Magnetism in fcc rhodium and palladium

Soft X-ray Fermi surface tomography of palladium and rhodium via momentum microscopy

Fermi surfaces of transition metals, which describe all thermodynamical and transport quantities of solids, often fail to be modeled by one-electron mean-field theory due to strong correlations among the valence electrons. In addition, relativistic spin-orbit coupling pronounced in heavier elements lifts the degeneracy of the energy bands and further modifies the Fermi surface. Palladium and rhodium, two 4d metals attributed to show significant spin-orbit coupling and electron correlations, are ideal for a systematic and fundamental study of the two fundamental physical phenomena and their interplay in the electronic structure. In this study, we explored the Fermi surface of the 4d noble metals palladium and rhodium obtained via high-resolution constant initial state momentum microscopy. The complete 3D-Fermi surfaces of palladium and rhodium were tomographically mapped using soft X-ray photon energies from 34 eV up to 660 eV. To fully capture the orbital angular momentum of states across the Fermi surface, the Fermi surface tomography was performed using p- and s- polarized light. Applicability and limitations of the nearly-free electron final state model in photoemission are discussed using a complex band structure model supported by experimental evidence. The significance of spin-orbit coupling and electron correlations across the Fermi surfaces will be discussed within the context of the photoemission results. State-of-the-art fully relativistic Korringa-Kohn-Rostoker (KKR) calculations within the one-step model of photoemission are used to support the experimental results.

The effect of negative pressures on the superconductivity of amorphous and crystalline bismuth

Materials may behave in non-expected ways when subject to unexpected conditions. For example, when Bi was turned into an amorphous phase (a-Bi) unexpectedly it became a superconductor at temperatures below 10 K. Using the superconductivity of the amorphous phase we provided an explanation as to why crystalline bismuth (c-Bi) had not been found to superconduct, and even predicted an upper limit for its superconducting transition temperature T_c. This was experimentally corroborated within the following year. We now decided to investigate what happens to the crystalline, Wyckoff structure, and amorphous Bi when pressures below the atmospheric are applied. Here it is shown that, within the BCS approach, under expansion the Wyckoff c-Bi increases its superconducting transition temperature minimally, whereas the amorphous phase decreases its T_c. The electron densities of states (eDoS), the vibrational densities of states (vDoS) and the Debye temperatures (θ_D) are calculated to perform this qualitative evaluation. Expansion can be obtained in the laboratory by chemically etching Bi-based alloys, for example, a process also known as dealloying.

Strained epitaxial interfaces of metal (Pd, Pt, Au) overlayers on nonpolar CdS ([Formula: see text]) surfaces from first-principles

The depositions of (1 1 1) and (1 0 0) overlayers of Pd, Pt and Au on the CdS (1 0 [Formula: see text] 0) surface are studied within epitaxial mismatches of 6%-7%, using spin-polarized density functional theory. For both compressively strained and tensile-strained interfaces, the (1 0 0) overlayers were found to be thermodynamically more stable owing to better interfacial matching, and higher surface uncoordination resulting in higher reactivity. Pt(1 1 1) exhibits slip dislocations even for five-atomic-layer thick Pt slabs. Along with the leading metal-S interaction, the interfacial charge transfers indicate a weak metal-Cd interaction which decreases in strength in the order Pd  >  Pt  ∼  Au. For the same substrate area, the accumulation of electronic charge for Pt overlayers is  ∼1.5-2 times larger than that of Pd and Au. The n-type Schottky barriers of Au overlayers with the minimum mismatch are within 0.1 eV of the predictions of Schottky-Mott rule, indicating a relatively ideal, scantily reactive interface structure. This is in clear contrast to the Pt epitaxial overlayers which deviate by 0.6-0.8 eV.

Magneto-Ionic Switching of Superparamagnetism

Electrochemical reactions represent a promising approach to control magnetization via electric fields. Favorable reaction kinetics have made nanoporous materials particularly interesting for magnetic tuning experiments. A fully reversible ON and OFF switching of magnetism in nanoporous Pd(Co) at room temperature is demonstrated, triggered by electrochemical hydrogen sorption. Comprehensive magnetic characterization in combination with high-resolution scanning transmission electron microscopy reveals the presence of Co-rich, nanometer-sized clusters in the nanoporous Pd matrix with distinct superparamagnetic behavior. The strong magneto-ionic effect arises from coupling of the magnetic clusters via a Ruderman-Kittel-Kasuya-Yoshida-type interaction in the Pd matrix which is strengthened upon hydrogen sorption. This approach offers a new pathway for the voltage control of magnetism, for application in spintronic or microelectromagnetic devices.

Asymmetric magnetic proximity effect in a Pd/Co/Pd trilayer system

In spintronic devices consisting of ferromagnetic/nonmagnetic systems, the ferromagnet-induced magnetic moment in the adjacent nonmagnetic material significantly influences the spin transport properties. In this study, such magnetic proximity effect in a Pd/Co/Pd trilayer system is investigated by x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity, which enables magnetic characterizations with element and depth resolution. We observe that the total Pd magnetic moments induced at the top Co/Pd interface are significantly larger than the Pd moments at the bottom Pd/Co interface, whereas transmission electron microscopy and reflectivity analysis indicate the two interfaces are nearly identical structurally. Such asymmetry in magnetic proximity effects could be important for understanding spin transport characteristics in ferromagnetic/nonmagnetic systems and its potential application to spin devices.

Magnetism in nanoparticles: tuning properties with coatings

This paper reviews the effect of organic and inorganic coatings on magnetic nanoparticles. The ferromagnetic-like behaviour observed in nanoparticles constituted by materials which are non-magnetic in bulk is analysed for two cases: (a) Pd and Pt nanoparticles, formed by substances close to the onset of ferromagnetism, and (b) Au and ZnO nanoparticles, which were found to be surprisingly magnetic at the nanoscale when coated by organic surfactants. An overview of theories accounting for this unexpected magnetism, induced by the nanosize influence, is presented. In addition, the effect of coating magnetic nanoparticles with biocompatible metals, oxides or organic molecules is also reviewed, focusing on their applications.

Electrically tunable anomalous Hall effect in Pt thin films

Pt is often considered to be an exchange-enhanced paramagnetic material, in which the Stoner criterion for ferromagnetism is nearly satisfied and, thus, external stimuli may induce unconventional magnetic characteristics. We report that a nonmagnetic perturbation in the form of a gate voltage applied via an ionic liquid induces an anomalous Hall effect (AHE) in Pt thin films, which resembles the AHE induced by the contact to Bi-doped yttrium iron garnet. Analysis of detailed temperature and magnetic field experiments indicates that the evolution of the AHE with temperature can be explained in terms of large local moments; the applied electric field induces magnetic moments as large as ~10 μ(B) that follow the Langevin function.

Water-soluble Pd nanoparticles capped with glutathione: synthesis, characterization, and magnetic properties

The synthesis, characterization, and magnetic properties of water-soluble Pd nanoparticles capped with glutathione are described. The glutathione-capped Pd nanoparticles were synthesized under argon and air atmospheres at room temperature. Whereas the former exhibits a bulklike lattice parameter, the lattice parameter of the latter is found to be considerably greater, indicating anomalous lattice expansion. Comparative structural and compositional studies of these nanoparticles suggest the presence of oxygen in the core lattice when Pd nanoparticles are prepared under an air atmosphere. Both Pd nanoparticles prepared under argon and air show ferromagnetism at 5 K, but the latter exhibits significantly greater coercivity (88 Oe) and magnetization (0.09 emu/g at 50 kOe). The enhanced ferromagnetic properties are explained by the electronic effect of the incorporated oxygen that increases the 4d density of holes at the Pd site and localizes magnetic moments.

Double σ-Aromaticity in a Surface-Deposited Cluster: Pd4 on TiO2 (110)

We report double σ-aromaticity in a surface-deposited cluster, Pd4, on the TiO2 (110) surface. In the gas phase, Pd4 adopts a tetrahedral structure. However, surface binding promotes a flat, σ-aromatic cluster. This is the first time aromaticity has been found in surface-deposited clusters. The concept of aromaticity is expected to become instrumental in predicting and interpreting properties of such systems, much like it is for isolated molecules and clusters.

Enhanced magnetic moment in Fe-doped Pd(n) clusters (n = 1-13): a density functional study

Here we report a systematic theoretical study of the equilibrium structures, electronic and magnetic properties of FePd(n-1) clusters with n = 1-13, within the framework of density functional theory. The results show that the doping of a single Fe impurity enhances the binding energies as well as the magnetic moment of the Pd(n) clusters. Interestingly, in the mid-size region (n = 5-7), Fe substitution in Pd(n) clusters results in a three fold enhancement in the magnetic moment. We find that the geometries of the host clusters do not change significantly after the addition of an Fe atom, except for n = 6, 7, 11, 12. In the lowest energy configurations, the Fe atom tries to increase its coordination number by moving from the convex to the interior site as the number of Pd atoms varies from 2 to 12.

Ferromagnetism in fcc twinned 2.4 nm size Pd nanoparticles

The onset of ferromagnetism has been experimentally observed in small Pd particles of average diameter 2.4 nm. High-resolution studies reveal that a high percentage of the fcc particle exhibits single and multiple twinning boundaries. The spontaneous magnetization close to 0.02 emu/g seems to indicate that only a small fraction of atoms holds a permanent magnetic moment and contributes to ferromagnetism. The possible origin of ferromagnetism is briefly discussed according to different models recently reported.

Structure and magnetism of neutral and anionic palladium clusters

The properties of neutral and anionic Pd(N) clusters were investigated with spin-density-functional calculations. The ground-state structures are three dimensional for N>3 and they are magnetic with a spin triplet for 2 < or = N < or = 7 and a spin nonet for N = 13 neutral clusters. Structural and spin isomers were determined and an anomalous increase of the magnetic moment with temperature is predicted for a Pd7 ensemble. Vertical electron detachment and ionization energies were calculated and the former agrees well with measured values for Pd(-)(N).