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The influence of a patterned substrate on obliquely deposited, exchange biased Co/CoO films was studied. It was found that substrates decorated with nanoparticle patterns provide the option to manipulate the orientation of the magnetic easy axis in obliquely deposited thin films. The complementary methods of SQUID magnetometry and polarized neutron reflectometry were used to disentangle the different contributions to the magnetic hysteresis of such complex magnetic systems.

The solid solubility of the isomorphous monosilicides during the silicide reaction of Ni-Pt, Ni-Pd and Pt-Pd alloys on Si(100) is comparatively studied in the full composition range. Our study reveals that PtSi and PdSi, exhibiting a minor lattice mismatch, directly form a solid solution. In contrast, for larger differences in lattice parameters such as is the case for NiSi-PtSi and NiSi-PdSi, the mutually soluble phases coexist, prior to the formation of a solid solution at increased temperatures. Hence, it appears that the direct formation of a ternary monosilicide solid solution is inherently related to the lattice mismatch of the binary monosilicides. This finding provides an explanation for important differences observed in the elemental redistribution between the three systems, considered to be very similar up to now. Moreover, the different formation of a solid solution results in a fundamentally different nucleation of PdSi: while Ni lowers the nucleation barrier by reducing the contribution of the interface energy, Pt strongly increases the entropy of mixing which triggers the formation of the Pd-monosilicide at surprisingly low temperatures. Our results show that the lattice mismatch is a crucial parameter and determines the phase formation sequence and elemental redistribution during the silicide reaction.

Herein, the magnetoelectric (ME) performance of epitaxial multilayer composite films built from nanometer‐thick layers of multiferroic BiFeO3 and ferroelectric BaTiO3 is reviewed. A successful implementation of shadow‐mask pulsed laser deposition considerably reduces the interface and surface roughness of the multilayers. In dependence of double‐layer thickness and the degree of structural perfection, the multilayers show high ME voltage coefficients up to 480 V cm−1 Oe−1 at 300 K and 0 T bias magnetic field. With decreasing double‐layer thickness, an interface‐driven effect critically enhances the ME coupling in this strain and charge-comediated system. Interestingly, the characteristics of temperature and DC magnetic field dependencies of magnetoelectric voltage coefficients change with the transition from the 2D to 3D character of the single layers, i.e., for BiFeO3 layers thicker than 5 nm within the multilayers. These changes are attributed to variations of the contributing ME coupling mechanisms. Furthermore, scanning transmission electron microscopy (STEM) with energy‐dispersive X‐ray (EDX) spectroscopy mapping‐based nanoanalysis indicates that chemical effects at the interfaces play an important role for the ME performance of the BiFeO3-BaTiO3 multilayer thin films.

We used lithographically fabricated nanoconstrictions made of Ag–Co granular thin films to achieve current densities above 10 A/cm during magnetotransport measurements. Systematically measured variations of the resistance with current under high applied magnetic fields did not reveal any dramatically large magnetoresistance effect, in contrast to the previous observations using mechanical point contact based techniques. Our experimental results on resistance switching by current injection in the Ag–Co nanoconstrictions reveal spin torque effects. The switching amplitudes are considerably smaller when compared to the results obtained for pillar shaped trilayers due to the large number of Co grains in the nanoconstrictions. We demonstrate that the use of point contact patterns with nanometer dimensions, where effects resulting from current induced mechanical distortions at high magnetic fields are negligible, has obvious advantages over the mechanical point contact technique for studying spin injection effects.

We report on the growth of thin NiSi films via the thermal reaction of Ni layers (13–35 nm) with Si(100) substrates modified by ion implantation. By introducing substrate damage or nitrogen impurities prior to the solid-phase reaction, several properties of the NiSi films can be modified: the formation temperature, texture, diffusion-limited growth rate and morphological stability. As some of the modifications to the NiSi films are rooted in the early silicide phases preceding the NiSi phase, particularly its formation temperature, special attention is devoted to the growth of the amorphous Ni-Si alloy and the crystalline δ-Ni2Si and θ-Ni2Si phases. We employed a number of experimental techniques, including in situ synchrotron x-ray diffraction (XRD), in situ Rutherford backscattering spectrometry (RBS), in situ sheet resistance measurements, ex situ ion beam channelling and ex situ pole figure measurements. We show that both the formation temperature of the NiSi films and the intensity of epitaxial and axiotaxial components of the NiSi texture can be either lowered or raised by selecting appropriate implantation conditions. Agglomeration of the NiSi films at high temperature (> 700 °C) can be slowed down, either by slowing down the mobility of the Ni and Si atoms, or by removing the morphologically destabilizing axiotaxial texture. Our results emphasize the strong interwoven nature of phase formation, texture and morphological degradation. We illustrate that the kinetics of the early stages of thin film reactions consist of more than just diffusion, i.e. nucleation can also play a crucial role.

© 2019 IOP Publishing Ltd. Solid-state amorphization, the growth of an amorphous phase during annealing, has been studied in a wide variety of thin film structures. Whereas research on the remarkable growth of such a metastable phase has mostly focused on strictly binary systems, far less is known about the influence of impurities on such reactions. In this paper, the influence of nitrogen, introduced via ion implantation, is studied on the solid-state amorphization reaction of thin (35 nm) Ni films with Si, using in situ x-ray diffraction (XRD), ex situ Rutherford backscattering spectrometry, XTEM, and synchrotron XRD. It is shown that due to small amounts of nitrogen (

Indium tin oxide (ITO) is broadly used as a transparent conducting material for electrodes in optoelectronic devices. Irradiation of ITO with low energy ions can result in the formation of periodic surface nanopatterns which can serve as an alternative for the polymer alignment layer in liquid crystal devices. We investigated the formation of the ion-induced surface nanopatterns on ITO with focus on the influence of the crystalline structure of the material. We find that the crystallinity plays a crucial role in the pattern formation, with no pattern developing on an amorphous ITO surface. We discuss these findings in the context of the state-of-the-art theory for ion-induced patterning. We show that the ion-induced pattern plays a critical role in the liquid crystal alignment on ITO surfaces.

Composite multiferroics are a new class of material where magneto-electric coupling is achieved by creating an interface between a ferromagnetic and a ferroelectric compound. The challenge of understanding the chemical and magnetic properties of such interface is a key to achieve good magneto-electric coupling. The unique possibilities offered by isotope sensitive techniques are used to selectively investigate the interface's chemistry and magnetism in Fe/BaTiO3 and Fe/LiNbO3 systems during the application of an electric field. With a large enough electric field, a strong oxidation of Fe is triggered, which creates a magnetically dead interface. This leads to an irreversible decrease of the magneto-electric coupling properties. Material parameters are identified that determine under which electric field the interface may be modified. The results are confirmed on the two systems and are expected to be widespread in this new class of hybrid material.

We present a fundamental study of the monolayer adsorption of sulfur on Ge(100) surfaces from aqueous (NH4)2S solution. This treatment shows promising perspectives for the passivation of high-mobility semiconductor surfaces and is therefore presently of great technological importance. The adsorption mechanisms as well as the adsorption geometry are thoroughly investigated at the atomic scale, by both experiment and theory, applying X-ray absorption spectroscopy and molecular dynamics simulations. Our findings indicate that sulfidation in solution results in the formation of Ge–S–Ge bridges along the [110] direction, with no indication for −SH surface groups. A S–Ge bond length of 2.25 ± 0.05 A was deduced, which is affected by the chemical environment of the sulfur atoms, i.e., by residual surface oxides. Our study provides novel insights into the surface termination and atomic structure of (NH4)2S-treated Ge(100) surfaces and discusses possible differences from in situ sulfur adsorption methods such as H...

In this contribution, we discuss the crystalline properties of strained and strain-relaxed CVD-grown GeSn layers with Sn content in the range 6.4-12.6 at.%. A positive deviation from Vegard's law was observed and a new experimental bowing parameter was extracted for GeSn: bGeSn = 0.041 Å (in excellent agreement with recent theoretical predictions). The GeSn critical thickness for strain relaxation as a function of Sn concentration was determined, resulting in significantly higher values than those predicted by equilibrium models. A composition-dependent strain relaxation mechanism was also found, with the formation of an increasing density of GeSn pyramidal islands in addition to misfit dislocations at lower Sn concentration. © 2013 The Electrochemical Society. ispartof: ECS Journal of Solid State Science and Technology vol:2 issue:4 pages:P134-P137 status: published

Germanium is considered to be a potential semiconductor to replace silicon in future high-performance microelectronic devices. Yet, when compared to Si, very little is known about the surface chemistry of Ge surfaces. In this article, we report on the oxygen adsorption on Ge(100)-(2 × 1) surfaces and the related changes in the density of surface states. In particular, the adsorption geometry is examined both in reciprocal- and real-space using reflection high-energy electron diffraction and scanning tunneling microscopy, respectively. Our findings reveal that the insertion of oxygen atoms into the Ge backbonds is not favored under the investigated reaction conditions, i.e., at a moderate O2 exposure. The O2-exposed surface exhibits a local (1 × 1) surface reconstruction, which implies that dimer bonds are destroyed upon oxygen adsorption. Surface states related to Ge dangling surface bonds present on the clean Ge(100) surface are found to be passivated. In addition, a high density of expelled substrate at...

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-...

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.

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.

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.

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.

© 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

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 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.

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 (

© 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.

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.

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.

We report on the characterization by X-ray scattering and atomic force microscopy of laterally nanostructured Pb/Ge multilayers in which a diffraction grating is introduced by a periodic square lattice of submicron holes (‘antidot lattice’). Experimental low- and high-angle specular X-ray scattering data reveal the layered structure and interface roughness and using model calculations, a slightly increased structural disorder within the Pb layers is observed, while still a good multilayer periodicity is maintained. Atomic force microscopy and diffuse X-ray reflectivity provide information about the lateral grating periodicity. It is shown that by combining atomic force microscopy and X-ray scattering, detailed information about the internal structure and the geometrical configuration of these metallic mesoscopic systems can be obtained.

High resolution scanning Hall probe microscopy has been used to study flux line pinning in a thin superconducting Pb film evaporated on top of an ordered array of ferromagnetic Co dots. After magnetisation along the easy axis, all the nanomagnets are seen to be in single domain magnetic states with dipole stray field distributions ( T > T c ). Upon cooling through the critical temperature of the superconducting film at H =0, we observe strong screening of the dipole fields which we attribute to fluxoid quantisation. After field cooling, we find that flux lines are selectively pinned at poles with the opposite sign of field and ordered structures which are commensurate with the underlying pinning array are observed. Each pinning site appears to be capable of trapping at least two flux lines, and no evidence has yet been seen for interstitial vortices at low temperatures. Results will be compared with those from the disordered pinning system found naturally in Cu-rich epitaxial YBCO thin films. These typically contain micron-sized normal Cu-rich precipitates, which have a similar density to other artificially patterned systems. We show that the precipitates act as strong pinning sites close to T c , and also give rise to weak magnetisation peaks at average matching fields.

We report on the growth and structure of Ag(001)/Fe(001) superlattices studied in situ by reflection high-energy electron diffraction (RHEED) and ex situ by Rutherford backscattering and channeling spectroscopy (RBS/C), X-ray diffraction (XRD) and atomic force microscopy (AFM). These complementary characterization methods have been compared and applied to a detailed investigation of the epitaxial quality and the interface roughness. The apparent inconsistency in the results is explained by the difference in length scale probed by the four characterization techniques.

We have imaged vortices in the conventional superconductors NbSe2 (crystal) and Nb (thin film) with a low temperature magnetic force microscope (MFM). The MFM detection is based on commercially available piezoresistive cantilevers. A considerably improved sensitivity (0.2 pN/nm) at 4.3 K has been obtained by using a higher flexural mode of the cantilevers. The operation at higher (2nd or 3rd) mechanical resonances improves the signal-to-noise ratio by a factor of 5. The improved sensitivity allows us to reduce the heat dissipation down to 0.05 mW in the cantilevers without lowering their performance, which is highly desirable for MFM applications at liquid helium temperatures. The magnetic tip coating was optimized by relying on Co/Au multilayers grown by oblique incidence molecular beam epitaxy. A magnetic field of 0.5–3 mT was used to induce the vortices under field-cooled conditions. In thin Nb films, we observed an irregular vortex arrangement and the imaged vortices are attached to individual pinning centers. On the cleaved surface of NbSe2 crystals, we observed an evolution from a disordered towards an ordered state of the vortex lattice. The possibility to image the Abrikosov vortex lattice in NbSe2 can be understood in terms of collective pinning effects.

The pinning of flux lines by two different types of regular arrays of submicron magnetic dots is studied in superconducting Pb films; rectangular Co dots with in-plane magnetization are used as pinning centers to investigate the influence of the magnetic stray field of the dots on the pinning phenomena, whereas multilayered Co/Pt dots with out-of-plane magnetization are used to study the magnetic interaction between the flux lines and the magnetic moment of the dots. For both types of pinning arrays, matching anomalies are observed in the magnetization curves versus perpendicular applied field at integer and rational multiples of the first matching field, which correspond to stable flux configurations in the artificially created pinning potential. By varying the magnetic domain structure of the Co dots with in-plane magnetization, a clear influence of the stray field of the dots on the pinning efficiency is found. For the Co/Pt dots with out-of-plane magnetization, a pronounced field asymmetry is observed in the magnetization curves when the dots are magnetized in a perpendicular field prior to the measurement. This asymmetry can be attributed to the interaction of the out-of-plane magnetic moment of the Co/Pt dots with the local field of the flux lines and indicates that flux pinning is stronger when the magnetic moment of the dot and the field of the flux line have the same polarity., Comment: 7 pages including figures; submitted for publication in Physica C (Proceedings ESF-Vortex Conference, 18-24 Sept. 1999, Crete, Greece)

Free charged and neutral Ag n clusters were produced by a laser vaporization cluster source and deposited onto SiO 2 substrates resulting in cluster films. These films were characterized by atomic force microscopy, X-ray diffraction and resistivity measurements to determine film topography, grain sizes, dimensions of the coherent scattering regions perpendicular to the film plane, and the mean free path length of the conduction electrons. An Ag film grown by molecular beam deposition was investigated using the same characterization techniques. The results show that the deposited clusters keep at least part of their shape upon impact on the substrate, giving rise to growth patterns and film properties distinctly different from those of conventionally grown films.

International audience; We have investigated the structural and optical properties of metastable amorphous and crystalline GeSn layers on Si substrates. The as-deposited amorphous layers crystallize during annealing at 500 degrees C. This transition leads to a significant change in the local environment of the Sn atoms and in the optical properties. The Ge-Sn bond length is decreased after crystallization. The as-deposited GeSn layers, with nominal 4.5% and 11.3% Sn content, do not show Sn segregation. For the crystallized GeSn with nominal 4.5%, the Sn appears to be substitutional, as no Sn clustering was observed. However, for the crystallized GeSn with nominal 11.3% Sn, Sn segration and the presence of beta-Sn is observed by EXAFS. A method to suppress Sn segregation and increase the substitutional Sn concentration is discussed. Furthermore, we determined the optical properties of amorphous and crystalline GeSn with nominal 4.5% Sn. The bandgap energy decreases significantly from 0.89 eV (1392 nm) +/- 0.05 eV for the amorphous layer to 0.52 eV (2383 nm) +/- 0.05 eV for crystalline GeSn, leading to significant reduction in penetration depth. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. All rights reserved

The possibility of tracking the average rotation sense of the magnetization vector of a ferromagnetic layer upon magnetization reversal by means of magnetotransport measurements is explored. It is demonstrated that the rotation sense of the ferromagnetic magnetization vector during a hysteresis loop can be determined for the polycrystalline Co/CoO exchange bias system by measuring the anisotropic magnetoresistance (AMR) with a specific choice of the measurement geometry. The AMR measurements reveal that the rotation direction of the magnetization vector can be reversed by performing an in-plane hysteresis loop with a magnetic field perpendicular to the cooling field. This reversal can be directly linked to the experimental fact that after training, i.e., after performing hysteresis loops with a field along the cooling field direction, the average orientation of the uncompensated magnetization of the granular CoO antiferromagnet can be largely rotated back to the initial orientation after field cooling by applying a perpendicular field with the appropriate amplitude and orientation.

The physical properties of superlattices are strongly affected by the chemical and physical properties of the individual layers and by the superlattice structure. In this paper the relationship between structure and properties will be illustrated by the giant magnetoresistance (GMR) effect, as well as the dimensional transitions and pinning mechanisms in respectively magnetic and superconducting superlattices. In the first example we will analyse the effect of the sample structure and interface quality on the GMR of polycrystalline and epitaxial Fe/Cr superlattices grown by molecular beam epitaxy. Secondly, the effect of thermal annealing and ion irradiation on the electrical and magnetic properties of Ag/Fe multilayers is discussed. In a third example, the influence of a very thin Fe interlayer on the coupling phenomena in Fe/Nb multilayers is analysed. Finally, it will be shown that an additional lateral modulation (lattice of submicron holes) in Pb/Ge multilayers substantially changes the critical superconducting parameters.

We report on an extensive and detailed study of the silicide reaction of Ni-W alloys on Si(100). The solid phase reaction when studied over the full composition range reveals the substantial impact of composition and microstructure on the silicide reaction properties, such as the phase formation sequence and formation temperatures. It was found that the microstructure of the as-deposited film depends crucially on the alloy composition, being polycrystalline below 45 at.% W and amorphous above 45 at.% W. The microstructure affects the elemental mobility substantially, resulting in a drastic increase in the silicide reaction temperature in the case of an amorphous thin film. To further investigate the effect of elemental mobility, Si was premixed in the as-deposited alloy, thereby excluding the need for long-range diffusion. As a result, the silicide reaction temperatures were lowered. However, what was more striking was the observation of a bilayer structure for epitaxial NiSi2 in contact with the Si substrate and a W-rich layer residing at the outermost layer at a temperature of only 300 degrees C. The results stress the importance of the composition and crystalline nature of the as-deposited film, with these being decisive for the reaction sequence.

This paper reports a study of cubic magnetic anisotropies of bcc-Fe and bcc-Co structures in Fe/Mn/Co trilayers grown by molecular beam epitaxy onto MgO(001) substrates. Parallel ferromagnetic resonance experiments, at microwave frequency of 33.9 GHz, reveal the presence of a large fourfold magnetic anisotropy in all studied films. Two uniform resonance modes, with fourfold magnetic anisotropy in both bcc-Fe and Co layers, are excited by the microwave field in samples with Mn thicknesses of 0.83 and 0.99 nm. Films with thick Mn layer (1.16-2.20 nm) do not exhibit independent Fe and Co uniform modes, a result associated with an enhancement of the roughness at the Mn/Co interface (Fe/Mn interface roughness was estimated by 57 Fe Mössbauer spectroscopy to be nearly constant for all films) at the Fe/Mn and Mn/Co interfaces. Magnetic anisotropy constants of bcc-Fe and bcc-Co layers of sample with 0.99 nm thick Mn layer were determined, respectively, as 4.5 × 10 5 and 6.4 10 5 erg/cm 3 . © 1965-2012 IEEE. ispartof: IEEE Transactions on Magnetics vol:49 issue:8 pages:4525-4529 ispartof: location:Buenos Aires: ARGENTINA status: published

We investigate the link between superconductivity and atomic vibrations in Nb3Sn films with a thickness ranging from 10 to 50 nm. The challenge of measuring the phonon density of states (PDOS) of these films has been tackled by employing the technique of nuclear inelastic scattering by 119Sn isotopes to reveal the Sn-partial phonon density of states. With the support of ab initio calculations, we evaluate the effect of reduced film thickness on the PDOS. This approach allows us to estimate the changes in superconducting critical temperature Tc induced by phonon confinement, which turned out to be limited to a few tenths of K. The presented method is successful for the Nb3Sn system and paves the way for more systematic studies of the role of phonon confinement in Sn-containing superconductors. ispartof: Physical Review B, Condensed Matter and Materials Physics vol:88 issue:4 status: published

The Pippard coherence length xi(0) (the size of a Cooper pair) in two extreme type-I superconductors (In and Sn) was determined directly through high-resolution measurement of the nonlocal electrodynamic effect combining low-energy muon spin rotation spectroscopy and polarized neutron reflectometry. The renormalization factor Z = m*(cp)/2m (m*(cp) and m are the mass of the Cooper pair and the electron, respectively) resulting from the electron-phonon interaction, and the temperature-dependent London penetration depth lambda(L)(T) were determined as well. An expression linking xi(0), Z, and lambda(L)(0) is introduced and experimentally verified. This expression allows one to determine experimentally the Pippard coherence length in any superconductor, independent of whether the superconductor is local or nonlocal, conventional or unconventional. DOI: 10.1103/PhysRevB.87.104508

Mesoporous In2O3-y materials have been implanted using Co ions to induce a moderate ferromagnetic response at room temperature, forming a >mesoporous oxide-diluted magnetic semiconductor> (MODMS). X-ray photoemission spectroscopy (XPS) reveals that implantation results in up to 1 at. % Co (for 6 × 1015 ions/cm2 at 40 keV) and 15 at. % Co (for 1 × 1017 ions/cm2 at 60 keV). This is in both cases accompanied by a pronounced increase in the amount of oxygen vacancies with respect to the pristine, nonimplanted, In2O 3-y. Further increase in the ion fluence (up to 2 × 10 17 ions/cm2 at 60 keV) results in the collapse of the mesoporous structure, i.e., loss of the 3D-ordered porous configuration. XPS also reveals that virtually no metallic Co is formed at 40 keV, while a mixture of Co2+ and Co0 states is detected after implantation at 60 keV. Most of the Co2+ is incorporated in the bixbyite structure of the In2O3-y matrix. These results are consistent with previous models suggesting that the origin of the obtained ferromagnetic response in oxide-diluted magnetic semiconductors can be ascribed to ferromagnetic exchange interactions mediated by oxygen vacancies. This work constitutes the first report on MODMS prepared by nanocasting followed by implantation of transition metal ions. © 2013 American Chemical Society., The authors acknowledge financial support from the MAT2011-27380-C02-01 and MAT2010-20616-C02 projects from the Spanish Ministry of Economy and Competitiveness, the 2009-SGR-1292 project from the Generalitat de Catalunya, the Fund for Scientific Research - Flanders (FWO), and the KU Leuven Concerted Action (GOA/09/006) programs. E.M. thanks the Fund for Scientific Research - Flanders (FWO) for financial support.

The unidirectional shift of the hysteresis loop and the athermal training effect are two key features of the exchange bias phenomenon in most polycrystalline FM/AFM bilayers. We show that, by using MuMax3 which is an open source micromagnetic simulation program, we are able to reproduce experimental data (bias field and coercivity) for a polycrystalline Co/CoO bilayer. We also demonstrate that rotatable uncompensated spins can be responsible for the athermal training effect and that the reversal mechanism between the ascending and descending branch in the first hysteresis loop can be different, as has frequently been noticed in experiments.

Critical current densities J c and normalized flux creep rate S (≡ dln M /dln t ) have been measured as a function of the perpendicular field H ⊥ in low- T c Pb/Ge multilayers, which can be used to model the superconducting properties of the layered high- T c cuprates. By taking a proper Ge separator thickness a field-induced crossover between two different pinning regimes can be observed. This transition leads to an anomalous behavior of J c ( H ⊥ ) and S ( H ⊥ ) in Pb/Ge multilayers, while a sing le Pb film demonstrates a monotonic suppression of J c by H ⊥ . These data have been analyzed in the framework of the collective pinning (CP) theory of magnetically coupled superconducting planes: the J c ( H ⊥ ) anomaly is interpreted as a crossover from three-dimensional (3D) flux lines in low fields to two-dimensional (2D) pancake vortices in higher fields. This interpretation has also been confirmed by the suppression of the J c ( H ⊥ ) anomaly in the Pb/Ge multilayers with a square lattice of holes, forming additional defects for the confinement of the flux lines and for the expansion of the 3D regime.

Amorphous W/Si multilayers, with single layer thickness varying between 10 and 100 A, have been prepared by UHV electron beam evaporation. The superconducting samples ( d w T c d w >30 A) have a positive coefficient. With decreasing temperature, the parallel critical field H c2∥ ( T ) shows a dimensional crossover which depends on the ratio between the total multilayer thickness and the in-plane superconducting coherence length. The amorphous character of the layers enables one to prepare continuous and smooth W layers with thickness down to d w =10 A . These ultrathin layers give rise to very high values of the parallel critical fields.

The layer magnetisation reorientation transition (spin-flop, SF) was studied in an artificial layer antiferromagnet (AF), namely in MgO(0 0 1)/[ 57 Fe(2.6 nm)/Cr(1.3 nm)] 20 epitaxial superlattice (SL) by synchrotron Mossbauer reflectometry and Kerr effect (SMOKE). The SF occurs simultaneously in the entire SL stack (bulk SF) in an increasing field of H SF =13 mT along the easy direction parallel to the layer magnetisations. It is recognised by the kink in the SMOKE loop and by the sharp up-rise of the AF Bragg peak in the delayed Mossbauer reflectivity. The moderate value of observed H SF is compared with estimations from a spin-chain model and interpreted as due to intraplane domain-wall motion during SF.

Fe films, 300 A thick, were deposited by molecular beam epitaxy on amorphous SiO2 substrates previously coated with an Ag buffer layer of varying thickness (0–50–100–150 A). The surface roughness of the Ag buffer layer grows with increasing thickness. Magnetization measurements revealed that both the magnetization reversal process and the coercivity are profoundly influenced by the surface roughness.

We report on measurements of the magnetoresistance as a function of the angle θ between magnetic field H and the basal plane in YBa2Cu3O7/PrBa2Cu3O7 superlattices with two different thicknesses of the separator layer (YBCO:PrBCO=1:1 and 1:3). The angular dependence of the magnetoresistance for different field intensities can be scaled into a single curve by introducing the reduced field, H(sin 2 θ+γ -2 cos 2 θ) 1 2 , with the anisotropy paramete γ. In the YBCO/PrBCO=1:3 superlattice, quite good scaling is achieved with only the perpendicular field component, which implies fully decoupled YBCO layers (γ=∞). In the YBCO/PrBCO=1:1 superlattice, however, a finite anisotropy (γ≈50) is necessary to obtain a convincing scaling. Measurements of the magnetoresistance for different directions between magnetic field and transport current in these superlattices have shown that the resistive transition broadening in the field is independent of the macroscopic Lorentz force. An interpretation of these data based on the two-dimensional fluctuation model is also presented.