AVS2018 Session AC+MI+SA-WeM: Magnetism, Complexity, and Superconductivity in the Actinides and Rare Earths

Systems exhibiting strong electron-electron interactions remain at the forefront of inquiry into complex properties of condensed matter systems due to their exciting properties (eg. superconductivity) and their resistance to being understood on a fundamental level. A bottleneck toward a better understanding has been the difficulty of the required many-body calculations for extended solids. Alternatively, calculations on small molecules require fewer and better approximations, potentially offering a better description. Although strong electron-electron interactions are well established in extended solids, recent work on lanthanide organometallic coordination compounds has demonstrated the importance of such interactions, fueled by the propensity for certain 4f orbitals to be partially delocalized. Meanwhile, recent work on the actinides challenges the canonical view that the 5f electrons can bond in the light actinides but are essentially localized in the heavier actinides. A major stumbling block for such work is the paucity of known structures for elements beyond Am in the periodic table.

For the discussion presented here, work on Ce and Yb organometallics will provide context in terms of f-occupancy and in bonding characteristics and the effect on magnetism. The role of strongly electron interactions will be described in terms of configuration interaction (CI) and related calculations. Occupancy is measured using Ln L_III-edge x-ray absorption near-edge structure (XANES) techniques, and local structure (EXAFS) measurements demonstrate the final effect on the bonding at the metal center. Of particular interest is what happens in formally Ce(IV) systems that exhibit strong interactions.

XANES measurements of actinides are more difficult to interpret and will be discussed. The focus will be, however, on EXAFS measurements across the An series in the presence of strongly oxidizing ligands. Chosen ligands include hydroxypyridonone (HOPO), with less oxidizing ligands, such as diethylenetriaminepentaacetic acid (DTPA) used for comparison. Cations include Th, U, Pu, Am, Cm, Bk, and Cf. Discussions will center on nearest-neighbor bond lengths, using DFT calculations as a guide. The surprising role of covalency in the late actinides will be considered, both in terms of the EXAFS results and in terms of the edge shifts.

This work was supported by the U.S. Department of Energy (DOE), Office of Science (OS), Office of Basic Energy Sciences (OBES), under Contract No. DE-AC02-05CH1123.

Uranium is the basic component of most nuclear fuels. The production of uranium-based films has advantage over bulk materials studies as it allows performing advanced physics and chemistry experiments on small amounts of radioactive material and on its clean and smooth surfaces. Other interesting field is uranium magnetism. Although uranium itself is non-magnetic, uranium compounds display a rich variety of magnetic phenomena intimately related to the variable character of the 5f electron states [1]. Additional degrees of freedom can be used in thin films, in which the reduced dimensionality and structure modifications far exceed the limits imposed by thermodynamics, obeyed in bulk systems. We review the achievements in the field of sputter-deposited films, in which variations of deposition conditions can dramatically suppress crystallinity of the deposited material. The 5f itinerant magnetic systems (as US or UN [2]) react to the low substrate temperatures and high deposition rates by decreasing ordering temperatures and eventually by the loss of U magnetic moments. The strong ferromagnetism of uranium hydride is, on the other hand, almost insensitive, which underlines its local-moment character.

The possibility to combine films of various materials on the nanostructure scale can also give rise to new functionalities. For example, the exchange bias (EB) effect [3], arising as a result of combination of a ferromagnet biased by exchange interaction at the interface to an antiferromagnet, is particularly interesting if uranium magnetics are involved. The new ingredient, strong spin-orbit interaction, can lead to very strong magnetic anisotropy, which represents an essential parameter. We have been systematically studying films of Fe₃O₄ (ferromagnet) grown using different substrates on the top UO₂, playing the role of biasing antiferromagnet [4]. The resulting high bias field (> 0.2 T) and a proximity effect, in which the high Curie temperature of Fe₃O₄ provides the EB functionality even at temperatures exceeding ordering of UO₂, demonstrate the promising aspects of using actinides in this non-traditional way.

The work is supported by the Czech Science Foundation under the project #18-02344S. Part of the work was supported by “Nano-materials Centre for Advanced Applications,” Project No.CZ.02.1.01/0.0/0.0/15_003/0000485, financed by ERDF.

[1] V. Sechovsky, L. Havela, in: Magnetic Materials, K.H.J. Buschow (Ed.), Elsevier, Amsterdam, 1998, Vol. 11, p. 1.

[2] L. Havela et al., JALCOM 408-412, p. 1320 (2006).

[3] W. H. Meiklejohn and C. P. Bean, Phys. Rev. B 102, 1413 (1956).

[4] E.A. Tereshina et al., Appl. Phys. Lett. 105(12),122405 (2014).

We determine the effects of an applied magnetic field. Since the order parameter describes an ordering in the x-y plane, the electronic properties of the system are anisotropic below the critical temperature THO. We show that the magnetic susceptibility becomes anisotropic below THO. Furthermore, for fields applied along a spontaneously chosen hard axis, THO decreases towards zero and that the HO transition changes from second order to first order at a large value of the magnetic field. Also, we find that the system undergoes a cascade of field-induced Lifshitz transitions and also show how these properties originate from the dependence of the quasi-particle bands on the orientation of the applied field. The good qualitative agreement with experimental findings provides strong support for the proposed description of the HO phase as a spin-dependent inter-orbital density wave phase.

Most of f-elements form with hydrogen both di- and trihydrides. Actinide and rare-earth dihydrides occur, as a rule, in the CaF₂ structure type. Uranium represents an exception, only UH₃ is present in the binary phase diagram. It exists in two different structure types. The metastable form α-UH₃ forms in the Cr₃Si structure type, which is in fact bcc U lattice filled with hydrogen. The stable form β-UH₃ has a larger cubic cell with two different U sites. Both forms are ferromagnets with the total U moment of ≈ 1 μ_B/U and the Curie temperature T_C ≈ 165 K. We have recently synthesized UH₃ thin films using a reactive sputter deposition. XRD analysis indicated the β-UH₃ structure, modified by a pronounced (00l) texture and compressive residual strains imposed by the deposition dynamics. Magnetization measurements proved T_C = 165 K.

The sputter deposition on a cooled substrate (T = 170 K) using Si wafer the crystal structure turned different. The deposited material is undoubtedly cubic, of the fcc type, and the lattice parameter a = 5.3598 ± 0.0014 Å is very close to that of PuH₂ (a = 5.359 Å) and NpH_2+x (a = 5.343-5.355 Å). Hence we can assume that UH₂ in the fluorite structure has been formed. The key role in stabilization plays likely the effect of substrate (Si has a = 5.431 Å) in combination with low temperature deposition. The UH₂ film was subsequently subjected to magnetization measurements, which indicated a ferromagnetic ground state with T_C ≈ 125 K. This is lower than in the UH₃ phases, although the U-U spacing in UH₂ should be higher, 3.78 Å, than in both UH₃ phases (3.31 and 3.60 Å for β- and α-UH₃, respectively). This fact points to the U-U interaction being more important than the U-U spacing. The ferromagnetic state is also the ground state obtained from ab-initio calculations. Scalar relativistic calculations (LDA) for experimental lattice parameter yield the spin moment μ_S = 2.0 μ_B/U. LDA+U (U = 2.25 eV) gives the equilibrium lattice parameter a = 539.9 Å, i.e. 0.7% larger than the experimental one, the ferromagnetic ground state with (111) easy-magnetization direction and the magnetic anisotropy energy E_a = 9 meV. The total moment 0.45 μ_B/U consists of 2.59 μ_S and -3.04 μ_L.

This work was supported by the Czech Science Foundation under the grant No. 18-02344S. The work at JRC Karlsruhe was supported by the European FP7 TALISMAN project, under contract with the European Commission. Part of the work was supported by the project “Nanomaterials centre for advanced applications”, Project No. CZ.02.1.01/0.0/0.0/15_003/0000485, financed by ERDF.

The sensitivity of the interactions in the intermetallic systems to modification of the crystal structure makes the experimental techniques involving alteration of the atomic arrangement especially important. Various studies under compression are well-known examples of such methods. From this point of view hydrogenation can be treated as a complementary technique that provides „negative“ pressure. Hydrides can be defined as compounds for which the hydrogen absorption leads to the modifications of the crystal structure, such as pure lattice expansion or the formation of a new structure. As a result, the new compounds (hydrides) exhibit qualitatively new physical properties and such modifications provide us with additional information on the peculiarities of interatomic interactions in the initial compounds.

As an example, we will compare the impact of H absorption on U- and RE-compounds using A₂T₂X (A = Rare-Earth (RE) or actinide, T = transition metal, X = p-metal) compounds crystallizing in the tetragonal Mo₂FeB₂ structure type (space group P4/mbm). U₂T₂X interact with H₂ only at high pressure (≈ 100 bar) reaching 2 H/f.u. The H absorption produces a lattice expansion (lower than 10 %), while the tetragonal structure is preserved. Higher H concentrations, which can be achieved in some RE₂T₂X compounds (up to 8 H atoms/f.u), lead to amorphization or structure symmetry changes (with volume expansion exceeding 20 %), imposed by a minimum H-H distance requirement.

Magnetic properties of U-compounds strongly depend on the U-U distances. Hydrogen intrusion modifies the lattice by expanding it without changing the crystal-structure type leading to a 5f band narrowing. As a consequence, doping of U intermetallics by interstitial hydrogen leads to stronger magnetic properties. On the other hand, the hydrogen absorption has opposite effect on magnetic properties of RE₂T₂X compounds. For RE compounds, hydrogenation affects mainly the inter-site exchange interaction, which is weakened presumably by reducing the concentration of conduction electrons, responsible for the RKKY interaction.

In the past decade, U-compounds crystallizing in the HoCoGa₅-type structure (P4/mmm), frequently referred to as 115, have been in the focus of attention in experimental and theoretical research. Vigorous activities have been motivated by the high superconducting transition temperature of T_c = 8.7K in PuRhGa₅ [1] and T_c = 18.5K in PuCoGa₅ [2]. No further superconductivity has been reported in neither U-115 nor in the closely related U₂TGa₈ compounds (T = transition metal). However, interesting magnetic properties have been observed: neutron scattering experiments revealed that UNiGa₅ exhibits the G-type antiferromagnetic (AFM) phase, while UPdGa₅ and UPtGa₅ exhibit the A-type AFM state. Note that G-type indicates a 3D Néel state, while A-type refers to a layered AF structure where spins align FM in the ab plane and AFM along the c axis [3]. The difference in the two magnetic structures is significant since it implies a sign change of the nearest-neighbor (NN) interaction.

Here we report on the magnetic structures of URhIn₅ and U₂RhIn₈, two new members of the U_nT_mX_3n+2m (X=In,Ga) family of compounds [4]. URhIn₅ displays AFM order below T_N = 98K. The observed increase of the resistivity for current parallel [100], [110] and [001] are reminiscent to a spin-density wave (SDW) type of transition with the gap opening first along the [001] direction [5]. U₂RhIn₈ enters the AFM state at T_N = 117K. No increase in resistivity in the vicinity of T_N is found which would hint to a SDW gap opening. Neutron diffraction experiments on URhIn₅ were performed at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching using the triple axis spectrometer PANDA. Single crystals with accumulated mass of 10mg where glued on an Al-plate. Our results confirmed the magnetic propagation vector k=(1/2,1/2,1/2) predicted by NMR experiments [6] and a magnetic moment of 1.65 µ_B/U³⁺. The neutron study on U₂RhIn₈ has been conducted at ILL, Grenoble using D10 on only one single crystal with m ≈ 1mg. Analysis revealed a propagation vector k=(1/2,1/2,0) and an ordered moment of 1.7 µ_B/U³⁺. UIn₃, URhIn₅ and U₂RhIn₈ all show G-type AFM phase. While the c-axis parameter differs significantly the a lattice parameter equals 4.601Å, 4.621Å and 4.6056Å respectively, being a change of less than 1% pointing to the fact that the NN coupling is important for the type of magnetic structure.

[1] F. Wastin et al., J.Phys.Condens.Matter 15, S2279 (2003)

[2] J.L. Sarrao et al., Nature (London) 420, 297 (2002)

[3] T. Hotta, Phys.Rev. B 70, 054405 (2004)

[4] A. Bartha et al., J.Magn.Magn.Mater. 381, 310 (2015)

[5] A. Bartha et al., Acta Phys.Pol. A 127, 339 (2015)

[6] H. Sakai et al. Phys.Rev. B 88, 045123 (2013)

Detailed dc magnetization (M) measurements of a 7.6 nm La_0.7Sr_0.3MnO₃ thin film samples is investigated. The sample was fabricated by pulsed laser deposition. Zero-field-cooled (ZFC) M vs. applied field (H) cooled down to T = 5 K reveal the presence of negative remanent magnetization (NRM) as well as in ZFC M vs. temperature (T) measurements in H = 50 Oe and 100 Oe. ZFC and FC (field-cooled) protocols are used to determine the blocking temperature T_B in different H. Isothermal hysteresis loops at different T are used to determine the temperature dependence of saturation magnetization (M_S), remanence (M_R) and coercivity H_C. The M_S vs. T data are fit to the Bloch law, M_S (T) = M₀ (1 – BT^3/2), showing a good fit for T < 100 K and yielding the nearest-neighbor exchange constant J/k_B ≅ 18 K. The variations of T_B vs. H and H_C vs. T are well described by the model often used for randomly oriented magnetic nanoparticles with magnetic domain diameter ≈ 9 nm present in the dead-layer of thickness d =1.4 nm. Finally, the data available from literature on the thickness (D) variation of Curie temperature (T_C) and M_S of LSMO films grown under 200, 150, and 0.38 mTorr pressures of O₂ are analyzed in terms of the finite-size scaling, with M_S vs. D data fit to M_S (D) = M_S(b)(1-d/D) yielding the dead layer thickness d = 1.1 nm, 1.4nm and 2.4 nm respectively.