Isolating the step contribution to the uniaxial magnetic anisotropy in nanostructured Fe/Ag(001) films

Ion Beam Nanopatterning of Biomaterial Surfaces

Ion beam irradiation of solid surfaces may result in the self-organized formation of well-defined topographic nanopatterns. Depending on the irradiation conditions and the material properties, isotropic or anisotropic patterns of differently shaped features may be obtained. Most intriguingly, the periodicities of these patterns can be adjusted in the range between less than twenty and several hundred nanometers, which covers the dimensions of many cellular and extracellular features. However, even though ion beam nanopatterning has been studied for several decades and is nowadays widely employed in the fabrication of functional surfaces, it has found its way into the biomaterials field only recently. This review provides a brief overview of the basics of ion beam nanopatterning, emphasizes aspects of particular relevance for biomaterials applications, and summarizes a number of recent studies that investigated the effects of such nanopatterned surfaces on the adsorption of biomolecules and the response of adhering cells. Finally, promising future directions and potential translational challenges are identified.

Tailoring of uniaxial magnetic anisotropy in Permalloy thin films using nanorippled Si substrates

In this work the investigation of in-plane uniaxial magnetic anisotropy induced by the morphology due to ion beam erosion of Si(1 0 0) has been done. Ion beam erosion at an oblique angle of incidence generates a well-ordered nanoripple structure on the Si surface and ripple propagates in a direction normal to ion beam erosion. Permalloy thin films grown on such periodic nanopatterns show a strong uniaxial magnetic anisotropy with easy axis of magnetization in a direction normal to the ripple wave vector. The strength of uniaxial magnetic anisotropy is found to be high for the low value of ripple wavelength; it is decreasing with increasing value of ripple wavelength. Similarly, the strength of uniaxial magnetic anisotropy decreases with increasing Permalloy film thickness. Grazing incidence small angle x-ray scattering data reveals an anisotropic growth of Permalloy thin films with preferential orientation of grains in the direction normal to the ripple wave vector. Permalloy thin film growth is highly conformal with the film surface replicating the substrate ripple morphology up to a film thickness of 50 nm has been observed. Correlation between observed uniaxial magnetic anisotropy to surface modification has been addressed.

Direct observation of temperature-driven magnetic symmetry transitions by vectorial resolved MOKE magnetometry

Angle- and temperature-dependent vectorial magnetometry measurements are necessary to disentangle the effective magnetic symmetry in magnetic nanostructures. Here we present a detailed study on an Fe(1 0 0) thin film system with competing collinear biaxial (four-fold symmetry) and uniaxial (two-fold) magnetic anisotropies, carried out with our recently developed full angular/broad temperature range/vectorial-resolved magneto-optical Kerr effect magnetometer, named TRISTAN. The data give direct views on the angular and temperature dependence of the magnetization reversal pathways, from which characteristic axes, remanences, critical fields, domain wall types, and effective magnetic symmetry are obtained. In particular, although the remanence shows four-fold angular symmetry for all investigated temperatures (15 K-400 K), the critical fields show strong temperature and angular dependencies and the reversal mechanism changes for specific angles at a given (angle-dependent) critical temperature, showing signatures of an additional collinear two-fold symmetry. This symmetry-breaking is more relevant as temperature increases to room temperature. It originates from the competition between two anisotropy contributions with different symmetry and temperature evolution. The results highlight the importance of combining temperature and angular studies, and the need to look at different magnetic parameters to unravel the underlying magnetic symmetries and temperature evolutions of the symmetry-breaking effects in magnetic nanostructures.

Note: Vectorial-magneto optical Kerr effect technique combined with variable temperature and full angular range all in a single setup

Here, we report on a versatile full angular resolved/broad temperature range/vectorial magneto optical Kerr effect (MOKE) magnetometer, named TRISTAN. Its versatility relies on its capacity to probe temperature and angular dependencies of magnetization reversal processes without the need to do any intervention on the apparatus during measurements. The setup is a combination of a vectorial MOKE bench and a cryostat with optical access. The cryostat has a motorized rotatable sample holder with azimuthal correction. It allows for simultaneous and quantitative acquisition of the two in-plane magnetization components during the hysteresis loop at different temperatures from 4 K up to 500 K and in the whole angular range, without neither changing magnet orientation nor opening the cryostat. Measurements performed in a model system with competing collinear biaxial and uniaxial contributions are presented to illustrate its capabilities.

Ion-Induced Nanoscale Ripple Patterns on Si Surfaces: Theory and Experiment

Nanopatterning of solid surfaces by low-energy ion bombardment has received considerable interest in recent years. This interest was partially motivated by promising applications of nanopatterned substrates in the production of functional surfaces. Especially nanoscale ripple patterns on Si surfaces have attracted attention both from a fundamental and an application related point of view. This paper summarizes the theoretical basics of ion-induced pattern formation and compares the predictions of various continuum models to experimental observations with special emphasis on the morphology development of Si surfaces during sub-keV ion sputtering.

The morphology of amorphous SiO(2) surfaces during low energy ion sputtering

The morphology of different amorphous or amorphized SiO(2) surfaces, including thermally grown films, fused silica, and single crystalline quartz, during low energy ion sputtering has been investigated by means of atomic force microscopy. For all three materials, the formation of periodic ripple patterns oriented normal to the direction of the ion beam is observed at intermediate incident angles. At near-normal incidence, the SiO(2) surfaces remain flat, whereas a rotation of the ripple patterns is observed at grazing incidence. At intermediate angles, the patterns on the different surfaces exhibit wavelength coarsening of different strengths, which can be attributed to different amounts of near-surface mass transport by the surface-confined ion-enhanced viscous flow. In the framework of the recent hydrodynamic model of ion erosion, the observed differences in ripple coarsening are consistent with this interpretation and indicate that the surface energies of thermally grown SiO(2) and amorphized quartz are lower and higher than that of fused silica, respectively.

Applications of metal surfaces nanostructured by ion beam sputtering

We review results relative to the formation of regular nanoscale patterns on metal substrates exposed to defocused ion beam irradiation. Particular emphasis is placed on work which demonstrates the possibility of controllably modifying chemico-physical properties of the material by tailoring the nanoscale morphology during IBS patterning. Starting from the well-established results found on single-crystal model systems, we show how the controlled modification of the atomic step termination can deeply affect chemical reactivity or magnetic anisotropy. We then look in greater detail at the more recent attempts focused on the extension of IBS patterning on supported polycrystalline metal films, a promising class of systems in view of potential applications. A modification of the functional properties of metal films can also be obtained by forcing a shape anisotropy of the nanostructures. The modification of the optical response of polycrystalline metal nanowires supported on anisotropic templates produced by IBS provides a clear example of this.

Ion beam sputtering induced nanostructuring of polycrystalline metal films

The development of fine scale nanostructures in polycrystalline metal films by off-normal ion beam sputtering (IBS) follows similar mechanisms to those in random targets, i.e. the pattern results from the interplay of curvature-dependent-roughening and various smoothing processes. By grazing angle IBS of the deposited metal films it is possible to fabricate metallic nanoripples, nanowires, and nanorods onto semiconductor or insulator substrates without using a template. It has been found that the rms roughness of the as-deposited film is substantially reduced under ion bombardment before the development of nanoscale patterns. The structural anisotropy of the sputtered morphology can have a great influence on the local physical properties of the underlying material. In this paper, we shall review the experimental results on the formation and characteristics of the IBS ripples on polycrystalline metal films prepared by the physical vapor deposition (PVD) technique.