy excitations. We show that the powder inelastic neutron scattering cross section should contain, in addition to the already-observed peak at 6.5 meV, a second weaker peak at about 14 meV.

Direct determination of the magnetic ground state in the square lattice S = 1/2 antiferromagnet Li2VOSiO4

Powder neutron diffraction and resonant x-ray scattering measurements from a single crystal have been performed to study the low-temperature state of the 2D frustrated, quantum-Heisenberg system Li2VOSiO4. Both techniques indicate a collinear antiferromagnetic ground state, with propagation vector k=(1 / 2 1 / 2 0), and magnetic moments in the a-b plane. Contrary to previous reports, the ordered moment at 1.44 K, m=0.63(3)micro(B), is very close to the value expected for the square lattice Heisenberg model ( approximately 0.6micro(B)). The magnetic order is three dimensional, with antiferromagnetic a-b layers stacked ferromagnetically along the c axis. Neither x-ray nor neutron diffraction shows evidence for a structural distortion between 1.6 and 10 K.

Quantum magneto-oscillations in a supramolecular Mn(II)-[3 x 3] grid

The magnetic grid molecule Mn(II)-[3 x 3] has been studied by high-field torque magnetometry at 3He temperatures. At fields above 5 T, the torque versus field curves exhibit an unprecedented oscillatory behavior. A model is proposed which describes these magneto-oscillations well.