Measuring magnetic hysteresis curves with polarized soft X-ray resonant reflectivity.

Calculations and measurements of polarization-dependent soft X-ray scattering intensity are presented during a magnetic hysteresis cycle. It is confirmed that the dependence of the intensity on the magnetic moment can be linear, quadratic or a combination of both, depending on the polarization of the incident X-ray beam and the direction of the magnetic moment. With a linearly polarized beam, the scattered intensity will have a purely quadratic dependence on the magnetic moment when the magnetic moment is parallel to the scattering plane. However, with the magnetic moment perpendicular to the scattering plane, there is also a linear component. This means that, when measuring the hysteresis with linear polarization during a hysteresis cycle, the intensity will be an even function of the applied field when the change in the magnetic moment (and field) is confined within the scattering plane but becomes more complicated when the magnetic moment is out of the scattering plane. Furthermore, with circular polarization, the dependence of the scattered intensity on the moment is a combination of linear and quadratic. With the moment parallel to the scattering plane, the linear component changes with the helicity of the incident beam. Surprisingly, in stark contrast to absorption studies, even when the magnetic moment is perpendicular to the scattering plane there is still a dependence on the moment with a linear component. This linear component is completely independent of the helicity of the beam, meaning that the hysteresis loops will not be inverted with helicity.

Quantifying the influence of 3d-4s mixing on linearly coordinated metal-ions by L2,3-edge XAS and XMCD.

The mixing valence d and s orbitals are predicted to strongly influence the electronic structure of linearly coordinated molecules, including transition metals, lanthanides and actinides. In specific cases, novel magnetic properties, such as single-ion magnetic coercivity or long spin decoherence times, ensue. Inspired by how the local coordination symmetry can engender such novel phenomena, in this study, we focus our attention on dopants (Mn, Fe, Co, Ni, Cu) in lithium nitride to accept innovation from molecular magnetism in a high symmetry P6/mmm solid-state crystal. The linear coordination environment results in strong 3d-4s mixing, proving to be an ideal series to investigate the role of d-s mixing and bonding on electronic structure and magnetism. It is shown that L2,3-edge XAS can be applied to experimentally identify the presence of 3d-4s mixing and the influence this has on the ligand-field splitting. XMCD specifies how spin-orbit coupling is affected. The combined spectroscopies are analysed to determine the effect of 4s mixing with support from ab initio calculations. The results provide new insight of relevance to future applications, including quantum information processing and the sustainable replacement of rare earths in magnets.

Revealing solid electrolyte interphase formation through interface-sensitive Operando X-ray absorption spectroscopy.

The solid electrolyte interphase (SEI) that forms on Li-ion battery anodes is critical to their long-term performance, however observing SEI formation processes at the buried electrode-electrolyte interface is a significant challenge. Here we show that operando soft X-ray absorption spectroscopy in total electron yield mode can resolve the chemical evolution of the SEI during electrochemical formation in a Li-ion cell, with nm-scale interface sensitivity. O, F, and Si K-edge spectra, acquired as a function of potential, reveal when key reactions occur on high-capacity amorphous Si anodes cycled with and without fluoroethylene carbonate (FEC). The sequential formation of inorganic (LiF) and organic (-(C=O)O-) components is thereby revealed, and results in layering of the SEI. The addition of FEC leads to SEI formation at higher potentials which is implicated in the rapid healing of SEI defects and the improved cycling performance observed. Operando TEY-XAS offers new insights into the formation mechanisms of electrode-electrolyte interphases and their stability for a wide variety of electrode materials and electrolyte formulations.

A Detailed Investigation of the Onion Structure of Exchanged Coupled Magnetic Fe3-δO4@CoFe2O4@Fe3-δO4 Nanoparticles.

Nanoparticles that combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core-shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide nanoparticles which hampers potential applications such as data storage or sensors. In this manuscript, we report on the design of nanoparticles with an onion-like structure which has been scarcely reported yet. These nanoparticles consist of a Fe3-δO4 core covered by a first shell of CoFe2O4 and a second shell of Fe3-δO4, e.g., a Fe3-δO4@CoFe2O4@Fe3-δO4 onion-like structure. They were synthesized through a multistep seed-mediated growth approach which consists consists in performing three successive thermal decomposition of metal complexes in a high-boiling-point solvent (about 300 °C). Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increasing. We investigated very precisely the structure of nanoparticles in performing high resolution (scanning) TEM imaging and geometrical phase analysis (GPA). The chemical composition and spatial distribution of atoms were studied by electron energy loss spectroscopy (EELS) mapping and spectroscopy. The chemical environment and oxidation state of cations were investigated by 57Fe Mössbauer spectrometry, soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The combination of these techniques allowed us to estimate the increase of Fe2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, whereas the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe3-δO4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core-shell nanoparticles, thus demonstrating the high potential of onion-like nanoparticles to accurately tune the magnetic properties of nanoparticles according to the desired applications.

Quenching and Restoration of Orbital Angular Momentum through a Dynamic Bond in a Cobalt(II) Complex.

Orbital angular momentum plays a vital role in various applications, especially magnetic and spintronic properties. Therefore, controlling orbital angular momentum is of paramount importance to both fundamental science and new technological applications. Many attempts have been made to modulate the ligand-field-induced quenching effects of orbital angular momentum to manipulate magnetic properties. However, to date, reported changes in the magnitude of orbital angular momentum are small in both molecular and solid-state magnetic materials. Moreover, no effective methods currently exist to modulate orbital angular momentum. Here we report a dynamic bond approach to realize a large change in orbital angular momentum. We have developed a Co(II) complex that exhibits coordination number switching between six and seven. This cooperative dynamic bond switching induces considerable modulation of the ligand field, thereby leading to substantial quenching and restoration of the orbital angular momentum. This switching mechanism is entirely different from those of spin-crossover and valence tautomeric compounds, which exhibit switching in spin multiplicity.

Macroscopic Polarization Change via Electron Transfer in a Valence Tautomeric Cobalt Complex.

Polarization change induced by directional electron transfer attracts considerable attention owing to its fast switching rate and potential light control. Here, we investigate electronic pyroelectricity in the crystal of a mononuclear complex, [Co(phendiox)(rac-cth)](ClO4)·0.5EtOH (1·0.5EtOH, H2phendiox = 9, 10-dihydroxyphenanthrene, rac-cth = racemic 5, 5, 7, 12, 12, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane), which undergoes a two-step valence tautomerism (VT). Correspondingly, pyroelectric current exhibits double peaks in the same temperature domain with the polarization change consistent with the change in dipole moments during the VT process. Time-resolved Infrared (IR) spectroscopy shows that the photo-induced metastable state can be generated within 150 ps at 190 K. Such state can be trapped for tens of minutes at 7 K, showing that photo-induced polarization change can be realized in this system. These results directly demonstrate that a change in the molecular dipole moments induced by intramolecular electron transfer can introduce a macroscopic polarization change in VT compounds.

Single-ion magnetism in the extended solid-state: insights from X-ray absorption and emission spectroscopy.

Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L2,3-edge and Kß X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine FeI dopant ions to be linearly coordinated, occupying a D 6h symmetry pocket. The dopant engages in strong 3dπ-bonding, resulting in an exceptionally short Fe-N bond length (1.873(7) Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner-Teller vibronic coupling and pseudo Jahn-Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L2,3-edge XAS from which the energy reduction of 3d z 2 due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L3-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm-1), that corresponds with Orbach relaxation via the first excited, M J = ±5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.

Temperature dependence of spherical electron transfer in a nanosized [Fe14] complex.

The study of transition metal clusters exhibiting fast electron hopping or delocalization remains challenging, because intermetallic communications mediated through bridging ligands are normally weak. Herein, we report the synthesis of a nanosized complex, [Fe(Tp)(CN)3]8[Fe(H2O)(DMSO)]6 (abbreviated as [Fe14], Tp-, hydrotris(pyrazolyl)borate; DMSO, dimethyl sulfoxide), which has a fluctuating valence due to two mobile d-electrons in its atomic layer shell. The rate of electron transfer of [Fe14] complex demonstrates the Arrhenius-type temperature dependence in the nanosized spheric surface, wherein high-spin centers are ferromagnetically coupled, producing an S = 14 ground state. The electron-hopping rate at room temperature is faster than the time scale of Mössbauer measurements (<~10-8 s). Partial reduction of N-terminal high spin FeIII sites and electron mediation ability of CN ligands lead to the observation of both an extensive electron transfer and magnetic coupling properties in a precisely atomic layered shell structure of a nanosized [Fe14] complex.

Na-doped ruthenium perovskite electrocatalysts with improved oxygen evolution activity and durability in acidic media.

The design of active and durable catalysts for the H2O/O2 interconversion is one of the major challenges of electrocatalysis for renewable energy. The oxygen evolution reaction (OER) is catalyzed by SrRuO3 with low potentials (ca. 1.35 VRHE), but the catalyst's durability is insufficient. Here we show that Na doping enhances both activity and durability in acid media. DFT reveals that whereas SrRuO3 binds reaction intermediates too strongly, Na doping of ~0.125 leads to nearly optimal OER activity. Na doping increases the oxidation state of Ru, thereby displacing positively O p-band and Ru d-band centers, weakening Ru-adsorbate bonds. The enhanced durability of Na-doped perovskites is concomitant with the stabilization of Ru centers with slightly higher oxidation states, higher dissolution potentials, lower surface energy and less distorted RuO6 octahedra. These results illustrate how high OER activity and durability can be simultaneously engineered by chemical doping of perovskites.

Tunable Magnetic Phases at Fe3O4/SrTiO3 Oxide Interfaces.

We demonstrate the emergence and control of magnetic phases between magnetite (Fe3O4), a ferrimagnetic halfmetal, and SrTiO3, a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. The Verwey transition ( TV) was detected to persist from bulk-like down to ultrathin Fe3O4 films, decreasing from 117 ± 4 K (38 nm) to 25 ± 4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent hard X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism techniques. We observe a reduction of Fe2+ ions with decreasing film thickness, accompanied by an increase of Fe3+ ions in both tetrahedral and octahedral sites and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γ-Fe2O3 intralayer. To manipulate the interfacial magnetic phase, a postannealing process causes the controlled reduction of the γ-Fe2O3 that finally leads to stoichiometric and ferrimagnetic Fe3O4/SrTiO3(001) heterointerfaces.

Enhancing magnetic ordering in Cr-doped Bi2Se3 using high-TC ferrimagnetic insulator.

We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi(2-x)Cr(x)Se(3), via the proximity effect using a high-TC ferrimagnetic insulator Y(3)Fe(5)O(12). The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced TC of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 to 50 K) penetration depth compared to that of diluted ferromagnetic semiconductors (DMSs), which could indicate a novel mechanism for the interaction between FMIs and the nontrivial TIs surface.

Complete polarization analysis of an APPLE II undulator using a soft X-ray polarimeter.

Two APPLE II undulators installed on the Diamond I10 beamline have all four magnet arrays shiftable and thus can generate linear polarization at any arbitrary angle from 0° to 180°, as well as all other states of elliptical polarization. To characterize the emitted radiation polarization state from one APPLE II undulator, the complete polarization measurement was performed using a multilayer-based soft X-ray polarimeter. The measurement results appear to show that the linear polarization angle offset is about 6° compared with other measurements at 712 eV, equivalent to an undulator jaw phase offset of 1.1 mm. In addition, the polarization states of various ellipticities have also been measured as a function of the undulator row phase.

Supramolecular control of the magnetic anisotropy in two-dimensional high-spin Fe arrays at a metal interface.

Magnetic atoms at surfaces are a rich model system for solid-state magnetic bits exhibiting either classical or quantum behaviour. Individual atoms, however, are difficult to arrange in regular patterns. Moreover, their magnetic properties are dominated by interaction with the substrate, which, as in the case of Kondo systems, often leads to a decrease or quench of their local magnetic moment. Here, we show that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5 nm square grid. Lateral coordination with the molecular ligands yields unsaturated yet stable coordination bonds, which enable chemical modification of the electronic and magnetic properties of the Fe atoms independently from the substrate. The easy magnetization direction of the Fe centres can be switched by oxygen adsorption, thus opening a way to control the magnetic anisotropy in supramolecular layers akin to that used in metallic thin films.

Soft X-ray coherent scattering: instrument and methods at ESRF ID08.

An experimental setup has been developed to perform soft x-ray coherent scattering at beamline ID08 of the European Synchrotron Radiation Facility. An intense coherent beam was obtained by filtering the primary beam with the monochromator and a circular pinhole. A pinhole holder with motorized translations was installed inside the UHV chamber of the diffractometer. The scattered intensity was recorded in reflection geometry with a back-illuminated charge coupled device camera. As a demonstration we report experimental results of resonant magnetic scattering using coherent beam. The degree of coherence is evaluated, and it is shown that, while the vertical coherence is much higher than the horizontal one at the source, the situation is reversed at the diffractometer. The intensity of the coherent beam is also discussed.