Magnetic tunnelling junctions increasingly enter the market for magnetic sensor applications. Thus, technological parameters such as the lifetime characteristics become more and more important. Here, an analysis of the lifetime characteristics of magnetic tunnelling junctions using the Weibull statistical distribution for CoFeB/MgO/CoFeB junctions is presented. The Weibull distribution is governed by two parameters, the characteristic lifetime eta of the population and the shape parameter beta, which gives information about the presence of an infant mortality. The suitability of the Weibull distribution is demonstrated for the description of dielectric breakdown processes in MgO-based tunnelling junctions at different voltages. A study of the dependence of the characteristic lifetime extrapolated to the low voltage regime, and the beta parameter on the nominal barrier thickness and the resistance x area product of the MgO barrier is shown. The influence of the RF deposition power for the MgO barrier and an annealing step on the Weibull parameters is also discussed.

We report on the experimental investigation of the influence of a seed layer on the magnetic properties of the full-Heusler alloy Co2FeAl0.4Si0.6 (CFAS). The studied magnetic films are grown epitaxially on MgO (1 0 0) substrates with Cr and/or MgO seed layers. By employing magneto-optical Kerr effect magnetometry we show that magnetic anisotropy can be tuned by choosing the proper seed layer. The results on CFAS show an overall uniaxial anisotropy plus a biaxial contribution which depends on the seed layer. In addition, if grown on MgO, a sharp increase in the coercive field H-C at a series of angles symmetric with respect to the easy axis is present. Scanning Kerr-microscope imaging is performed during the magnetization reversal process in order to reveal the type of magnetic domain formation taking place at these angles.

Electron spin resonance measurements in EuFe2As2 single crystals revealed an absorption spectrum of a single resonance with Dysonian line shape. Above the spin-density wave (SDW) transition at T-SDW = 190 K the spectra are isotropic and the Eu spins relax via the conduction electrons resulting in a Korringa-type increase in the linewidth. Below T-SDW, a distinct anisotropy develops and the relaxation behavior of the Eu spins changes drastically into one with characteristic properties of a magnetic insulating system, where dipolar and crystal-field interactions dominate. This indicates a spatial confinement of the conduction electrons to the FeAs layers in the SDW state.

The long and short range order of chemically prepared Co2FeGa Heusler nanoparticles with various sizes are determined by x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) spectroscopy. Specifically, EXAFS fittings reveal the size dependent crystal structure and short range order of the Heusler type Co2FeGa nanoparticles. With decreasing particle size, the degree of L21 order in the nanoparticles decreases and the probability of B2 disorder increases simultaneously. The consequences of antisite disorder on the size correlated structure of Co2FeGa nanoparticles are also discussed.

The discovery of new high-temperature superconductors based on FeAs has led to a new 'gold rush' in high-T(C) superconductivity. All of the new superconductors share the same common structural motif of FeAs layers and reach T(C) values up to 55 K (ref. 2). Recently, superconductivity has been reported in FeSe (ref. 3), which has the same iron pnictide layer structure, but without separating layers. Here, we report the magnetic and electronic phase diagram of beta-Fe(1.01)Se as a function of temperature and pressure. The superconducting transition temperature increases from 8.5 to 36.7 K under an applied pressure of 8.9 GPa. It then decreases at higher pressures. A marked change in volume is observed at the same time as T(C) rises, owing to a collapse of the separation between the Fe(2)Se(2) layers. No static magnetic ordering is observed for the whole p-T phase diagram. We also report that at higher pressures (starting around 7 GPa and completed at 38 GPa), Fe(1.01)Se transforms to a hexagonal NiAs-type structure and exhibits non-magnetic behaviour.

Gd-155-Mossbauer spectroscopy was applied to study the magnetic properties of GdPdSb with hexagonal LiGaGe structure and of GdNiSb in the cubic MgAgAs-type structure as well as in the hexagonal AlB2-type structure. In GdPdSb magnetic ordering is observed at 13.0 K with indications of a tilted spin structure at lower temperatures. In the cubic phase of GdNiSb magnetic ordering is observed at 9.5 K and in the hexagonal phase around 3.5 K. These results are discussed in conjunction with previous investigations of these samples. (C) 2008 Elsevier B.V. All rights reserved.

In this Letter we show that superconducting Fe(1.01)Se undergoes a structural transition at 90 K from a tetragonal to an orthorhombic phase but that nonsuperconducting Fe(1.03)Se does not. High resolution electron microscopy at low temperatures further reveals an unexpected additional modulation of the crystal structure of the superconducting phase that involves displacements of the Fe atoms, and that the nonsuperconducting composition shows a different, complex nanometer-scale structural modulation. Finally, we show that magnetism is not the driving force for the phase transition in the superconducting phase.

In this letter, we report that the superconductivity transition temperature in beta-Fe1.01Se increases from 8.5 to 36.7 K under applied pressure of 8.9 GPa. It then decreases at higher pressure. A dramatic change in volume is observed at the same time Tc rises, due to a collapse of the separation between the Fe2Se2 layers. A clear transition to a linear resistivity normal state is seen on cooling at all pressures. No static magnetic ordering is observed for the whole p-T phase diagram. We also report that at higher pressure (starting around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal NiAs-type structure and displays non-magnetic, insulating behavior. The inclusion of electron correlation in band structure caculations is necessary to describe this behavior, signifying that such correlations are important in this chemical system. Our results strongly support unconventional superconductivity in beta-Fe1.01Se.

The interplay between electronic structure, band nesting and antiferromagnetic fluctuations is of general interest for understanding the unconventional superconductivity. For the present study, the tetragonal phase of the superconducting FeSe was selected as model system. The electronic structure of the low-temperature high-pressure phase of FeSe was investigated by means of {\it ab-initio} calculations to find the relation between the band nesting and the critical temperature as a function of pressure. The crystal structure for the highest $T_{\rm C}$ was determined according to the criterion of the {\it optimum} pairing conditions within the unconventional superconductivity regime which was suggested for this type of material by many authors. Those {\it optimum} conditions are found by studying the band structure as function of lattice parameters that corresponds to the application of pressure. As a striking result, two sorts of antiferromagnetic fluctuations corresponding to the intra-layer as well as the inter-layer Fe-Fe coupling are identified. The structure corresponding to a strongest contribution of both mechanisms to the superconducting pairing is found to be in a very good agreement with the experimental crystal structure corresponding to a maximum of $T_{\rm C}$ and qualitatively explains the pressure dependence observed in experiments.

This paper has been withdrawn, due a wrong analysis of the obtained results.

Electron spin resonance measurements in EuFe2As2 single crystals revealed an absorption spectrum of a single resonance with Dysonian lineshape. Above the spin-density wave transition at T_SDW = 190 K the spectra are isotropic and the spin relaxation is strongly coupled to the CEs resulting in a Korringa-like increase of the linewidth. Below T_SDW, a distinct anisotropy develops and the relaxation behavior of the Eu spins changes drastically into one with characteristic properties of a magnetic insulating system, where dipolar and crystal-field interactions dominate. This indicates a spatial confinement of the conduction electrons to the FeAs layers in the SDW state.

The family of half-Heusler compounds offers a variety of half-metallic ferromagnetic materials. We have applied the Mössbauer spectroscopy to study the atomic order, local surroundings and hyperfine fields to several half-Heusler compounds. 121Sb Mössbauer study of the compound CoMnSb revealed the presence of two nonequivalent antimony positions in the elementary cell and enabled to identify the structure. 119mSn, 155Gd and 197Au Mössbauer spectroscopic studies were used to characterize the properties of ferromagnetic granular material based on the half-Heusler ferromagnet MnAuSn in the antiferromagnetic GdAuSn matrix.

We prepared thin films of the Heusler compound Co2Cr0.6Fe0.4Al with the B2 structure on a-plane (112¯0) Al2O3 by sputtering. Films grown at high temperatures (T⩾600∘C) on Al2O3 are fully epitaxial with the (110) and (11¯0) planes of the film parallel to the (112¯0) and (0001) planes of the substrate, respectively. These epitaxial films possess a higher surface roughness than films grown at room temperature. The films show nearly rectangular hysteresis loops with coercive fields of the order of 10mT. Magnetooptical Kerr measurements show an in-plane anisotropy of the magnetization with the easy axis in {001} direction. Hall measurements show a strong anomalous Hall effect and a weak normal Hall voltage signalling the existence of a compensated Fermi surface.

Zeitschrift für anorganische und allgemeine ChemieVolume 630, Issue 11 p. 1715-1715 Poster Dünne epitaktische Filme der Heusler-Phase Co2Cr0,6Fe0,4Al F. Casper, F. Casper Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität MainzSearch for more papers by this authorG. Jakob, G. Jakob Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 7, 55099 MainzSearch for more papers by this authorS. Wurmehl, S. Wurmehl Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität MainzSearch for more papers by this authorH. J. Elmers, H. J. Elmers Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 7, 55099 MainzSearch for more papers by this authorC. Felser, C. Felser Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität MainzSearch for more papers by this author F. Casper, F. Casper Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität MainzSearch for more papers by this authorG. Jakob, G. Jakob Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 7, 55099 MainzSearch for more papers by this authorS. Wurmehl, S. Wurmehl Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität MainzSearch for more papers by this authorH. J. Elmers, H. J. Elmers Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 7, 55099 MainzSearch for more papers by this authorC. Felser, C. Felser Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität MainzSearch for more papers by this author First published: 31 August 2004 https://doi.org/10.1002/zaac.200470047AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume630, Issue11September 2004Pages 1715-1715 RelatedInformation