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 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

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

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

Direct observation of frozen moments in the NiFe/FeMn exchange bias system

We detect the presence of frozen magnetic moments in an exchange biased NiFe ferromagnet at the NiFe/FeMn ferromagnet/antiferromagnet interface by magnetic circular dichroism in x-ray absorption and resonant reflectivity experiments. Frozen moments are detected by means of the element-specific hysteresis loops. A weak dichroic absorption with unidirectional anisotropy can be linked to frozen magnetic moments in the ferromagnet. A more pronounced exchange bias for increasing the thickness of the FeMn layer correlates with an increase in orbital moment for interface Ni atoms carrying a frozen moment. These atoms compose about a single monolayer, but only a fraction of the atoms contributes by means of a strongly enhanced orbital moment to the macroscopic exchange bias phenomenon. The microscopic spin-orbit energy associated with these few interface frozen moment atoms appears to be sufficient to account for the macroscopic exchange bias energ

Direct observation of frozen moments in the NiFe/FeMn exchange bias system

We detect the presence of frozen magnetic moments in an exchange biased NiFe ferromagnet at the NiFe/FeMn ferromagnet/antiferromagnet interface by magnetic circular dichroism in x-ray absorption and resonant reflectivity experiments. Frozen moments are detected by means of the element-specific hysteresis loops. A weak dichroic absorption with unidirectional anisotropy can be linked to frozen magnetic moments in the ferromagnet. A more pronounced exchange bias for increasing the thickness of the FeMn layer correlates with an increase in orbital moment for interface Ni atoms carrying a frozen moment. These atoms compose about a single monolayer, but only a fraction of the atoms contributes by means of a strongly enhanced orbital moment to the macroscopic exchange bias phenomenon. The microscopic spin-orbit energy associated with these few interface frozen moment atoms appears to be sufficient to account for the macroscopic exchange bias energ

Direct Observation of Ensemble Averaging of the Aharonov-Bohm Effect in Normal-Metal Loops

Aharonov-Bohm magnetoconductance oscillations have been measured in series arrays of 1, 3, 10, and 30 submicron-diameter Ag loops. At constant temperature, the amplitude of the h/e oscillations is observed to decrease as the square root of number of loops, while the amplitude of h/2e conductance oscillations, measured in the same samples, is independent of the number of series loops. This is direct confirmation of the ensemble averaging properties of h/e oscillations in multiloop systems. The amplitude of the h/e oscillations is in good agreement with recent calculations

Equilibrium properties of the mixed state in superconducting niobium in a transverse magnetic field: Experiment and theoretical model

Equilibrium magnetic properties of the mixed state in type-II superconductors were measured with high purity bulk and film niobium samples in parallel and perpendicular magnetic fields using dc magnetometry and scanning Hall-probe microscopy. Equilibrium magnetization data for the perpendicular geometry were obtained for the first time. It was found that none of the existing theories is consistent with these new data. To address this problem, a theoretical model is developed and experimentally validated. The new model describes the mixed state in an averaged limit, i.e. %without detailing the samples' magnetic structure and therefore ignoring interactions between vortices. It is quantitatively consistent with the data obtained in a perpendicular field and provides new insights on properties of vortices. % and the entire mixed state. At low values of the Ginzburg-Landau parameter, the model converts to that of Peierls and London for the intermediate state in type-I superconductors. It is shown that description of the vortex matter in superconductors in terms of a 2D gas is more appropriate than the frequently used crystal- and glass-like scenarios.Comment: 8 pages, 9 figure