Localized magnetic states of Fe, Co, and Ni impurities on alkali metal films

Ferromagnetism on an atom-thick & extended 2D metal-organic coordination network

Ferromagnetism is the collective alignment of atomic spins that retain a net magnetic moment below the Curie temperature, even in the absence of external magnetic fields. Reducing this fundamental property into strictly two-dimensions was proposed in metal-organic coordination networks, but thus far has eluded experimental realization. In this work, we demonstrate that extended, cooperative ferromagnetism is feasible in an atomically thin two-dimensional metal-organic coordination network, despite only ≈ 5% of the monolayer being composed of Fe atoms. The resulting ferromagnetic state exhibits an out-of-plane easy-axis square-like hysteresis loop with large coercive fields over 2 Tesla, significant magnetic anisotropy, and persists up to TC ≈ 35 K. These properties are driven by exchange interactions mainly mediated by the molecular linkers. Our findings resolve a two decade search for ferromagnetism in two-dimensional metal-organic coordination networks.

A Stunt of Sustainability: Artificial Humic Substances Can Generate and Stabilize Single Fe0 Species on Mineral Surfaces

Iron species are omnipresent in fertile soils and contribute to biological and geological redox processes. Here, we show by advanced electron microscopy techniques that an important, but previously not considered iron species, single atom Fe0 stabilized on clay mineral surfaces, is contained in soils when humic substances are present. As the concentration of neutral iron atoms is highest under frost logged soil conditions, their formation can be attributed to the action of a then reductive microbiome. The Fe0 /Fe2+ couple is with -0.04 V standard potential highly suited for natural environmental remediation and detoxification, and its occurrence can help to explain the sustained auto-detoxification of black soils.

On-Surface Design of a 2D Cobalt-Organic Network Preserving Large Orbital Magnetic Moment

The design of antiferromagnetic nanomaterials preserving large orbital magnetic moments is important to protect their functionalities against magnetic perturbations. Here, we exploit an archetype H₆HOTP species for conductive metal-organic frameworks to design a Co-HOTP one-atom-thick metal-organic architecture on a Au(111) surface. Our multidisciplinary scanning probe microscopy, X-ray absorption spectroscopy, X-ray linear dichroism, and X-ray magnetic circular dichroism study, combined with density functional theory simulations, reveals the formation of a unique network design based on threefold Co⁺² coordination with deprotonated ligands, which displays a large orbital magnetic moment with an orbital to effective spin moment ratio of 0.8, an in-plane easy axis of magnetization, and large magnetic anisotropy. Our simulations suggest an antiferromagnetic ground state, which is compatible with the experimental findings. Such a Co-HOTP metal-organic network exemplifies how on-surface chemistry can enable the design of field-robust antiferromagnetic materials.

Iron L3-edge energy shifts for the full range of possible 3d occupations within the same oxidation state of iron halides

Oxidation states are integer in number but dⁿ configurations of transition metal centers vary continuously in polar bonds. We quantify the shifts of the iron L₃ excitation energy, within the same formal oxidation state, in a systematic L-edge X-ray absorption spectroscopy study of diatomic gas-phase iron(II) halide cations, [FeX]⁺,where X = F, Cl, Br, I. These shifts correlate with the electronegativity of the halogen, and are attributed exclusively to a fractional increase in population of 3d-derived orbitals along the series as supported by charge transfer multiplet simulations and density functional theory calculations. We extract an excitation energy shift of 420 meV ± 60 meV spanning the full range of possible 3d occupations between the most ionic bond in [FeF]⁺ and covalently bonded [FeI]⁺.

Slow Magnetic Relaxation of Dy Adatoms with In-Plane Magnetic Anisotropy on a Two-Dimensional Electron Gas

We report on the magnetic properties of Dy atoms adsorbed on the (001) surface of SrTiO3. X-ray magnetic circular dichroism reveals slow relaxation of the Dy magnetization on a time scale of about 800 s at 2.5 K, unusually associated with an easy-plane magnetic anisotropy. We attribute these properties to Dy atoms occupying hollow adsorption sites on the TiO2-terminated surface. Conversely, Ho atoms adsorbed on the same surface show paramagnetic behavior down to 2.5 K. With the help of atomic multiplet simulations and first-principles calculations, we establish that Dy populates also the top-O and bridge sites on the coexisting SrO-terminated surface. A simple magnetization relaxation model predicts these two sites to have an even longer magnetization lifetime than the hollow site. Moreover, the adsorption of Dy on the insulating SrTiO3 crystal leads, regardless of the surface termination, to the formation of a spin-polarized two-dimensional electron gas of Ti 3dxy character, together with an antiferromagnetic Dy-Ti coupling. Our findings support the feasibility of tuning the magnetic properties of the rare-earth atoms by acting on the substrate electronic gas with electric fields.

Mapping Orbital-Resolved Magnetism in Single Lanthanide Atoms

Single lanthanide atoms and molecules are promising candidates for atomic data storage and quantum logic due to the long lifetime of their magnetic quantum states. Accessing and controlling these states through electrical transport requires precise knowledge of their electronic configuration at the level of individual atomic orbitals, especially of the outer shells involved in transport. However, no experimental techniques have so far shown the required sensitivity to probe single atoms with orbital selectivity. Here we resolve the magnetism of individual orbitals in Gd and Ho single atoms on MgO/Ag(100) by combining X-ray magnetic circular dichroism with multiplet calculations and density functional theory. In contrast to the usual assumption of bulk-like occupation of the different electronic shells, we establish a charge transfer mechanism leading to an unconventional singly ionized configuration. Our work identifies the role of the valence electrons in determining the quantum level structure and spin-dependent transport properties of lanthanide-based nanomagnets.

Revealing noncollinear magnetic ordering at the atomic scale via XMCD

Mass-selected V and Fe monomers, as well as the heterodimer \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Fe}}_1{\text{V}}_1$$\end{document}Fe1V1, were deposited on a Cu(001) surface. Their electronic and magnetic properties were investigated via X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. Anisotropies in the magnetic moments of the deposited species could be examined by means of angle resolving XMCD, i.e. changing the X-ray angle of incidence. A weak adatom-substrate-coupling was found for both elements and, using group theoretical arguments, the ground state symmetries of the adatoms were determined. For the dimer, a switching from antiparallel to parallel orientation of the respective magnetic moments was observed. We show that this is due to the existence of a noncollinear spin-flop phase in the deposited dimers, which could be observed for the first time in such a small system. Making use of the two magnetic sublattices model, we were able to find the relative orientations for the dimer magnetic moments for different incidence angles.

Molecular Approach for Engineering Interfacial Interactions in Magnetic/Topological Insulator Heterostructures

Controlling interfacial interactions in magnetic/topological insulator heterostructures is a major challenge for the emergence of novel spin-dependent electronic phenomena. As for any rational design of heterostructures that rely on proximity effects, one should ideally retain the overall properties of each component while tuning interactions at the interface. However, in most inorganic interfaces, interactions are too strong, consequently perturbing, and even quenching, both the magnetic moment and the topological surface states at each side of the interface. Here, we show that these properties can be preserved using ligand chemistry to tune the interaction of magnetic ions with the surface states. By depositing Co-based porphyrin and phthalocyanine monolayers on the surface of Bi₂Te₃ thin films, robust interfaces are formed that preserve undoped topological surface states as well as the pristine magnetic moment of the divalent Co ions. The selected ligands allow us to tune the interfacial hybridization within this weak interaction regime. These results, which are in stark contrast with the observed suppression of the surface state at the first quintuple layer of Bi₂Se₃ induced by the interaction with Co phthalocyanines, demonstrate the capability of planar metal-organic molecules to span interactions from the strong to the weak limit.

Strain effect on orbital and magnetic structures of Mn ions in epitaxial Nd0.35Sr0.65MnO3/SrTiO3 films using X-ray diffraction and absorption

This study probes the temperature-dependent strain that is strongly correlated with the orbital and magnetic structures of epitaxial films of Nd_0.35Sr_0.65MnO₃ (NSMO) that are fabricated by pulsed laser deposition with two thicknesses, 17 (NS17) and 103 nm (NS103) on SrTiO₃ (STO) substrate. This investigation is probed using X-ray diffraction (XRD) and absorption-based techniques, X-ray linear dichroism (XLD) and the X-ray magnetic circular dichroism (XMCD). XRD indicates a significant shift in the (004) peak position that is associated with larger strain in NS17 relative to that of NS103 at both 30 and 300 K. Experimental and atomic multiplet simulated temperature-dependent Mn L_3,2-edge XLD results reveal that the stronger strain in a thinner NS17 film causes less splitting of Mn 3d e_g state at low temperature, indicating an enhancement of orbital fluctuations in the band above the Fermi level. This greater Mn 3d orbital fluctuation can be the cause of both the enhanced ferromagnetism (FM) as a result of spin moments and the reduced Néel temperature of C-type antiferromagnetism (AFM) in NS17, leading to the FM coupling of the canted-antiferromagnetism (FM-cAFM) state in NSMO/STO epitaxial films at low temperature (T = 30 K). These findings are also confirmed by Mn L_3,2-edge XMCD measurements.

Large orbital magnetic moments of small, free cobalt cluster ions Co[Formula: see text] with n [Formula: see text]

The size dependent electronic structure and separate spin and orbital magnetic moments of free Co[Formula: see text] ([Formula: see text]) cluster ions have been investigated by x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. A very large orbital magnetic moment of [Formula: see text] per atom was determined for Co[Formula: see text], which is one order of magnitude larger than in the bulk metal. Large orbital magnetic moments per atom of  ≈1 [Formula: see text] were also found for Co[Formula: see text], Co[Formula: see text], and Co[Formula: see text]. The orbital contribution to the total magnetic moment shows a non-monotonic cluster size dependence: The orbital contribution increases from a local minimum at n  =  2 to a local maximum at n  =  5 and then decreases with increasing cluster size. The 3d spin magnetic moment per atom is nearly constant and is solely defined by the number of 3d holes which shows that the 3d majority spin states are fully occupied, that is, 3d hole spin polarization is 100%.

Magnetic properties of supported metal atoms and clusters

Clusters are small systems ranging from a few atoms up to several thousand atoms. They are of high interest in basic research, but also for applications due to their specific electronic, magnetic or chemical properties depending on size and composition. For small clusters, quantum size effects play an important role and specific material properties might be tailored by choosing a special size or composition of the cluster. Here, we review the magnetic properties of adatoms and supported small mass-selected transition-metal clusters in the few-atom limit investigated by x-ray magnetic circular dichroism spectroscopy in the soft x-ray regime. The influence of cluster size, composition, the cluster-surface and intra-cluster interaction on the spin and orbital magnetic moments will be discussed.

Magnetic remanence in single atoms

A permanent magnet retains a substantial fraction of its saturation magnetization in the absence of an external magnetic field. Realizing magnetic remanence in a single atom allows for storing and processing information in the smallest unit of matter. We show that individual holmium (Ho) atoms adsorbed on ultrathin MgO(100) layers on Ag(100) exhibit magnetic remanence up to a temperature of 30 kelvin and a relaxation time of 1500 seconds at 10 kelvin. This extraordinary stability is achieved by the realization of a symmetry-protected magnetic ground state and by decoupling the Ho spin from the underlying metal by a tunnel barrier.

The high-field magnet endstation for X-ray magnetic dichroism experiments at ESRF soft X-ray beamline ID32

A new high-field magnet endstation for X-ray magnetic dichroism experiments has been installed and commissioned at the ESRF soft X-ray beamline ID32. The magnet consists of two split-pairs of superconducting coils which can generate up to 9 T along the beam and up to 4 T orthogonal to the beam. It is connected to a cluster of ultra-high-vacuum chambers that offer a comprehensive set of surface preparation and characterization techniques. The endstation and the beam properties have been designed to provide optimum experimental conditions for X-ray magnetic linear and circular dichroism experiments in the soft X-ray range between 400 and 1600 eV photon energy. User operation started in November 2014.

Origin of Perpendicular Magnetic Anisotropy and Large Orbital Moment in Fe Atoms on MgO

We report on the magnetic properties of individual Fe atoms deposited on MgO(100) thin films probed by x-ray magnetic circular dichroism and scanning tunneling spectroscopy. We show that the Fe atoms have strong perpendicular magnetic anisotropy with a zero-field splitting of 14.0±0.3  meV/atom. This is a factor of 10 larger than the interface anisotropy of epitaxial Fe layers on MgO and the largest value reported for Fe atoms adsorbed on surfaces. The interplay between the ligand field at the O adsorption sites and spin-orbit coupling is analyzed by density functional theory and multiplet calculations, providing a comprehensive model of the magnetic properties of Fe atoms in a low-symmetry bonding environment.

Magnetic moments of chromium-doped gold clusters: the Anderson impurity model in finite systems

The magnetic moment of a single impurity atom in a finite free electron gas is studied in a combined x-ray magnetic circular dichroism spectroscopy, charge transfer multiplet calculation, and density functional theory study of size-selected free chromium-doped gold clusters. The observed size dependence of the local magnetic moment can be understood as a transition from a local moment to a mixed valence regime. This shows that the Anderson impurity model essentially describes finite systems even though the discrete density of states introduces a significant deviation from a bulk metal, and the free electron gas is only formed by less than 10 electrons. Electronic shell closure in the gold host minimizes the interaction of localized impurity states with the confined free electron gas and preserves the magnetic moment of 5 μ_{B} fully in CrAu_{2}^{+} and almost fully in CrAu_{6}^{+}. Even for open-shell species, large local moments are observed that scale with the energy gap of the gold cluster. This indicates that an energy gap in the free electron gas stabilizes the local magnetic moment of the impurity atom.

Tailoring the magnetism of Co atoms on graphene through substrate hybridization

We determine the magnetic properties of individual Co atoms adsorbed on graphene (G) with x-ray absorption spectroscopy and magnetic circular dichroism. The magnetic ground state of Co adatoms strongly depends on the choice of the metal substrate on which graphene is grown. Cobalt atoms on G/Ru(0001) feature exceptionally large orbital and spin moments, as well as an out-of-plane easy axis with large magnetic anisotropy. Conversely, the magnetic moments are strongly reduced for Co/G/Ir(111), and the magnetization is of the easy-plane type. We demonstrate how the Co magnetic properties, which ultimately depend on the degree of hybridization between the Co 3d orbitals and graphene π bands, can be tailored through the strength of the graphene-substrate coupling.

Reaching the magnetic anisotropy limit of a 3d metal atom

Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom's magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.

Titanium dxy ferromagnetism at the LaAlO3/SrTiO3 interface

A number of recent transport and magnetization studies have shown signs of ferromagnetism in the LaAlO3/SrTiO3 heterostructure, an unexpected property with no bulk analogue in the constituent materials. However, no experiment thus far has provided direct information on the host of the magnetism. Here we report spectroscopic investigations of the magnetism using element-specific techniques, including X-ray magnetic circular dichroism and X-ray absorption spectroscopy, along with corresponding model calculations. We find direct evidence for in-plane ferromagnetic order at the interface, with Ti(3+) character in the dxy orbital of the anisotropic t2g band. These findings establish a striking example of emergent phenomena at oxide interfaces.

Ferromagnetic coupling of mononuclear Fe centers in a self-assembled metal-organic network on Au(111)

The magnetic state and magnetic coupling of individual atoms in nanoscale structures relies on a delicate balance between different interactions with the atomic-scale surroundings. Using scanning tunneling microscopy, we resolve the self-assembled formation of highly ordered bilayer structures of Fe atoms and organic linker molecules (T4PT) when deposited on a Au(111) surface. The Fe atoms are encaged in a three-dimensional coordination motif by three T4PT molecules in the surface plane and an additional T4PT unit on top. Within this crystal field, the Fe atoms retain a magnetic ground state with easy-axis anisotropy, as evidenced by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. The magnetization curves reveal the existence of ferromagnetic coupling between the Fe centers.

Spin coupling and orbital angular momentum quenching in free iron clusters

Magnetic spin and orbital moments of size-selected free iron cluster ions Fe{n}{+} (n=3-20) have been determined via x-ray magnetic circular dichroism spectroscopy. Iron atoms within the clusters exhibit ferromagnetic coupling except for Fe{13}{+}, where the central atom is coupled antiferromagnetically to the atoms in the surrounding shell. Even in very small clusters, the orbital magnetic moment is strongly quenched and reduced to 5%-25% of its atomic value while the spin magnetic moment remains at 60%-90%. This demonstrates that the formation of bonds quenches orbital angular momenta in homonuclear iron clusters already for coordination numbers much smaller than those of the bulk.

Dilute doping, defects, and ferromagnetism in metal oxide systems

Over the past decade intensive research efforts have been carried out by researchers around the globe on exploring the effects of dilute doping of magnetic impurities on the physical properties of functional non-magnetic metal oxides such as TiO(2) and ZnO. This effort is aimed at inducing spin functionality (magnetism, spin polarization) and thereby novel magneto-transport and magneto-optic effects in such oxides. After an early excitement and in spite of some very promising results reported in the literature, this field of diluted magnetic semiconducting oxides (DMSO) has continued to be dogged by concerns regarding uniformity of dopant incorporation, the possibilities of secondary ferromagnetic phases, and contamination issues. The rather sensitive dependence of magnetism of the DMSO systems on growth methods and conditions has led to interesting questions regarding the specific role played by defects in the attendant phenomena. Indeed, it has also led to the rapid re-emergence of the field of defect ferromagnetism. Many theoretical studies have contributed to the analysis of diverse experimental observations in this field and in some cases to the predictions of new systems and scenarios. In this review an attempt is made to capture the scope and spirit of this effort highlighting the successes, concerns, and questions.

Correlated electrons step by step: itinerant-to-localized transition of fe impurities in free-electron metal hosts

High-resolution photoemission spectroscopy and ab initio calculations have been employed to analyze the onset and progression of d-sp hybridization in Fe impurities deposited on alkali metal films. The interplay between delocalization, mediated by the free-electron environment, and Coulomb interaction among d electrons gives rise to complex electronic configurations. The multiplet structure of a single Fe atom evolves and gradually dissolves into a quasiparticle peak near the Fermi level with increasing host electron density. The effective multiorbital impurity problem within the exact diagonalization scheme describes the whole range of hybridizations.

Longitudinal detection of ferromagnetic resonance using x-ray transmission measurements

We describe a setup for the x-ray detection of ferromagnetic resonance in the longitudinal geometry using element-specific transmission measurements. Thin magnetic film samples are placed in a static magnetic field collinear with the propagation direction of a polarized soft x-ray beam and driven to ferromagnetic resonance by a continuous wave microwave magnetic field perpendicular to it. The transmitted photon flux is measured both as a function of the x-ray photon energy and as a function of the applied static magnetic field. We report experiments performed on a 15 nm film of doped Permalloy (Ni(73)Fe(18)Gd(7)Co(2)) at the L(3)/L(2)-edges of Fe, Co, and Ni. The achieved ferromagnetic resonance sensitivity is about 0.1 monolayers/square root(Hz). The obtained results are interpreted in the framework of a conductivity tensor based formalism. The factors limiting the sensitivity as well as different approaches for the x-ray detection of ferromagnetic resonance are discussed.

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.

No quenching of magnetic moment for the GenCo (n=1-13) clusters: first-principles calculations

The authors predict that for the Ge(n)Co (n=1-13) clusters the magnetic moment does not quench, which is dark contrast to the previous results with transition-metal-doped Si(n) clusters. It may be due to the unpaired electrons of the Co atom in the clusters. For the ground state structures of the Ge(n)Co (n>or=9) clusters, the Co atom completely falls into the center of the Ge outer frame, forming metal-encapsulated Ge(n) cages. The doping of the Co atom enhances the stability of the host Ge(n) clusters. The Ge(10)Co cluster with the bicapped tetragonal antiprism structure is more stable than others, which agrees very well with the results of the experiment of the Co/Ge binary clusters by the laser vaporization.

Magnetic and structural investigation of Mn(2+)-doped ZnSe semiconductor nanoparticles

X-ray magnetic circular dichroism (XMCD) experiments on diluted magnetic semiconductor nanocrystals were carried out to study the local electronic structure and magnetic properties of Mn(2+) embedded in the lattice of ZnSe nanoparticles. It is shown that Mn(2+) is exclusively present in the bulk of ZnSe nanoparticles. Neither Mn-Mn coupling nor traces of oxidation to higher Mn oxidation states was observed. This result, which is consistent with EPR spectroscopic data, provides clear proof of the location of Mn(2+) in semiconductor nanoparticles. Further, it is shown that the magnetic ions are highly polarised inside the nanocrystals, where they reach about 50 % of the theoretical value of a pure d(5) state under identical conditions.

Effective Kondo model for a trimer on a metallic surface

I consider a Hubbard-Anderson model which describes localized orbitals in three different atoms hybridized both among themselves and with a continuum of extended states. Using a generalized Schrieffer-Wolf transformation, I derive an effective Kondo model for the interaction between the doublet ground state of the isolated trimer and the extended states. For an isoceles trimer with distances a, l, l between the atoms, the Kondo temperature is very small for l<a and has a maximum for finite l>a when a is small. The results agree with experiments for a Cr trimer on Au(111).

Surface diffusion of Cr adatoms on Au(111) by quantum tunneling

The low-temperature surface diffusion of isolated Cr adatoms on Au(111) has been determined using nonperturbing x rays. Changes in the x-ray magnetic circular dichroism spectral line shape together with Monte Carlo calculations demonstrate that adatom nucleation proceeds via quantum tunneling diffusion rather than over-barrier hopping for temperatures <40K. The jump rates are shown to be as much as 35 orders of magnitude higher than that expected for thermal over-barrier hopping at 10 K.

Direct observation of orbital magnetism in cubic solids

We present x-ray magnetic circular dichroism determinations of the orbital/spin magnetic moment ratios of dilute 3d-series impurities in Au (and Cu) host matrices. This is the first direct measurement of considerable orbital moments in cubic symmetry for a localized impurity in a bulk metal host. It is shown that the unquenching of orbital magnetism depends on a delicate balance of hybridization effects between the local impurity with the host and the filling of the 3d states of the impurity. The results are accompanied by ab initio calculations that support our experimental findings.

Spin and orbital magnetic moments of Fe3O4

We present measurements of the spin and orbital magnetic moments of Fe3O4 by using SQUID and magnetic circular dichroism in soft x-ray absorption. The measurements show that Fe3O4 has a noninteger spin moment, in contrast to its predicted half-metallic feature. Fe3O4 also exhibits a large unquenched orbital moment. Calculations using the local density approximation including the Hubbard U method and the configuration interaction cluster-model suggest that strong correlations and spin-orbit interaction of the 3d electrons result in the noninteger spin and large orbital moments of Fe3O4.

X-ray Magnetic Circular Dichroism of Pseudomonas aeruginosa Nickel(II) Azurin

We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L(3)-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) mu(B). Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d(x2-y2) and 3d(z2) are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d(x2-y2) having much greater ligand character.

High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO(2-delta)

The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

Giant magnetic anisotropy of single cobalt atoms and nanoparticles

The isotropic magnetic moment of a free atom is shown to develop giant magnetic anisotropy energy due to symmetry reduction at an atomically ordered surface. Single cobalt atoms deposited onto platinum (111) are found to have a magnetic anisotropy energy of 9 millielectron volts per atom arising from the combination of unquenched orbital moments (1.1 Bohr magnetons) and strong spin-orbit coupling induced by the platinum substrate. By assembling cobalt nanoparticles containing up to 40 atoms, the magnetic anisotropy energy is further shown to be dependent on single-atom coordination changes. These results confirm theoretical predictions and are of fundamental value to understanding how magnetic anisotropy develops in finite-sized magnetic particles.