Polyoxometallates

metallate Polyoxometallates (POMs) respresent a large class of nanosized, polynuclear metal-oxo anions with a wide compositional and structural variety. POMs based on noble metal addenda could be attractive as catalysts and pre-catalysts of different chemical processes as well as precursors for nanoparticles synthesis.

We recently started to investigate Pd containing POMs which were synthsesised by Dr. Natalya Izarova from the FZ Jülich by means of x-ray absorption spectroscopy and its associated magnetic circular and linear dichroism.

Collaborations:
    Dr. Natalya Izarova, FZ Jülich (PGI-6)
    Dr. Detlef Schmitz, Helmholtz-Zentrum Berlin

Oxide nanoparticles

magnetite nanoparticles Magnetite (Fe3O4) nanoparticles are objects of intense research activities due to their broad range of applications covering technological, medical, and environmental applications. Magnetite is the Fe oxide with the highest net magnetic moment and crystallises in a cubic inverse spinel structure. In a simple picture, it consists of Fe2+ ions on octahedral lattice sites, Fe3+ on octahedral and Fe3+ on tetrahedral lattice sites. Although it is well-known to date that due to hybridisation effects the charges of the ions differ from the nominal values, this notation is still used to distinguish between the different Fe species.

The nanoparticles we analysed were synthesised by Dr. Masih Darbandi using a water-in-oil microemulsion technique. Small water droplets are stabilised in an organic solvent by surfactants and act as nanoreactors for the synthesis of nanoparticles. The size of the water droplets can be tuned by the molar ratio of water to surfactant and determines the size of the nanoparticles. After precipitation of the particles, they were redispersed in ethanol and the surfactants could be washed off.

We studied the influence of the surface on the magnetic properties like spin and orbital magnetic moments, spin canting, and the so-called Verwey transition, a phase transition that occurs in pure magnetite only. Parts of our results are supported by DFT calculations performed by Soumyajyoti Haldar and Sumanta Bhandary at the Uppsala University.

Selected publications: Collaborations:
    Dr. Detlef Schmitz, Helmholtz-Zentrum Berlin
    Dr. Masih Darbandi, Vanderbilt U.
    Prof. Dr. Heiko Wende, U. Duisburg-Essen
    Dr. Biplab Sanyal, Uppsala U.

Multiferroic nanocomposites

multiferroic nanoparticles Multiferroic materials showing both magnetic and electric ordering allow an additional degree of freedom in the design of actuators, transducers and storage devices and thus have attracted scientific interest from the technological perspective as well as from basic research. Because the choice of single-phase multiferroic materials being suitable at room temperature is limited, the use of magnetoelectric two-phase composites has proven to be more promising.

In our work, we studied nanocomposites of ferroelectric BaTiO3 (BTO) and ferrimagnetic CoFe2O4 (CFO). By pulsed laser deposition (PLD) CFO nanopillers in a BTO matrix -a so-called (1,3)-type composite - were grown epitaxially by Dr. Pavel Borisov in the group of Prof. W. Kleemann. Starting from CFO and BTO nanoparticles, ceramic samples of CFO grains in a BTO matrix ((0,3)-type) as well as BTO grains in a CFO matrix ((3,0)-type) were obtained by sintering and annealing in the group of Prof. D.C. Lupascu. The coupling between CFO and BTO was studied by x-ray absorption spectroscopy at the Ti and Co L3,2 absorption edges and its linear and circular dichroisms. For the epitaxial (1,3)-type composite we were able to demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the structures and discussed it in terms of off-diagonal magnetostrictive-piezo-electric coupling. This coupling creates staggered in-plane components of the electric polarisation, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BTO matrix.

By comparison of (0,3)- and(3,0)-type composites we showed that the latter exhibits improved magnetic properties while the good ferroelectric characteristics were retained. In addition, a good qualitative agreement between the magnetic field dependence of the electric polarisation obtained from XLD with measurements of the electrically induced magnetization using ac-SQUID susceptometry was obtained.

Selected publications: Collaborations:
    Prof. Dr. Wolfgang Kleemann, Universität Duisburg-Essen
    Dr. Pavel Borisov, West Virginia U.
    Prof. Dr. Doru C. Lupascu, Universität Duisburg-Essen
    Dr. Detlef Schmitz, Helmholtz-Zentrum Berlin
    Prof. Dr. Heiko Wende, U. Duisburg-Essen

Metallic nanoparticles

FePt cluster Our research in the field of metallic nanoparticles focusses on Fe-based systems like pure Fe nanoparticles, FePt, and FeRh nanoparticles. All were wet-chemically synthesised by colleagues from chemistry groups, in particular Frédéric Pelletier and Diana Ciuculescu from the group of Prof. Catherine Amiens at the LCC Toulouse and Prof. Shouheng Sun. We used the x-ray absorption spectroscopy to determine structural and magnetic properties.

Fe nanoparticles. Fe nanoparticles can be used as catalysts, e.g. for the Haber-Bosch process and in Fischer-Tropsch reactions. By analysis of the extended x-ray absorption fine structure (EXAFS), boron incorporations in Fe nanoparticles were identified and quantified. With this results, the effectiveness of amine-borane as reducing agent for the synthesis of iron nanoparticles has been investigated. Furthermore, the reactivity of amine-borane and amino-borane complexes in the presence of pure Fe nanoparticles was studied.

FePt nanoparticles. FePt in its chemically orderes phase has a very high magnetocrystalline anisotropy that stbilises the magnetisation direction even at the nanoscale. Thus, ensembles of FePt nanoparticles are the prime candidate for ultra-high density magnetic storage media. In our work, we studied the possibility to tailor the magnetic properties of FePt nanoparticles by capping. For instance, experimental results on oxide-free FePt nanoparticles embedded in Al were compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allowed us to present three rules to achieve desired magnetic properties for various applications.

FeRh nanoparticles. The 4d metal Rh almost fulfills the Stoner criterion for the occurence of ferromagnetic order. In contact to ferromagnets, it can be easily spin polarised and exhibits sizeable magnetic moments. In the group of Prof. Catherine Amiens at the Laboratoire de Chimie de Coordination, core/shell nanoparticles of Fe and Rh were synthesised. By analysis of the extended x-ray absorption fine structure (EXAFS), we studied the quality of the core/shell structure and used the results to interpret the magnetic moments of Fe and Rh atoms as determined by x-ray magnetic circular dichroism (XMCD) for different synthesis methods.

Pd nanoparticles. Commercially available Pd nanoparticles are investigated by x-ray absorption spectroscopy to study the influence of hydrogen loading on structural and electronic properties. Since the Pd absorption edges accessible in the soft x-ray regime are close to the oxygen K absorption egde, this is an additional challenge from the technical perspective.

Selected publications: Collaborations:
    Prof. Dr. Catherine Amiens, LCC Toulouse
    Dr. Markus Gruner, U. Duisburg-Essen
    Prof. Dr. Michael Farle, U. Duisburg-Essen
    Prof. Dr. Heiko Wende, U. Duisburg-Essen
    Prof. Dr. Kai Fauth, U. Würzburg
    Prof. Dr. Shouheng Sun, Brown U.