Site-selective holographic imaging of iron arrangements in magnetite

Complex gamma-ray holograms were recorded by tuning to the nuclear absorption lines of 57Fe in magnetite corresponding to different hyperfine fields. The numerical reconstruction of the holograms to real space provided three-dimensional images of local iron arrangements in octahedral and tetrahedral sublattices of magnetite. This direct site-selective imaging of atomic structure was performed using a tabletop experimental setup.     

In-situ conversion electron Mössbauer spectroscopy on Fe(110)-surfaces and thin films

Using in-situ conversion electron Mössbauer spectroscopy the layer-by-layer analysis of epitaxial 21-monolayer Fe(110)-films on W(110) was carried out. The hyperfine interaction parameters near the Ag-coated and free Fe(110) surfaces could be measured with the monolayer resolution. The quadrupole interaction deviates strongly from zero in the topmost iron monolayer. The reduction of hf-magnetic field for the free surface is demonstrated. The temperature dependence of the hf-magnetic field across the Ag-coated films is discussed.This work was supported by the Stiftung Volkswagenwerk.     

CEMS analysis of Fe thin films

The application of the Conversion Electron Mössbauer Spectroscopy (CEMS) to the structural and magnetic analysis of ultrathin films and their interfaces is reported. Fe(110) films were deposited on W(110) under UHV conditions and analyzed in situ using CEMS. The changes ofB _hf from layer to layer across the film are discussed with respect to the modifications of magnetic properties caused both by the finite film thickness and by the specific electronic structure of the interfaces.     

Onset of magnetization in monolayer films

Correlations between structural and magnetic properties for ultra-thin iron films are discussed. Some selected methods of structural and magnetic analysis are reviewed. The onset of magnetism for submonolayer Fe(110) film on W(110) is explained based on STM (scanning tunneling microscopy) imaging. Hyperfine magnetic fields in iron films sandwiched between Cr show unusual temperature behavior due to the influence of magnetic ordering in Cr. Magnetic properties of metastable iron phases on Cu and Ru are discussed, based on CEMS measurements.     

In-plane magnetic surface anisotropies in Fe(110)

In-plane magnetic surface anisotropies have been detected for Fe(110) on W(110) using in situ Conversion Electron Mössbauer Spectroscopy (CEMS). The phenomenon used for the determination of this anisotropy was a switching of the spontaneous magnetizationJ _s from [001] to [1¯10] with decreasing thickness. Analysis of the data is performed using a homogeneous magnetization approximation for competing surface and bulk anisotropies, which is justified by a micromagnetic analysis and established experimentally by CEMS. In-plane surface anisotropy constants for the clean Fe(110) surface, the Fe metal-interface and the FeGaAs interface are determined toK _s,p ^FeUHV =0.065 erg·cm^–2,K _s,p ^FeMetal =0.040 erg ·cm^–2, andK _s,p ^FeGaAs =0.047 erg ·cm^–2, all with an estimated accuracy of the order of 10%.     

γ-Ray Holography – Three-Dimensional Imaging of a Local Atomic Structure

γ-ray holography is a novel method for three-dimensional (3D) imaging of a local atomic structure, which utilizes the internal reference idea of holography. The principles of this method will be presented with emphasis on the phase information gained by means of complex holograms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of densification on the density of vibrational states of glasses

We studied the effect of densification on the vibrational dynamics of a Na(2)FeSi(3)O(8) glass. The density of vibrational states (DOS) has been measured using nuclear inelastic scattering. The corresponding changes in the microscopic, intermediate-range, and macroscopic properties have also been investigated. The results reveal that, in the absence of local structure transformations, the Debye level and the glass-specific excess of vibrational states above it have the same dependence on density, and the evolution of the DOS is fully described by the transformation of the elastic medium.     

Chemical proprieties of the iron-quinone complex in mutated reaction centers of Rb. sphaeroides

We investigated type II bacterial photosynthetic reaction centers, which contain a quinone - iron complex (Q_A-Fe-Q_B) on their acceptor side. Under physiological conditions it was observed mainly in a reduced high spin state but its low spin ferrous states were also observed. Therefore, it was suggested that it might regulate the dynamical properties of the iron–quinone complex and the protonation and deprotonation events in its neighbourhood. In order to get insight into the molecular mechanism of the NHFe low spin state formation, we preformed M?ssbauer studies of a wild type of Rb. sphaeroides and its two mutated forms. Our M?ssbauer measurements show that the hydrophobicity of the Q_A binding site can be crucial for stabilization of the high spin ferrous state of NHFe.