Direct determination of large spin-torque nonadiabaticity in vortex core dynamics

We use a pump-probe photoemission electron microscopy technique to image the displacement of vortex cores in Permalloy discs due to the spin-torque effect during current pulse injection. Exploiting the distinctly different symmetries of the spin torques and the Oersted-field torque with respect to the vortex spin structure we determine the torques unambiguously, and we quantify the amplitude of the strongly debated nonadiabatic spin torque. The nonadiabaticity parameter is found to be β=0.15±0.07, which is more than an order of magnitude larger than the damping constant α, pointing to strong nonadiabatic transport across the high magnetization gradient vortex spin structures.     

Relationship between nonadiabaticity and damping in permalloy studied by current induced spin structure transformations

By direct imaging we determine spin structure changes in Permalloy wires and disks due to spin transfer torque as well as the critical current densities for different domain wall types. Periodic domain wall transformations from transverse to vortex walls and vice versa are observed, and the transformation mechanism occurs by vortex core displacement perpendicular to the wire. The results imply that the nonadiabaticity parameter beta does not equal the damping alpha, in agreement with recent theoretical predictions. The vortex core motion perpendicular to the current is further studied in disks revealing that the displacement in opposite directions can be attributed to different polarities of the vortex core.     

Site occupancy and molecular complex formation of 19F ions in fullerenes C60 and C70 studied by TDPAD

TDPAD spectroscopy measurements have been performed on two fullerene samples, C₆₀ and C₇₀, with recoil implanted ¹⁹F ions as probe nuclei. Quadrupole coupling frequencies of ν_Q= 54(1) MHz for C₆₀ and ν_Q= 57(1) MHz for C₇₀ were observed. These frequencies are associated with a C–F molecular complex formation. The C₇₀ sample was also examined for possible effects due to variation of temperature. No significant changes were observed. An additional frequency of ν_Q= 13(1) MHz registered for both samples has to be attributed to the formation of H–F complexes. Finally, both samples were analysed for proton dose dependent effects, due to the (p,p'γ) implantation process but no effects have shown up. Specifically, computational studies based on quantum chemical methods were performed to assist in the interpretation of the experimental results, using the code MOPAC. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature-dependent electronic structure of gadolinium

We investigate the electronic structure of the ferromagnetic 4f-metal Gadolinium by use of a many-body evaluation of a generalized model of magnetism, the one-particle part of which is derived from an ASW-LSDA bandstructure calculation. A striking temperature-dependence of the conduction band states is traced back to a 4f-(5d, 6s) interband exchange. The conduction electron polarization (0.63 _B atT=0) decreases forTT _c very similar to the 4f-magnetization. A red shift of the lower -band edge of about 0.25 eV appears upon cooling fromT=T _c toT=0. — The quasiparticle band-structure exibits a remarkable non uniform magnetic behaviour at different positions in the Brillouin zone, and in particular for different subbands. Weakly correlated (s-like) dispersions show a Stoner-likeT-dependence of the exchange splitting. On the other hand, stronger correlated (d-like) dispersions split belowT _c into four branches, two for each spin direction. TheirT-dependence mainly concerns the spectral weights of the quasiparticle peaks and not so much the energetic positions. An exchange caused splitting remains even forT<T _c .     

Pressure-volume relationship,quasiparticle density of states and hybridization gap of mixed valence EuO

Thes-f model, extended by a hybridization term, is used to investigate the pressure-dependence of the quasiparticle density of states of paramagnetic EuO (T=300K). We find a semiconductor-metal transition of first order atp _c =301 kbar, which manifests itself by a striking discontinuity in the pressure-volume relationship, in almost exact agreement with the experiment [5]. The transition is due to the valence-change Eu²⁺Eu³⁺ of the Europium ion.-Special attention is devoted to the competing influence of thes-f exchange interaction and thes-f hybridization between extendeds-band states and localizedf-levels on characteristic details of the phase transition. Exchange and hybridization are therefore taken into account exactly, while the electron hopping is treated in an approximate many body theory.-As a typical feature we find a small hybridization gap in the quasiparticle density of states. Forp<p _c the chemical potential lies always within this gap, so that the material is a narrow gap semiconductor. Forp<p _c the gap does survive, but then is below the gap allowing metallic conductivity.     

The electronic spectrum of protonated adenine: theory and experiment

In this work we present the results of a combined experimental and theoretical study concerned with the question how a proton changes the electronic spectrum and dynamics of adenine. In the experimental part, isolated adenine ions have been formed by electro-spray ionisation, stored, mass-selected and cooled in a Paul trap and dissociated by resonant photoexcitation with ns UV laser pulses. The S(0)-S1 spectrum of protonated adenine recorded by fragment ion detection lies in a similar energy range as the first pipi* transition of neutral 9H-adenine. It shows a flat onset with a broad substructure, indicating a large S(0)-S1 geometry shift and an ultra-short lifetime. In the theoretical part, relative energies of the ground and the excited states of the most important tautomers have been calculated by means of a combined density functional theory and multi-reference configuration interaction approach. Protonation at the nitrogen in position 1 of the neutral 9H-adenine tautomer yields the most stable protonated adenine species, 1H-9H-A+. The 3H-7H-A+ and the 3H-9H-A+ tautomers, formed by protonation of 7H- and 9H-adenine in 3-position, are higher in energy by 162 cm(-1) and 688 cm(-1), respectively. Other tautomers lie at considerably higher energies. Calculated vertical absorption spectra are reported for all investigated tautomers whereas geometry optimisations of excited states have been carried out only for the most interesting ones. The S1 state energies and geometries are found to depend on the protonation site. The theoretical data match best with the experimental onset of the spectrum for the 1H-9H-A+ tautomer although we cannot definitely exclude contributions to the experimental spectrum from the 3H-7H-A+ tautomer at higher energies. The vertical S(0)--> S1 excitation energy is similar to the one in neutral 9H-adenine. As for the neutral adenine, we find a conical intersection of the S1 of protonated adenine with the ground state in an out-of-plane coordinate but at lower energies and accessible without barrier.     

Ferromagnetismus in schmalen Energiebändern

The method of spectral moments is applied to the Hubbard-model for six various densities of states. We find for each density of states a criterion for ferromagnetism as a function of the electron concentrationn. The most surprising result may be that sometimes there are two distinct ferromagnetic solutions, a fact which was pointed out also by Meyer and Schweitzer using the Roth-method. Looking for the reason of the possibility of magnetic ordering we discuss the behaviour of some important physical quantities (energy, magnetization, spin dependent band shift, band width and Fermi-energy).     

A new type of temperature driven reorientation transition in magnetic thin films

We present a new type of temperature driven spin reorientation transition (SRT) in thin films. It can occur when the lattice and the shape anisotropy favor different easy directions of the magnetization. Due to different temperature dependencies of the two contributions the effective anisotropy may change its sign and thus the direction of the magnetization as a function of temperature may change. Contrary to the well-known reorientation transition caused by competing surface and bulk anisotropy contributions the reorientation that we discuss is also found in film systems with a uniform lattice anisotropy. The results of our theoretical model study may have experimental relevance for film systems with positive lattice anisotropy, as e.g. thin iron films grown on copper.     

Investigations on the applicability of two ELISA types for the determination of chlorpyrifos in soil samples compared with a gas chromatographic method

Two commercially available ELISA kits for water analysis, a microtiter plate-ELISA based on polyclonal antibodies (p-ELISA) and a magnetic particle tube-ELISA based on monoclonal antibodies (t-ELISA), were used to determine chlorpyrifos residues in soils. Comparison with a gas chromatographic method frequently applied was carried out by fortification experiments and by analyses of real soil samples. At concentration levels of 1.0, 0.1, 0.05 and 0.01 mg/kg, chlorpyrifos was reliably determined by the GC method. Application of the p-ELISA did not permit a reliable quantitation, while the t-ELISA was applicable in a concentration range of 0.05–1.0 mg chlorpyrifos/kg dry soil.