W. Keune

Antiferromagnetism of fcc Fe thin films

Thin iron films (∼18 A and 90% enriched in Fe57) were prepared on (001) Cu single‐crystal substrates. The fcc structure was verified by electron microscopy. By Mossbauer spectroscopy it was found that antiferromagnetic ordering begins at around 80±10 K with hyperfine fields of about 16–20 kOe. Additional proof for the existence of anitferromagnetism has been obtained by measuring the films in a longitudinal (parallel to the γ‐ray direction) external magnetic field. The apparent discrepancy in the literature of ferromagnetic and antiferromagnetic ordering in fcc films might be resolved by taking into account the differences concerning the film orientations.

Selective oxidation of benzene to phenol in the liquid phase with amorphous microporous mixed oxides

Abstract Amorphous microporous mixed oxides (AMM) have been investigated as catalysts for the direct oxidation of benzene to phenol with hydrogen peroxide in the liquid phase. The catalysts have been prepared by an acidic sol–gel process. The most active AMM-catalysts found have been based on Fe as active centre. The catalysts have been characterised by a variety of methods including Ar-adsorption isotherms, MAS-NMR, EXAFS, XANES and HRTEM. The most promising catalytic system was AMM-Fe 3 Al 3 Si in acetonitrile as solvent. The effect of various reaction parameters such as solvent, reaction temperature, molar ratio of benzene:hydrogen peroxide, catalyst concentration, and reaction orders have been studied. The iron containing catalysts shows substantial leaching under reaction conditions. Among several attempts to stabilise the active centres in the amorphous network doping with Al-ions was most successful. The drastic effect of Al on the nature of the active Fe in the amorphous silica framework could be documented by Moessbauer spectroscopy. Reaction calorimetry studies revealed that acetonitrile is not an inert solvent but participates directly in the reaction.

Mössbauer‐effect study of Co57 and Fe57 impurities in ferroelectric LiNbO3

The Mossbauer effect observed with LiNbO3:Co57 (source) and LiNbO3:Fe57 (absorber) crystals showed the existence of high‐spin Fe2+ and Fe3+ valence states. The Fe2+/Fe3+ ratio could be changed by reducing or oxidizing heat treatment. Fe3+ in sources and absorbers shows Mossbauer spectra which are typical for slow electronic relaxation between the crystal field states of the 6S5/2 state ion. For both Fe3+ and Fe2+, the principal axis of the electric field gradient is found to be parallel to the crystallographic c axis. For ferric iron Vzz is positive, while Vzz is negative and strongly temperature dependent for ferrous iron. The impurity site substitution and charge compensation mechanism are discussed.

Nanoscale size effect on surface spin canting in iron oxide nanoparticles synthesized by the microemulsion method

Uniformly sized and crystalline iron oxide nanoparticles (IONPs) with spinel structure and mean diameters of about 3, 6 and 9 nm were synthesized in high yield using the microemulsion route at room temperature. The nanoparticles (NPs) were stabilized in situ by organic surfactant molecules which acted both as a stabilizer of the microemulsion system and as a capping layer of the NP surface. NP size control was attained by careful adjustment of the preparation conditions. The structure, morphology and NP size distribution were investigated by x-ray diffraction, transmission electron microscopy and scanning electron microscopy. A particular effort was devoted in this work to study the effect of size and capping of these NPs on their magnetic structure by in-field Mossbauer spectroscopy at 4.2 K. The mean canting angle (relative to the applied field direction) of the Fe spins was observed to increase with decreasing NP size due to the enhanced surface-to-volume ratio. Comparing bare and capped NPs of the same diameter, we verified that the spin canting was not affected by the organic capping. This implied almost identical magnetic orientations of bare and capped NPs. Simultaneously, the capping material was capable of preventing agglomeration effects which can occur in case of direct particle contact. Using a core/shell model, we showed that spin canting originated from the surface shell of the NPs. Furthermore, the Mossbauer spectral parameters provided evidence for the existence of a high fraction of Fe3O4 (magnetite) in the IONP.

Electron spin injection into GaAs from ferromagnetic contacts in remanence

We demonstrate electrical spin injection into a (GaIn)As∕GaAs light-emitting diode from the remanent state of ferromagnetic contacts in perpendicular geometry. Using a Fe∕Tb multilayer structure with perpendicular magnetic anisotropy and a reverse-biased Schottky contact, we achieve a circular polarization degree of the emitted light of 0.75% at 90K.

Magneto-volume effects in γ-Fe ultrathin films and small particles

Abstract Experimental results from 57 Fe Mossbauer-effect investigations on epitaxial, γ-Fe ultrathin films and on γ-Fe precipitates are presented. Depending on the preparation conditions, low-moment anti-ferromagnetic Fe or high-moment ferromagnetic Fe is observed in fcc-Fe (0 0 1) films on Cu (0 0 1), and high-moment Fe in fcc-Fe (0 0 1) films on Cu 3 Au (0 0 1). Isomer-shift values suggest that this behavior is related to different lattice strains. The saturation hyperfine-field of antiferromagnetic coherent γ-Fe precipitates in a Cu 100− x Al x matrix (0⩽ x ⩽14) was found to increase remarkably with increasing x , i.e., rising lattice expansion of the matrix. For γ-Fe precipitates in Cu, the magnetic transition temperature and the saturation hyperfine-field show an anomalous increase above a critical particle size indicating a suggested structural phase transition. Very small precipitates show superparamagnetism.

Structural and magnetic properties of NixFe1-x evaporated thin films

Abstract Thin Ni x Fe 1- x alloy films of various compositions ( x x > 0.46) were vacuum deposited onto quartz crystal substrates by electron beam evaporation. Structural properties of the Ni x Fe 1- x films were obtained by electron microprobe analysis (EPMA), X -ray diffraction and TEM-investigation. Magnetic properties were determined by M ( H , T )- and conversion electron Mossbauer spectroscopic (CEMS) measurements. These gave the concentration dependence of the saturation magnetization M s ( x ) and the mean hyperfine field B hf (x) of the evaporated films. The results are compared to those found for bulk Ni x Fe 1- x alloys and sputtered Ni x Fe 1- x alloys films, which had been investigated by other authors . The comparison of the magnetic behavior of the evaporated and sputtered Ni x Fe 1- x films yields the possibility to discuss more intensively the influence of Ar which had been suggested to be decisive in changing the magnetic properties of sputtered Ni x Fe 1- x films.

Element-Resolved Thermodynamics of Magnetocaloric LaFe 13−x Si x

By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe(13-x)Si(x). These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties.

Electrical detection of photoinduced spins both at room temperature and in remanence

We demonstrate a photodetector with ferromagnetic contacts which can electrically detect the polarization degree of incoming light using spin filtering of photoinduced spin-polarized electron currents. Our structure is a pin diode with a single GaAs quantum well as active region and a Fe∕Tb multilayer on top of a MgO tunnel barrier as n-contact where the spin-polarized electron current is filtered. The photocurrent depends on the magnetization of the contacts and on the polarization of the injected light. We prove that even in remanence and at room temperature the degree of circular polarization of the incident light can be unambiguously determined by the photocurrent intensity.

Room temperature electrical spin injection in remanence

We demonstrate electrical spin injection from ferromagnetic Fe/Tb multilayer structures with remanent perpendicular magnetization into GaAs-based light-emitting diodes at room temperature. Using a reverse-biased Schottky contact and a MgO tunnel contact, respectively, we achieve spin injection at remanence. The maximum degree of circular polarization of the emitted light is 3% at room temperature.