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Gallium nitride (GaN), grown by HVPE, was implanted with 300 keV Eu ions and then annealed at 1000 oC . Deep level transient spectroscopy (DLTS) and Laplace DLTS (L-DLTS) were used to characterise the ion implantation induced defects in GaN. Two of the implantation induced defects, E1 and E2, with DLTS peaks in the 100 – 200 K temperature range, had DLTS signals that could be studied with L-DLTS. We show that these two defects, with energy levels of 0.18 eV and 0.27 eV below the conduction band, respectively, are two configurations of a metastable defect. These two defect states can be reproducibly removed and re-introduced by changing the pulse, bias and temperature conditions, and the transformation processes follow first order kinetics.

Pronounced room temperature vertical shifts in the magnetic hysteresis loops of granular, highly polycrystalline and ferromagnetic-like BiFeO3 thin films are observed upon field-cooling from a temperature above the Neel temperature of bulk BiFeO3. This is ascribed to the interplay between the preferential alignment, established by the field-cooling process, of the net magnetic moment, which arises from uncompensated antiferromagnetic spins, and the pinning of a fraction of these spins at the particle boundaries. Conversely, field-cooling of an epitaxially grown BiFeO3 film results in no vertical shift, confirming the effective role played by the particle boundaries (i.e., morphology) of the granular-like BiFeO3 films in the process of spin frustration.

In this paper, we propose the fabrication of whole strained Ge complementary metal-oxide-semiconductor (CMOS) with Ge1-xSnx materials as embedded stressors to outperform the state-of-the-art strained Si CMOS. Ge1-xSnx materials have larger lattice constant than that of Ge, which can apply the strain into Ge channel region. Compatibility of Ge1-xSnx (with 2-8% Sn) materials with source/drain engineering processes (B implantation and activation, in situ Ga doping, and Ni(Ge1-ySny) formation) is characterized. A low thermal budget has been determined for those processes on Ge1-xSnx alloys: temperatures must be lower than 600{degree sign}C for B activation and lower than 450{degree sign}C for Ni(Ge1-ySny) formation.

High mobility channels are currently being explored to replace the Si channel in future technology nodes. However, until now the promising bulk properties are very difficult to translate into reality due to a bad passivation of the interface between the gate stack and the substrate. In this paper we want to emphasize that gate stack passivation is the result of a complex interplay between the surface treatment and the ALD deposition. During ALD deposition on GaAs, the removal of surface oxides is observed. However, this effect does not lead to a good passivation. For Ge, the S-passivation of the interface is studied in combination with different high-κ. It is clear that the Ge/S/Al2O3 interface is superior to the Ge/S/ZrO2 interface.

This paper reports on the advantage of using artificial neural networks (ANNs) to analyze large sets of real-time Rutherford backscattering spectrometry (RBS) data. Real-time RBS, i.e. collecting RBS spectra at periodic time intervals during a thermal treatment, probes the full response of a thin film to the annealing in situ . Although very valuable insights can be gained by this technique, the time-consuming analysis of the vast amount of RBS spectra acquired during real-time RBS measurements has so far prevented the widespread use of real-time RBS. Setting up an ANN is quite an intensive process as well, but once trained, these ANNs can handle the analysis of large data sets practically instantaneously. As such, the beneficial combination of real-time RBS and ANN analysis forms a perfect synergy. In this test case, a network was trained and applied to analyze the Ni silicide growth during annealing of a thin 80 nm Ni film on Si(1 0 0). The ANN performance was validated by comparing the ANN results with the conventional analysis performed on the same data set.

A new passivation approach integrating the III-V/Ge MOSFET gate stack processes for a complete CMOS solution is proposed. We explore the In0.53Ga0.47As and Ge MOS electrical properties and present the common gate stack (CGS) concept based on the complementary nature of the asymmetric oxide-In0.53Ga0.47As and the oxide-Ge interface state distributions. In addition, this approach requires neither the interfacial passivation layer (IPL) such as Si, nor the native oxide such as the GeO2, between the gate dielectric and the channels. The oxide-semiconductor interface properties of the common gate stack III-V/Ge MOS system have been carefully investigated and MOS transistors have been fabricated to validate the proposed common gate stack concept. It has been demonstrated that the above common gate stack MOS system can achieve high-performance n-channel operation on In0.53Ga0.47As substrates and p-channel operation on Ge substrates.

In this work, an in-depth growth study with organic vapor phase deposition of the n-type semiconductor N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) is presented. The organic vapor phase deposition technique allows independent control of more parameters than traditional vapor thermal evaporation, namely, not only deposition flux and substrate temperature but also chamber pressure can be changed. We study the influence of these parameters on the morphology and microstructure of PTCDI-C13 thin films, and correlate them with electrical properties. Films of PTCDI-C13 on SiO2 surfaces modified with poly-(α-methylstyrene) exhibit Stranski−Krastanov growth. Upon increasing deposition flux, the resulting surface morphology changes from rough films, characterized by needle growth, to smoother films consisting of small, uniform grains. Notably, increasing the pressure shifts this morphology transition toward lower deposition fluxes. All X-ray reflectivity measurements are indicative of PTCDI-...

The capabilities of artificial neural networks (ANNs) have been investigated for the analysis of nuclear resonant scattering (NRS) data obtained at a synchrotron source. The major advantage of ANNs over conventional analysis methods is that, after an initial training phase, the analysis is fully automatic and practically instantaneous, which allows for a direct intervention of the experimentalist on-site. This is particularly interesting for NRS experiments, where large amounts of data are obtained in very short time intervals and where the conventional analysis method may become quite time-consuming and complicated. To test the capability of ANNs for the automation of the NRS data analysis, a neural network was trained and applied to the specific case of an Fe/Cr multilayer. It was shown how the hyperfine field parameters of the system could be extracted from the experimental NRS spectra. The reliability and accuracy of the ANN was verified by comparing the output of the network with the results obtained by conventional data analysis.

Using complementary surface analysis techniques, we have studied the chemical and structural properties of the S/Ge(100) passivation layer formed upon sulfidation in an aqueous (NH4)2S solution. Our experiments have revealed that only S-Ge bonds are formed upon the adsorption of S and that the surface is essentially free of other S-containing species (i.e. Sn or SOx). The passivation treatment was found to reduce, but not to fully inhibit Ge-O bonds on the surface. Moreover, we have shown that the top-layer of the passivated surface shows a high degree of disorder even though it is associated with a spotty (1x1) RHEED pattern. Based on our experimental findings, we conclude that the passivation layer can be best described as a thin amorphous GeSxOy film atop a partially ordered interfacial layer, a surface model that has been proposed previously in literature.

We present a study of the transport properties of films consisting of Co n clusters embedded in a Au matrix for different Co cluster concentrations. The films were prepared by low-energy cluster beam deposition of Co clusters, combined with simultaneous thermal evaporation of Au. By varying the deposition rates of both Co clusters and Au, films with different Co concentrations were obtained. We investigated the influence of the Co concentration on the low-temperature resistance behavior, thus probing the dominant scattering mechanisms determining the film resistance. The change in low-temperature resistance behavior reveals a transition from films with a low Co concentration, where the low-temperature resistance is dominated by phonon scattering to increasingly ferromagnetically behaving films, in which electron–electron interactions are dominant. The Debye temperature of the films was found to scale with the Co concentration in the films. The low-temperature electron mean free path was determined, which allowed to investigate the influence of the film granularity on the resistance. The temperature dependence of the resistivity reflects the increasing granularity as Au is gradually replaced by Co grains.

Our understanding of the gas exchange mechanisms in plant organs critically depends on insights in the three-dimensional (3-D) structural arrangement of cells and voids. Using synchrotron radiation x-ray tomography, we obtained for the first time high-contrast 3-D absorption images of in vivo fruit tissues of high moisture content at 1.4-μm resolution and 3-D phase contrast images of cell assemblies at a resolution as low as 0.7 μm, enabling visualization of individual cell morphology, cell walls, and entire void networks that were previously unknown. Intercellular spaces were always clear of water. The apple (Malus domestica) cortex contains considerably larger parenchyma cells and voids than pear (Pyrus communis) parenchyma. Voids in apple often are larger than the surrounding cells and some cells are not connected to void spaces. The main voids in apple stretch hundreds of micrometers but are disconnected. Voids in pear cortex tissue are always smaller than parenchyma cells, but each cell is surrounded by a tight and continuous network of voids, except near brachyssclereid groups. Vascular and dermal tissues were also measured. The visualized network architecture was consistent over different picking dates and shelf life. The differences in void fraction (5.1% for pear cortex and 23.0% for apple cortex) and in gas network architecture helps explain the ability of tissues to facilitate or impede gas exchange. Structural changes and anisotropy of tissues may eventually lead to physiological disorders. A combined tomography and internal gas analysis during growth are needed to make progress on the understanding of void formation in fruit.

Epitaxial multiferroic BaTiO₃-BiFeO₃ composite thin films exhibit a correlation between the magnetoelectric (ME) voltage coefficient αME and the oxygen partial pressure during growth. The ME coefficient αME reaches high values up to 43 V/(cm·Oe) at 300 K and at 0.25 mbar oxygen growth pressure. The temperature dependence of αME of the composite films is opposite that of recently-reported BaTiO₃-BiFeO₃ superlattices, indicating that strain-mediated ME coupling alone cannot explain its origin. Probably, charge-mediated ME coupling may play a role in the composite films. Furthermore, the chemically-homogeneous composite films show an oxygen vacancy superstructure, which arises from vacancy ordering on the {111} planes of the pseudocubic BaTiO₃-type structure. This work contributes to the understanding of magnetoelectric coupling as a complex and sensitive interplay of chemical, structural and geometrical issues of the BaTiO₃-BiFeO₃ composite system and, thus, paves the way to practical exploitation of magnetoelectric composites. ispartof: Materials vol:9 issue:1 pages:1-13 ispartof: location:Switzerland status: published

The structure of titanyl phthalocyanine (TiOPc) thin films is correlated with photovoltaic properties of planar heterojunction solar cells by pairing different TiOPc polymorph donor layers with C60 as an acceptor. Solvent annealing and the insertion of two different templating layers, namely 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and CuI, prove to be effective methods to control the TiOPc thin film structure. The crystal phase of TiOPc thin films was identified by combining X-ray reflectivity (XRR) measurements with spectroscopic techniques, including absorption and micro-Raman measurements. Implementation of a donor layer with an absorption spectrum extending into the near-infrared (NIR) led to solar cells with external quantum efficiencies (EQEs) above 27% from λ = 600 - 890 nm, with the best device yielding a power conversion efficiency (PCE) of 2.6%. Our results highlight the need to understand the relationship between processing parameters and thin film structure, as these have important consequences on device performance.

© 2015, Springer Science+Business Media New York. This paper reports a thorough microstructural investigation of bismuth ferrite (BFO) thin films subjected to various processing conditions and discusses their influence on the stability of the BiFeO3 perovskite phase. The formation of secondary phases in BFO thin films is studied as a function of annealing temperature and time, film thickness, Bi excess, and Ti substitution. While films annealed at 600 °C consist of the desired BiFeO3 phase, higher temperatures induce the decomposition leading to a significant amount of secondary phases, particularly the iron-rich Bi2Fe4O9 phase. A longer annealing time at 700 °C further enhances the decomposition of BiFeO3. Qualitative microstructural analysis of the films is performed by electron backscattered diffraction which provides phase analysis of individual grains. The morphology of the single-crystalline Bi2Fe4O9 grains that are embedded in the BiFeO3 matrix drastically changes as a function of the film thickness. Nucleation of these Bi2Fe4O9 grains probably occurs at the film/substrate interface, after which grain growth continues toward the surface of the film through the depletion of the BFO phase. Addition of Bi excess or the substitution of Fe with Ti in the precursor solutions significantly reduces the formation of an iron-rich secondary phase. Influence of the secondary phases as well as Ti substitution on magnetic properties of BFO films was investigated. ispartof: Journal of Materials Science vol:50 issue:13 pages:4463-4476 status: published

© 2015 AIP Publishing LLC. We present an extended X-ray absorption fine structure investigation of the local environment of Sn atoms in strained and relaxed Ge1-xSnx layers with different compositions. We show that the preferred configuration for the incorporation of Sn atoms in these Ge1-xSnx layers is that of a α-Sn defect, with each Sn atom covalently bonded to four Ge atoms in a classic tetrahedral configuration. Sn interstitials, Sn-split vacancy complexes, or Sn dimers, if present at all, are not expected to involve more than 2.5% of the total Sn atoms. This finding, along with a relative increase of Sn atoms in the second atomic shell around a central Sn atom in Ge1-xSnx layers with increasing Sn concentrations, suggests that the investigated materials are homogeneous random substitutional alloys. Within the accuracy of the measurements, the degree of strain relaxation of the Ge1-xSnx layers does not have a significant impact on the local atomic surrounding of the Sn atoms. Finally, the calculated topological rigidity parameter a∗∗= 0.69 ± 0.29 indicates that the strain due to alloying in Ge1-xSnx is accommodated via bond stretching and bond bending, with a slight predominance of the latter, in agreement with ab initio calculations reported in literature. ispartof: Journal of Applied Physics vol:117 issue:9 pages:1-11 status: published

Magnetization reversal in exchange biased Co/CoO dot arrays has been investigated systematically as a function of shape anisotropy, dot size, temperature, cooling field and training effect. Polycrystalline 24?nm Co films have been sputtered on a pre-patterned Si/SiO2 substrate with the lateral dimensions of the dots varying from 200 to 900?nm at a fixed spacing of 800?nm. The Co dots have been oxidized using pure oxygen to create a Co/CoO bilayer. Below the N?el temperature, the hysteresis loop of this bilayer displays a normal unidirectional shift due to the exchange bias in the continuous film, while the patterned film displays an additional anomaly of the upper part of the hysteresis loop. This anomaly, which is shape dependent, increases with decreasing temperature and cooling field. On the other hand, it decreases for the trained loops with increasing aspect ratio (length/width) of the patterned dot. This anomaly is probably caused by incomplete biasing due to the competition between magnetostatic inter-dot interactions and exchange bias.

An (0.2 nm 57Fe/2.6 nm Ag)75 granular multilayer sample and heterostructures with additional continuous Fe layers in different sequences were studied in magnetic field applied at different temperatures. The broadening of the superparamagnetic lines was found to be very similar for the three samples in applied fields both parallel and perpendicular to the sample plane. While the layer sequence has no significant effect on the superparamagnetic properties, the continuous magnetic layers follow a different approach to saturation in perpendicular magnetic fields.

The influence of patterning on exchange bias has been investigated using arrays of micron-sized Co/CoO dots with different lateral confinement and length-to-width ratio. The patterned samples show higher coercive and exchange bias fields than a continuous Co/CoO bilayer. As in unpatterned film, magnetization reversal mechanisms on opposite sides of the hysteresis loops of the microstructured samples are different. However, with the increase of lateral confinement and shape anisotropy of the dots, the asymmetry in the magnetization reversal starts to differ from that observed in continuous Co/CoO films.

Patterned Co/CoO thin film structures have been investigated by magnetization and polarized neutron reflectivity measurements in order to study the influence of finite size and shape anisotropy effects on the magnetization reversal. An anomaly was found in the upper branch of the hysteresis loops, probably caused by incomplete bias in the patterned structures. The asymmetry in magnetization reversal mechanism commonly found in the two branches of the hysteresis loops of unpatterned Co/CoO layers is altered in the patterned structures, consistent with the existence of interfacial domains.

Fe(x)/Ag(2.6 nm) multilayer heterostructures containing Fe layers of periodically altered thickness have been prepared by molecular beam evaporation to study the influence of ferromagnetic layers on the magnetic properties of granular multilayers. The magnetic properties were examined by SQUID magnetization measurements and Mossbauer spectroscopy, and the magnetoresistance was measured in parallel and transversal magnetic fields up to 12 T. Samples with x=0.2 and 1.5 nm were prepared both as separated and as interleaved multilayer heterostructures. The average blocking temperature of the granular Fe layers (x=0.2 nm) is not altered when the thickness of every fourth layer is increased to 1.5 nm. On the other hand the temperature dependence of the magnetoresistance was found to be significantly different for the separated and the interleaved samples. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarized neutron reflectometry with neutron spin analysis is used to investigate the magnetic properties of large arrays of ferromagnetic rectangular Co bars with in-plane magnetization, as well as exchange-biased Co/CoO islands. The off-specular neutron scattering signal contains satellite peaks due to the lateral periodicity of the dot array. Using polarized neutrons, the intensity of the satellite peaks is measured as a function of an external magnetic field applied in the sample plane. Spin analysis of the scattered neutrons reveals the magnetization reversal and saturation within the bars.

Fe films with thickness varying from 46 to 1750 A have been deposited on oxidized Si substrates coated with a Ag buffer layer. The surface morphology of both the Ag buffer layer and the Fe films was investigated by atomic force microscopy. Analysis of the images shows that the surface roughness of the Fe layers is completely dominated by the surface roughness of the Ag buffer layer. Magnetization measurements revealed that the magnetic properties of the Fe films are strongly affected by the surface roughness. The change in the coercivity as a function of the film thickness depends on the ratio of the film thickness and the typical size of the height variations of the surface. When this ratio is smaller than 1, the coercivity increases with increasing film thickness. For a ratio larger than 1, the coercivity decreases monotonically with the film thickness.

The electronic structure of silver–palladium heterostructures is investigated, both experimentally and through ab-initio simulations. Synchrotron-induced photoelectron spectroscopy characterisations of the work function and the valence band structure are compared to and explained by simulations of slab and bulk heterostructures. Work functions and equilibrium geometries are shown to be in agreement with XPS, XRD, and STM measurements, and confirm in particular the pseudomorphic growth of Ag on Pd. Further insight into the differing behavior of the two terminating metal surfaces is extracted from the simulations.

We report on the use of specular and off-specular polarized neutron reflectometry (PNR) for the study of magnetization reversal in patterned magnetic structures. We discuss the instrumental requirements and the measurement method and illustrate its potential by applying it to lithographically structured trilayer systems of the form (7.5 nm Au/20 nm Co/7.5 nm Au) prepared on oxidized Si wafers. In order to study the influence of the shape of the structures on the magnetic properties we prepared rectangular bars with a width of 1 μm and a length of 4 μm. The dots were arranged in a square lattice symmetry with a period of 10 μm. By using a two-dimensional position sensitive detector (PSD) we were able to record not only the specularly but also the off-specularly scattered intensity which has maxima at particular angles due to the in-plane periodicity of the discs. The second system that we studied is that of wires with a width of 2 μm and a period of 15 μm consisting of an exchange biased Co/CoO bilayer structure. By applying neutron spin analysis and monitoring the neutron reflectivity at a fixed angle of incidence as a function of an external magnetic field we were able to effectively study the magnetization reversal process in both systems.

We demonstrate how room temperature scanning Hall probe microscopy (SHPM) can be applied to noninvasively probe the (meta-)stable magnetic domain states of individual mesoscopic Co bars as a function of applied field. These data are correlated with magneto-optical measurements of the hysteresis loop as well as higher spatial resolution magnetic force microscopy imaging. In particular, we show how a combination of visualization techniques is essential for fully understanding the mechanism of magnetization reversal in such microstructures. Micromagnetic calculations of the magnetic states and their corresponding stray fields are correlated with the observations. These experiments prove the usefulness of a noninvasive and quantitative technique such as room temperature SHPM in an applied magnetic field to study magnetic domain stability and switching field distributions.

We studied the off-specular intensity in polarized neutron reflectivity from regular arrays of circular and rectangular Co dots with sizes in the micrometer range. The dots were lithographically prepared and placed on a square lattice with a period of 10 μm, the rectangular dots having an aspect ratio of 4:1. In both cases, we observed enhanced intensity at particular angles off the specular condition. The positions of these satellites are correlated to the lateral periodicity of the ferromagnetic dots. The magnetic information could be revealed by measuring the off-specular intensity as a function of the magnetic field applied in-plane parallel to the rows of the dots. The magnetization reversal process could be analyzed by separating the spin-flip and non-spin-flip contributions to the off-specular signal. The measured spin-flip intensities clearly prove that for both samples the magnetization reversal is dominated by domain wall movement and not by a uniform rotation of single-domain particles.

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (TAs) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (TGa) is negligible. TAs is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding TAs versus TGa occupancy of interstitial Mn in (Ga,Mn)As.

© 2015 American Physical Society. We studied surface morphology induced changes of magnetic anisotropy, magnetization reversal, and symmetry of the anisotropic magnetoresistance (AMR) in ion sputtered Ni films grown on MgO (001). Grazing-incidence ion sputtering generally develops anisotropic surface roughness of the Ni films, i.e., nanometer wide ripples parallel to the ion beam direction, giving rise to uniaxial magnetic anisotropy with the easy axis along the ion beam direction. The formed ripples act as domain wall nucleation and pinning sites during magnetization reversal, while two-jump domain wall motion dominates in the as-grown Ni films. More importantly, the azimuthal angular dependence of the AMR indicates a superposition of twofold symmetry and fourfold symmetry. By relying on grazing-incidence ion sputtering along specific crystallographic directions, we are able to tailor the relative weight of twofold and fourfold symmetry of AMR. We demonstrate that in contrast to the bulk case, the symmetry of the AMR becomes also sensitive to the surface morphology in thin films, which is in particular relevant for the design of magnetotransport based sensors. ispartof: Physical Review B vol:91 issue:10 pages:1-7 status: published

© 2015 AIP Publishing LLC. Magnetic spin structures in epitaxial BiFeO3 single layer and an epitaxial BaTiO3/BiFeO3 multilayer thin film have been studied by means of nuclear resonant scattering of synchrotron radiation. We demonstrate a spin reorientation in the 15 × [BaTiO3/BiFeO3] multilayer compared to the single BiFeO3 thin film. Whereas in the BiFeO3 film, the net magnetic moment m → lies in the (1-10) plane, identical to the bulk, m → in the multilayer points to different polar and azimuthal directions. This spin reorientation indicates that strain and interfaces play a significant role in tuning the magnetic spin order. Furthermore, large difference in the magnetic field dependence of the magnetoelectric coefficient observed between the BiFeO3 single layer and multilayer can be associated with this magnetic spin reorientation. ispartof: Applied Physics Letters vol:106 issue:8 pages:1-4 status: published

We have investigated the magnetic and superconducting properties of a nanostructured ferromagnet/superconductor hybrid system, consisting of a Co layer with a square array of rectangular holes (antidots), covered with a thin continuous (nonperforated) superconducting Pb layer. The Co film with an antidot lattice shows a strong in-plane magnetic shape anisotropy determined by the array configuration and the rectangular shape of the antidots. Remanent domain structures in the uncovered Co antidot array are imaged at room temperature by magnetic force microscopy. The superconducting pinning properties of the hybrid ferromagnet/superconductor system are studied for different magnetic states of the Co antidot lattice. We demonstrate that the contribution to the superconducting pinning potential caused by the stray field from the domain walls is stronger than that arising from the periodic modulation due to the underlying antidot lattice.We have investigated the magnetic and superconducting properties of a nanostructured ferromagnet/superconductor hybrid system, consisting of a Co layer with a square array of rectangular holes (antidots), covered with a thin continuous (nonperforated) superconducting Pb layer. The Co film with an antidot lattice shows a strong in-plane magnetic shape anisotropy determined by the array configuration and the rectangular shape of the antidots. Remanent domain structures in the uncovered Co antidot array are imaged at room temperature by magnetic force microscopy. The superconducting pinning properties of the hybrid ferromagnet/superconductor system are studied for different magnetic states of the Co antidot lattice. We demonstrate that the contribution to the superconducting pinning potential caused by the stray field from the domain walls is stronger than that arising from the periodic modulation due to the underlying antidot lattice.

Fe and Fe 40 Co 60 thin films (5–60 nm) with a bcc structure have been prepared on a -axis oriented sapphire substrates by molecular beam epitaxy. The structural properties have been characterized in situ by reflection high-energy electron diffraction and ex situ by X-ray diffraction and atomic force microscopy. A two-dimensional magneto-optical Kerr effect set-up has been used to determine the in-plane magnetization components and to investigate the magnetic anisotropy and the orientation dependence of the magnetization reversal process. The Fe films and Fe 40 Co 60 alloy films both display a uniaxial in-plane anisotropy. They also exhibit a comparable increase of the coercive field along the easy axis with increasing thickness. We have evaluated this dependence using the results of the structural characterization, indicating that the enhancement of the coercive field is linked to the growing surface roughness and decreasing structural coherence.

We demonstrate the possibility to reliably image vortices in superconducting Nb films with a low-temperature magnetic-force microscope. Our force microscope enables to monitor the surface topography as well, allowing to correlate the location of the vortices with specific topographic features. For Nb films of different thickness (32 nm − 87 nm) and different Tc (7.9 K − 9.1 K) we studied how the vortex configuration changes when changing the applied magnetic field under field-cooled conditions (1 mT − 5 mT). We find that the vortex pinning preferentially occurs in between the grains appearing at the film surface. This is consistent with a distribution of pinning centers which is governed by the columnar growth of the Nb films. For thicker Nb films the vortex arrangement is no longer dominated by pinning alone, and there appear short-range correlations in the vortex lattice. The short-range correlations are enhanced when increasing the applied magnetic field.

We have studied the magnetic and superconducting properties of a nanostructured magnetic/superconducting hybrid system, consisting of a Co layer with a square array of rectangular antidots, covered with a thin 500 A superconducting Pb layer. The magnetic force microscopy data have revealed that the domain walls are located between neighbouring antidots and are pinned at the antidot corners. The superconducting pinning properties of the hybrid system are studied by means of SQUID magnetisation measurements for different magnetic states of the Co antidot lattice. The results show that the stray field, coming from the domain walls in the antidot array, contributes more to the pinning potential than the periodic modulation of the underlying Co layer. The flux lines seem to be pinned at the domain walls between the antidots. In this way, possible matching effects are lost. The network of domain walls thus creates a new type of an artificial pinning array.

We succeeded to image with magnetic force microscopy individual vortices in thin Nb films as well as in thin YBa 2 Cu 3 O 7− δ (YBCO) films. Relying on the imaging of microfabricated gold loops, we are able to identify the exact location of the vortices within 10 nm with respect to topographic features. The center of most of the vortices is located in between the grains appearing at the film surface. For the Nb films with a larger coherence length, the pinning can be linked to the reduced film thickness in between the protruding grains. For the YBCO films with a very small coherence length, the pinning is likely to be dominated by line defects at the trenches in between the growth islands. On the other hand, Fourier analysis of the imaged vortex distribution shows that for thicker Nb films not all of the vortices are pinned in between the grains. Due to the vortex–vortex repulsion, the vortex lattice reveals short-range correlations which become more pronounced at higher fields.

This paper gives an overview of pinning phenomena in superconductors with artificial magnetic pinning arrays. Different ferromagnetic structures have been considered in combination with a low Tc type-II superconducting film. Magnetic force microscopy is used to characterise the magnetic patterns. The pinning properties (critical current, matching effects) are investigated by macroscopic measurements (SQUID, electrical transport) as well as by local observation using scanning Hall probe microscopy. It is shown that, besides structural and geometric modulations, the magnetic interaction of flux lines (FLs) with the ferromagnetic patterns plays an essential role in their pinning properties. The local magnetic stray field affects the order parameter and determines the pinning through the magnetic interaction with the flux lines. To understand this interaction, one should take into account fluxoid quantisation effects and the possibility of having additional induced flux quanta in the superconductor. The magnetic moments of the dots and their mutual alignment with the applied magnetic field are especially important for the asymmetric pinning by perpendicularly magnetised dot- and antidot arrays.

© 2017 Author(s). Nanostructured and disordered superconductors exhibit many exotic fundamental phenomena, and also have many possible applications. We show here that films of superconducting lead nanoparticles with a wide range of particle coverages, exhibit non-linear V(I) characteristics that are consistent with percolation theory. Specifically, it is found that V (I-Ic)a, where a = 2.1 ± 0.2, independent of both temperature and particle coverage, and that the measured critical currents (I c ) are also consistent with percolation models. For samples with low normal state resistances, this behaviour is observable only in pulsed current measurements, which suppress heating effects. We show that the present results are not explained by vortex unbinding [Berezinskii-Kosterlitz-Thouless] physics, which is expected in such samples, but which gives rise to a different power law behaviour. Finally, we compare our results to previous calculations and simulations, and conclude that further theoretical developments are required to explain the high level of consistency in the measured exponents a. ispartof: Journal of Applied Physics vol:122 issue:22 status: published

Synchrotron Mossbauer Reflectometry (SMR) is a novel tool for studying the magnetic structure of multilayers. The orientation of the layer magnetisation in an antiferromagnetically coupled multilayer is determined from the intensity of the pure nuclear reflection in specular time-integral SMR experiments. The value of the saturation field is estimated with high accuracy. The bulk spin-flop transition in an Fe/Cr superlattice of fourfold in-plane magnetocrystalline anisotropy is demonstrated. The width of the off-specular (diffuse) scattering peak is a measure of the in-plane antiferromagnetic domain size. The domain correlation length of 2.6 μm measured in remanence on the Fe/Cr superlattice following magnetic saturation is in good agreement with semi-empirical model calculations.

The solid-phase reaction of ultrathin (

We describe an experimental setup for conversion electron Mossbauer spectroscopy (CEMS) at low temperature. The setup is composed of a continuous flow cryostat (temperature range of 4.2-500 K), detector housing, three channel electron multipliers, and corresponding electronics. We demonstrate the capabilities of the setup with CEMS measurements performed on a sample consisting of a thin enriched 57Fe film, with a thickness of 20 nm, deposited on a silicon substrate. We also describe exchangeable adaptations (lid and sample holder) which extend the applicability of the setup to emission Mossbauer spectroscopy as well as measurements under an applied magnetic field.

Room temperature magnetoelectric BiFeO3-BaTiO3 superlattices with strong out-of-plane magnetic anisotropy have been prepared by pulsed laser deposition. We show that the out-of-plane magnetization component increases with the increasing number of double layers. Moreover, the magnetoelectric voltage coefficient can be tuned by varying the number of interfaces, reaching a maximum value of 29 V/cm Oe for the 20×BiFeO3-BaTiO3 superlattice. This enhancement is accompanied by a high degree of perpendicular magnetic anisotropy, making the latter an ideal candidate for the next generation of data storage devices.

© 2014 American Physical Society. We investigated the training effect and magnetization reversal in CoO/Co bilayer films grown epitaxially on MgO (001) substrates. The asymmetry of the magnetization reversal, which appears due to the exchange bias after field cooling, survives after training, in contrast to the case of polycrystalline bilayers. By applying hysteresis loops with the magnetic field perpendicular to the cooling field, we are able to modify the orientation of the average uncompensated magnetization of the antiferromagnetic CoO. Subsequently, the untrained state can be partially restored, and more importantly the magnetization reversal asymmetry can be inverted by starting the perpendicular loop with the appropriate field polarity. Consequently, we succeeded in manipulating the magnetization reversal asymmetry and even in achieving opposite reversal asymmetries in the same exchange bias system. ispartof: Physical Review B, Condensed Matter and Materials Physics vol:90 issue:21 pages:1-5 status: published

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., The interdependence between training and magnetization reversal in granular Co-CoO exchange bias (EB) systems prepared by O ion implantation in Co thin films is demonstrated by polarized neutron reflectometry. While high-fluence O-implanted thin films show reduced relative training values and no asymmetry in magnetization reversal (all reversals take place by domain wall nucleation and motion), low-fluence O ion implantation results in an increased relative training and a magnetization reversal asymmetry between the first descending and the first ascending branches. Whereas the untrained decreasing field reversal occurs mainly by domain wall nucleation and motion, traces of a domain rotation contribution are evidenced in the increasing field reversal. This is explained by the evolution of the CoO structure and the contribution of the out-of-plane magnetization with ion implantation. The amount of incorporated O, which determines the threshold between both behaviors, is around 20 at.%. This reveals that the interdependence between training and magnetization reversal is insensitive to the morphology of the constituents (i.e., granular or layered), indicating that this is an intrinsic EB effect, which can be conveniently tailored by the interplay between the intrinsic properties of the investigated materials and ion implantation., This work was financed by the Research Foundation - Flanders (FWO), the KU Leuven Concerted Action (GOA/09/006 and GOA/14/007) programs, the 2009-SGR-1292 project of the Generalitat de Catalunya, the MAT2010-20616-C02 project of the Spanish Ministerio de Economía y Competitividad, and the European Commission under the 7th Framework Programme through the “Research Infrastructure” action of the “Capacities” Programme, NMI3-II Grant No. 283883. We thank HZB (Proposal No. PHY-04-2130) and ESRF (Proposal No. HC-1012, BM20 beamline) for the allocation of neutron and synchrotron radiation beamtime, respectively, and C. Bähtz for the assistance during the synchrotron measurements. E.M. and L.M.C.P. also thank the FWO for financial support. T.D. thanks the CNPq agency (Project No. 245897/2012-7) for financial support.

We give an overview on the historical development and current program for lattice location studies at CERN’s ISOLDE facility, where the EC-SLI (Emission Channeling with Short-Lived Isotopes) collaboration maintains several setups for this type of experiments. We illustrate that the three most decisive factors for the success of the technique are access to facilities producing radioactive isotopes, position-sensitive detectors for the emitted decay particles, and reliable simulation codes which allow for quantitative analysis.

We investigated the pinning of flux lines in a superconducting film by a regular array of magnetic antidots. The sample consists of a Co/Pt multilayer with perpendicular magnetic anisotropy in which a regular pattern of submicron holes is introduced and which is covered by a type-II superconducting Pb film. The resulting ferromagnetic/superconducting heterostructure shows a pronounced asymmetric magnetization curve with respect to the field polarity. This asymmetry clearly demonstrates that the magnetic contribution dominates the pinning potential imposed by the magnetic antidots on the superconducting film., Comment: 7 pages, 6 figures, including erratum with 1 page, 1 figure

Lateral nanostructuring is an efficient tool to control vortex confinement in superconductors. This will be illustrated by studying pinning phenomena in type-II superconducting Pb films with a lattice of submicron magnetic dots. We consider rectangular Co dots with in-plane magnetization and circular Co/Pt dots with out-of-plane magnetization. The domain structure of the Co dots can be changed from multi- to single-domain, resulting in an enhancement of their stray field. After covering this Co dot array with a Pb film, we demonstrate the influence of the local magnetic stray field of the dots on their flux pinning efficiency. The Co/Pt dots have a single-domain structure with their magnetic moment out of plane. Depending on the magnetic history, the magnetic moment of all dots can be aligned in positive or negative direction, or a random distribution of positive and negative magnetic moments of the dots can be achieved. For a Pb film covering this Co/Pt dot array, we observe an asymmetric magnetization loop due to the magnetic interactions between the vortices and the magnetic dots.

We have performed magnetic force microscopy measurements on isolated 35 nm thick rectangular Co structures. The structures have a length L ranging between 0.25 and 10 μm and a width W ranging between 0.25 and 5.5 μm, covering aspect ratios m=L/W between 1 and 40. This enables us to map the transition from a magnetic single-domain state towards a magnetic multidomain state when increasing the size of the structures. This transition depends on the size as well as the aspect ratio of the structures. Our results can be interpreted in terms of the theoretical model developed by A. Aharoni [J. Appl. Phys. 63, 5879 (1988)].

A Cr(001)/Fe(001) superlattice with ten bilayers grown by molecular beam epitaxy on a Ag(001) substrate is studied by in situ scanning tunneling microscopy (STM) and ex situ x-ray diffraction (XRD). Layer-resolved roughness parameters determined from STM images taken in various stages of the superlattice fabrication are compared with average values reported in the literature or obtained from the fits of our XRD data. Good agreement is found for the rms roughnesses describing vertical roughness and for the lateral correlation lengths characterizing correlated as well as uncorrelated interface roughness if peculiarities of STM and XRD are taken into account. We discuss in detail (i) the possible differences between the STM topography of a free surface and the morphology of a subsequently formed interface, (ii) contributions due to chemical intermixing at the interfaces, (iii) the comparison of XRD parameters averaged over all interfaces versus layer-resolved STM parameters, and (iv) the question of the coherent field of view for the determination of rms values. (C) 2001 American Institute of Physics.

The pinning of vortices is studied in a superconducting Pb film with a regular array of submicron dots (Co dots with in-plane magnetization and Co/Pt dots with out-of-plane magnetization) or antidots. Matching anomalies are observed in the critical current and magnetization behavior at integer and rational multiples of the first matching field, corresponding to stable vortex configurations. The pinning force of Co dots with a single-domain state is enhanced compared to multi-domain dots. For Co/Pt dots with out-of-plane magnetisation a clear asymmetry is present in the magnetisation curves due to the direct interaction of the vortices with the magnetic moment of the dots. In superconducting films with a square array of antidots, Shapiro voltage steps are observed in the voltage-current characteristics in the presence of rf-radiation. These steps clearly reveal the presence of mobile interstitial vortices coexisting with immobile vortices strongly pinned at the antidots.