Universal conductance fluctuations and low temperature 1/f noise in mesoscopic AuFe spin glasses

We report on intrinsic time-dependent conductance fluctuations observed in mesoscopic AuFe spin glass wires. These dynamical fluctuations have a 1/f-like spectrum and appear below the measured spin glass freezing temperature of our samples. The dependence of the fluctuation amplitude on temperature, magnetic field, voltage and Fe concentration allows a consistent interpretation in terms of quantum interference effects which are sensitive to the slowly fluctuating spin configuration.Comment: 4 pages, 4 figure

Magnetic anisotropy and reversal in epitaxial Fe/MgO(001) films revisited

We investigate the magnetization reversal in Fe/MgO(001) films with fourfold in-plane magnetic anisotropy and an additional uniaxial anisotropy whose orientation and strength are tuned using different growth geometries and post growth treatments. The previously adopted mechanism of 180^{o} domain wall nucleation clearly fails to explain the observed 180^{o} magnetization reversal. A new reversal mechanism with two successive domain wall nucleations consistently predicts the switching fields for all field orientations. Our results are relevant for a correct interpretation of magnetization reversal in many other epitaxial metallic and semiconducting thin films.Comment: 4 pages, 4 figure

Magnetic flux density and the critical field in the intermediate state of type-I superconductors

To address unsolved fundamental problems of the intermediate state (IS), the equilibrium magnetic flux structure and the critical field in a high purity type-I superconductor (indium film) are investigated using magneto-optical imaging with a 3D vector magnet and electrical transport measurements. The least expected observation is that the critical field in the IS can be as small as nearly 40% of the thermodynamic critical field $H_c$. This indicates that the flux density in the \textit{bulk} of normal domains can be \textit{considerably} less than $H_c$, in apparent contradiction with the long established paradigm, stating that the normal phase is unstable below $H_c$. Here we present a novel theoretical model consistently describing this and \textit{all} other properties of the IS. Moreover, our model, based the rigorous thermodynamic treatment of observed laminar flux structure in a tilted field, allows for a \textit{quantitative} determination of the domain-wall parameter and the coherence length, and provides new insight into the properties of all superconductors.Comment: 5 pages, 5 figure

Reversing the training effect in exchange biased CoO/Co bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) are consistent with nucleation of magnetic domains in the antiferromagnetic CoO layer during the first magnetization reversal. This accounts for the enhanced spin rotation observed in the ferromagnetic Co layer for all subsequent reversals. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.Comment: 4 pages, 4 figure

Diseño editorial de una revista para la promoción de emprendimientos y proyectos creativos emergentes en la ciudad de Cuenca

Este documento expone la investigación, conceptualización y desarrollo de un proyecto editorial enfocado a personas interesadas en la cultura, áreas creativas y muchos más ámbitos del arte en la ciudad de Cuenca. La metodología proyectual de Bruno Munari fue utilizada, al ser líneal, que divide “el problema” inicial en partes o subproblemas con el fin de obtener un resultado óptimo con un esfuerzo mínimo. El desarrollo del trabajo de graduación investiga el porqué de una revista cultural creativa, las partes y métodos de una revista, homólogos pertinentes y por último, se desarrolla el branding y una primera edición, enfocada al arte y oficio del Diseño Gráfico. Se realizó entrevistas con personas en el área y en varias subáreas,se desarrolló un calendario con eventos importantes para la rama del diseño gráfico entre otros segmentos en la revista. Una diagramación llamativa era el punto de partida para la línea gráfica de la revista.This document shows the investigation, conceptualization and development of an editorial project for people interested in culture, creative areas and other specialties in art in the city of Cuenca. The projectual methodology developed by Bruno Munari was used as it’s a lineal methodology that divides the main “problem” in parts or subproblems to reach the best result with the smallest amount of effort. In the process of the developing this project, various topics are investigated: the why of a cultural and creative magazine, the parts and methods of a magazine, important homologues and finally the branding of the magazine, along with its first edition, focused on the art and profession that is Graphic Design. Interviews with graphic designers were conducted, a calendar with important events was made among other sections of the magazine. An interesting and striking layout was the foundation for the graphic language of de magazine.Diseñador GráficoCuenc

Thermal Depinning of Abrikosov Vortices in a Nb Polycrystalline Bulk Absorber for Gamma-Ray Superconducting Detector

Abstract The threshold temperature at which the thermal depinning of Abrikosov vortices starts to be pronounced, defines the upper temperature limit for secure operation of a gamma-ray superconducting detector based on Abrikosov vortices. Indeed, because of the flux creep phenomenon, unwanted spontaneous vortex jumps can take place concurrently with those resulting from the gamma-ray photon absorption, resulting in the appearance of dark counts. Low temperature magnetic force microscopy (MFM) was applied for the evaluation of the threshold temperature for a 0.3 mm thick Nb polycrystalline bulk absorber with dimensions of 5×5 mm2, which was chosen for the fabrication of Josephson tunnel junctions serving as vortex sensor element of the gamma-ray detector. Vortices were generated by cooling the sample to 4.3 K in a small magnetic field. A field of 0.1 mT was chosen in order to produce more than two vortices within the 7×7 μm2 scan area, but with sufficiently large inter-vortex spacing such that vortex-vortex interactions would be negligible. The threshold temperature associated with the thermal depinning of a single vortex was found to be 6.3 ± 0.2 K, whereas the threshold temperature associated with the thermal depinning of half of vortices was found to be 7.2 ± 0.2 K

Surface morphology and magnetic anisotropy of Fe/MgO(001) films deposited at oblique incidence

We have studied surface morphology and magnetic properties of Fe/MgO(001) films deposited at an angle varying between 0o and 60o with respect to the surface normal and with azimuth along the Fe[010] or the Fe[110] direction. Due to shadowing, elongated grains appear on the film surface for deposition at sufficiently large angle. X-ray reflectivity reveals that, depending on the azimuthal direction, films become either rougher or smoother for oblique deposition. For deposition along Fe[010] the pronounced uniaxial magnetic anisotropy (UMA) results in the occurrence of reversed two-step and of three-step hysteresis loops. For deposition along Fe[110] the growth-induced UMA is much weaker, causing a small rotation of the easy axes.Comment: 3 pages, 4 figure

Noninvasive Embedding of Single Co Atoms in Ge(111)2x1 Surfaces

We report on a combined scanning tunneling microscopy (STM) and density functional theory (DFT) based investigation of Co atoms on Ge(111)2x1 surfaces. When deposited on cold surfaces, individual Co atoms have a limited diffusivity on the atomically flat areas and apparently reside on top of the upper pi-bonded chain rows exclusively. Voltage-dependent STM imaging reveals a highly anisotropic electronic perturbation of the Ge surface surrounding these Co atoms and pronounced one-dimensional confinement along the pi-bonded chains. DFT calculations reveal that the individual Co atoms are in fact embedded in the Ge surface, where they occupy a quasi-stationary position within the big 7-member Ge ring in between the 3rd and 4th atomic Ge layer. The energy needed for the Co atoms to overcome the potential barrier for penetration in the Ge surface is provided by the kinetic energy resulting from the deposition process. DFT calculations further demonstrate that the embedded Co atoms form four covalent Co-Ge bonds, resulting in a Co4+ valence state and a 3d5 electronic configuration. Calculated STM images are in perfect agreement with the experimental atomic resolution STM images for the broad range of applied tunneling voltages.Comment: 19 pages, 15 figures, 3 table