Gerrit van der Laan

Nature of the 5f states in actinide metals

Actinide elements produce a plethora of interesting physical behaviors due to the 5f states. This review compiles and analyzes progress in understanding of the electronic and magnetic structure of the 5f states in actinide metals. Particular interest is given to electron energy-loss spectroscopy and many-electron atomic spectral calculations, since there is now an appreciable library of core d -> valence f transitions for Th, U, Np, Pu, Am, and Cm. These results are interwoven and discussed against published experimental data, such as x-ray photoemission and absorption spectroscopy, transport measurements, and electron, x-ray, and neutron diffraction, as well as theoretical results, such as density-functional theory and dynamical mean-field theory.

Microscopic origin of magnetocrystalline anisotropy in transition metal thin films

We investigate the relation between the magnetocrystalline anisotropy energy (MAE) and the electronic structure for transition metal thin films and surfaces which can display enhanced orbital magnetic moments. When the spin-orbit interaction is treated in second order, the MAE is proportional to the expectation value of the orbital magnetic moment as given by Brunos model. However, there are additional terms which are related to the spin-subband orbital moment and to the magnetic dipole operator due to the anisotropy of the field of the spin. The latter term accounts for the spin-flip excitations between the exchange split majority and minority spin bands. A conjecture is proposed which relates the MAE to the expectation values of the orbital moments and the magnetic dipole term. It is shown how the different terms can be obtained experimentally with (transverse) magnetic circular x-ray dichroism. The model explains the experimentally observed perpendicular magnetic anisotropy in Co and Fe based multilayers and thin films.

Rapid magnetosome formation shown by real-time x-ray magnetic circular dichroism.

Magnetosomes are magnetite nanoparticles formed by biomineralization within magnetotactic bacteria. Although there have been numerous genetic and proteomic studies of the magnetosome-formation process, there have been only limited and inconclusive studies of mineral-phase evolution during the formation process, and no real-time studies of such processes have yet been performed. Thus, suggested formation mechanisms still need substantiating with data. Here we report the examination of the magnetosome material throughout the formation process in a real-time in vivo study of Magnetospirillum gryphiswaldense, strain MSR-1. Transmission EM and x-ray absorption spectroscopy studies reveal that full-sized magnetosomes are seen 15 min after formation is initiated. These immature magnetosomes contain a surface layer of the nonmagnetic iron oxide-phase hematite. Mature magnetite is found after another 15 min, concurrent with a dramatic increase in magnetization. This rapid formation result is contrary to previously reported studies and discounts the previously proposed slow, multistep formation mechanisms. Thus, we conclude that the biomineralization of magnetite occurs rapidly in magnetotactic bacteria on a similar time scale to high-temperature chemical precipitation reactions, and we suggest that this finding is caused by a biological catalysis of the process.

Microbial engineering of nanoheterostructures: Biological synthesis of a magnetically recoverable palladium nanocatalyst

Precious metals supported on ferrimagnetic particles have a diverse range of uses in catalysis. However, fabrication using synthetic methods results in potentially high environmental and economic costs. Here we show a novel biotechnological route for the synthesis of a heterogeneous catalyst consisting of reactive palladium nanoparticles arrayed on a nanoscale biomagnetite support. The magnetic support was synthesized at ambient temperature by the Fe(III)-reducing bacterium, Geobacter sulfurreducens , and facilitated ease of recovery of the catalyst with superior performance due to reduced agglomeration (versus conventional colloidal Pd nanoparticles). Surface arrays of palladium nanoparticles were deposited on the nanomagnetite using a simple one-step method without the need to modify the biomineral surface, most likely due to an organic coating priming the surface for Pd adsorption, which was produced by the bacterial culture during the formation of the nanoparticles. A combination of EXAFS and XPS showed the Pd nanoparticles on the magnetite to be predominantly metallic in nature. The Pd(0)-biomagnetite was tested for catalytic activity in the Heck reaction coupling iodobenzene to ethyl acrylate or styrene. Rates of reaction were equal to or superior to those obtained with an equimolar amount of a commercial colloidal palladium catalyst, and near complete conversion to ethyl cinnamate or stilbene was achieved within 90 and 180 min, respectively.

Direct determination of cation site occupancies in natural ferrite spinels by L2,3 X-ray absorption spectroscopy and X-ray magnetic circular dichroism

Abstract Cation distributions in natural ferrite spinels, some containing large amounts of Mg, Ti, Mn, and Zn, have been investigated using the element-, site-, and symmetry-selective spectroscopic techniques of L2,3 X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). By comparing XMCD data with calculated spectra, the site occupancies of the Fe cations have been determined. From the analysis of natural ferrite spinels with formulae very close to that of pure magnetite (Fe3O4), a standard XMCD spectrum for natural magnetite is proposed. Magnetites with small numbers of cation vacancies due to oxidation (solid solutions with maghemite, γ-Fe2O3) show that all the vacancies occur in octahedral sites. Ti L2,3 XAS of oxidized Ti-bearing magnetites (hereafter referred to titanomagnetites) shows that Ti is tetravalent occurring on the octahedral site with 10Dq ~2eV; Fe L2,3 XMCD spectra indicate that the vacancies occur in both tetrahedral and octahedral sites. Mn L2,3 XAS of the Mn-rich ferrite spinels shows that Mn is predominantly ordered onto the tetrahedral site with an Mn2+:Mn3+ ratio of 0.85:0.15. Mn- and Zn-rich ferrite spinels have an excess of cations over 3.0 per 4-oxygen formula unit. The sign of the XMCD for Mn corresponds to a parallel alignment of the Mn moments with the Fe3+ moments in the tetrahedral sublattice. This work demonstrates clearly that combined XAS and XMCD provides direct information on the distribution of multivalent cations in chemically complex magnetic spinels.

Probing the site occupancies of Co-, Ni-, and Mn-substituted biogenic magnetite using XAS and XMCD

Abstract Ferrimagnetic nanoparticles have many uses in industry including in magnetic recording media and transformers, however these particles are often expensive to synthesize. In this study, the Fe3+- reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis were used to synthesize spinel ferrite nanoparticles of the general chemical formula MxFe3-xO4, where M is either Co, Ni, Mn, Zn, or a combination of Mn and Zn. This was done at ambient temperatures through the dissimilatory reduction of Fe3+-oxyhydroxides containing the appropriate substitutional cations. A combination of L-edge and K-edge X-ray absorption spectroscopy (XAS) and L-edge X-ray magnetic circular dichroism (XMCD) was used to determine the site occupancies, valence, and local structure of the Fe and substitutional cations within the spinels. The Ni and Co ferrites produced using each bacterium were very similar and therefore this study concludes that, despite the difference in reduction mechanism of the bacteria used, the end-product is remarkably unaltered. Nickel ferrites contained only Ni2+, with at least 80% in Oh coordination. Cobalt ferrites contained only Co2+ but with a significant proportion (up to 45%) in Td coordination, showing a slight preference for Td sites. The Mn-ferrites contained Mn2+ only on the Oh sites but a mixture of Mn2+ and Mn3+ on Td sites when the amount of Mn exceeded 3% (compared to the amount of Fe) or some Zn was also present. This study successfully produced a range of nanoparticulate ferrites that could be produced industrially using relatively environmentally benign methodologies.

Cation site occupancy of biogenic magnetite compared to polygenic ferrite spinels determined by X-ray magnetic circular dichroism

Ferrite spinels, especially magnetite (Fe3O4), can be formed either by geological, biological or chemical processes leading to chemically similar phases that show different physical characteristics. We compare, for the first time, magnetite produced by these three different methods using X-ray magnetic circular dichroism (XMCD), a synchrotron radiation based technique able to determine the site occupancy of Fe cations in the ferrite spinels. Extracellular nanoscale magnetite produced by different Fe(Ill)reducing bacteria was shown to have different degrees of stoichiometry depending on the bacteria and the method of formation, but all were oxygen deficient due to formation under anoxic conditions. Intracellular nano-magnetite synthesized in the magnetosomes of magnetotactic bacteria was found to have a Fe cation site occupancy ratio most similar to stoichiometric magnetite, possibly due to the tight physiological controls exerted by the magnetosome membrane. Chemically-synthesised nano-magnetite and bulk magnetite produced as a result of geological processes were both found to be cation deficient with a composition between magnetite and maghemite (oxidised magnetite).

Fe site occupancy in magnetite-ulvöspinel solid solutions: A new approach using X-ray magnetic circular dichroism

Abstract Ordering of Fe3+ and Fe2+ cations between octahedral and tetrahedral sites in synthetic members of the magnetite-ulvöspinel (Fe3O4-Fe2TiO4) solid-solution series was determined using Fe L2,3-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L2,3-edge and Fe K-edge X-ray absorption spectroscopy (XAS), and unit-cell parameters. Microprobe analyses, cell edges, and chemical FeO determinations showed that bulk compositions were stoichiometric magnetite-ulvöspinel solid solutions. XMCD showed that the surface was sensitive to redox conditions, and samples required re-equilibration with solid-solid buffers. Detailed site-occupancy analysis gave Fe2+/Fe3+ XMCD-intensity ratios close to stoichiometric values. L2,3-edge XAS confirmed that Ti4+ was restricted to octahedral sites. XMCD showed that significant Fe2+ only entered the tetrahedral sites when Ti content was >0.40 atoms per formula unit (apfu), whereas Fe2+ in octahedral sites increased from 1 apfu in magnetite to a maximum of ~1.4 apfu when Ti content was 0.45 apfu. As Ti content increased, a steady increase in Fe2+ in tetrahedral sites was observable in the XMCD spectra, concurrent with a slow decrease in Fe2+ in octahedral sites. Calculated magnetic moments decreased rapidly from magnetite (4.06 μB) to USP45 (1.5 μB), then more slowly toward ulvöspinel (0 μB). Two synthesized samples were maghemitized by re-equilibrating with an oxidizing buffer. XMCD showed that Fe2+ oxidation, with concomitant vacancy formation, was restricted to octahedral sites. Through the direct measurement of Fe oxidation states, XMCD results can be used to rationalize the magnetic properties of titanomagnetites, along with oxidized titanomaghemitized analogs, in Earth’s crustal rocks.

Time-resolved synchrotron powder X-ray diffraction study of magnetite formation by the Fe(III)-reducing bacterium Geobacter sulfurreducens

Abstract The Fe(III)-reducing bacterium Geobacter sulfurreducens produces nanometer-sized magnetite by transferring electrons from organic matter or hydrogen to ferrihydrite, resulting in the reductive transformation of the ferrihydrite to magnetite, and the generation of ATP for growth. Electron transfer can occur by direct contact between the cell surface and the mineral or via a soluble “electron shuttle,” for example a quinone-containing humic species. The minerals produced at different stages of ferrihydrite reduction during two experiments, one with and one without the humic analog anthraquinone-2, 6-disulphonate (AQDS), were measured using high-resolution synchrotron powder X-ray diffraction. Amorphous 2-line ferrihydrite converts to goethite, then to a mixture of goethite and magnetite, and finally to magnetite. Samples with and without AQDS showed the same general mineralogical trends, and the rate of reaction was faster in the presence of AQDS. In addition, two transient minerals structurally similar to goethite and magnetite were observed to form as intermediates between ferrihydrite and goethite and goethite and magnetite, but only in samples produced in the absence of the electron shuttle. These transient minerals were named proto-goethite and proto-magnetite. Proto-goethite has a shorter c-axis [4.467(20) Å] than crystalline goethite, a function of size (<2 nm) where quantum properties prevail. Proto-magnetite is identified by long tetrahedral (2.113 Å) and short octahedral (1.943 Å) Fe-O bonds compared to stoichiometric magnetite, possibly indicative of a coordination crossover caused by charge density [Fe(II)] migration to tetrahedral sites. Fe(II) in solution or sorbed to the mineral surface is considered to be the catalyst causing the mineral transformations. The Fe(II) is thought to form predominantly from the reductive dissolution of 2-line ferrihydrite by G. sulfurreducens.

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.