Estructuras híbridas de silicio poroso y metal/óxido de metal: síntesis, caracterización y aplicaciones en biomedicina

Tesis doctoral inédita. Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 17-06-201

Influence of the sputtering flow regime on the structural properties and magnetic behavior of Fe-Ga thin films (Ga ∼ 30 at.%)

In this paper we analyze the structure of Fe-Ga layers with a Ga content of ∼30 at.% deposited by the sputtering technique under two different regimes. We also studied the correlation between the structure and magnetic behavior of the samples. Keeping the Ar pressure fixed, we modified the flow regime from ballistic to diffusive by increasing the distance between the target and the substrate. X-ray diffraction measurements have shown a lower structural quality when growing in the diffusive flow. We investigated the impact of the growth regime by means of x-ray absorption fine structure (XAFS) measurements and obtained signs of its influence on the local atomic order. Full multiple scattering and finite difference calculations based on XAFS measurements point to a more relevant presence of a disordered A2 phase and of orthorhombic Ga clusters on the Fe-Ga alloy deposited under a diffusive regime; however, in the ballistic sample, a higher presence of D0_3/B2 phases is evidenced. Structural characteristics, from local to long range, seem to determine the magnetic behavior of the layers. Whereas a clear in-plane magnetic anisotropy is observed in the film deposited under ballistic flow, the diffusive sample is magnetically isotropic. Therefore, our experimental results provide evidence of a correlation between flow regime and structural properties and its impact on the magnetic behavior of a rather unexplored compositional region of Fe-Ga compounds

Local and medium range order influence on the magnetic behavior of sputtered ga-rich fega thin films

We have investigated the influence of the growth power on the structural properties of Fe_100_(-x)Ga_x (x ca. 29) films sputtered in the ballistic regime in the oblique incidence. By means of different structural characterizations, mainly X-ray diffraction and X-ray absorption spectroscopy, we have reached a deeper understanding about the influence of the local and medium range order on the magnetic behavior of Ga-rich FeGa thin films. On the one hand, the increase of the growth power reduces the crystallite size (medium order) that promotes the decrease of the coercive field of the layers. On the other hand, the growth power also determines the local order as it controls the formation of the A2, B2, and D0_3 structural phases. The increase of the uniaxial in-plane magnetic anisotropy with growth power has been correlated with the enhancement of both Ga pairs and a tetragonal distortion. The results presented in this work give more evidence about the magnetic anisotropy sources in Ga-rich FeGa alloys, and therefore, it helps to understand how to achieve a better control of the magnetic properties in this family of alloys

The role of surface to bulk ratio on the development of magnetic anisotropy in high Ga content Fe100-xGax thin films

In this work we show the development of bulk in-plane magnetic anisotropy in high Ga-content (Ga = 28 at. %) Fe_(100-x)Ga_x thin films as the layer thickness increases. This result is in clear contrast with the generally reported decrease of this anisotropy with the film thickness. We propose the interrelation between the enhancement of the Ga-pair correlations and a collinear distortion of the bcc structure within the sample plane as the origin of the magnetic anisotropy. Our results have been obtained by employing a combination of long and local range structural probe techniques with bulk and surface magnetic characterization techniques. The key point shown in this work is that the in-plane structural anisotropy and hence, the magnetic anisotropy, are developed as the layer thickness increases. This fact strongly suggests that the surface to bulk free energy ratio plays a key role in the formation of ordered phases with a distorted bcc cell in Fe_(100-x)Ga_x films with x around 28 at. %. Our work also shows the arising of new phenomena in these high Ga content alloys due to the close correlation between structural and magnetic properties

GaSb/Mn multilayers structures fabricated by DC magnetron sputtering: Interface feature and nano-scale surface topography

The multilayer structure is a well-studied architecture for electronic and optoelectronic applications and more recently in spintronic devices. In this work, we present the structural, morphological, topographical, and magnetic properties of GaSb/Mn multilayers deposited via DC magnetron sputtering at room temperature and 423 K. Raman measurements evidence the formation of p-type GaSb layers with a contribution of electrons in the multilayer due to the neighboring Mn layer and the formation of effective interlayers. HR-SEM measurements show the multilayer architecture with columnar microstructure in the layer’s formation, while AFM micrographs allowed observing the changes in grain sizes (between 129 and 187 nm) and roughness (between 1.47 nm and 6.28 nm) with increasing number of layers. The formation of the interlayers between the GaSb and Mn layer was assayed in-depth spectroscopically via Rutherford backscattering studies. These interlayers were associated with diffusion processes during deposition and contributed to the magnetic behavior of multilayers. A ferromagnetic-like behavior was observed in the multilayer

Unveiling the different physical origins of magnetic anisotropy and magnetoelasticity in ga-rich fega thin films

The aim of this work is to clarify how in-plane magnetic anisotropy and magnetoelasticity depend on the thickness of Ga-rich FeGa layers. Samples with an Fe72Ga28 composition were grown by sputtering in the ballistic regime in oblique incidence. Although for these growth conditions uniaxial magnetic anisotropy could be expected, in-plane anisotropy is only present when the sample thickness is above 100 nm. By means of differential X-ray absorption spectroscopy, we have determined the influence of both Ga pairs and tetragonal cell distortion on the evolution of the magnetic anisotropy with the increase of FeGa thickness. On the other hand, we have used the cantilever beam technique with capacitive detection to also determine the evolution of the magnetoelastic parameters with the thickness increase. In this case, experimental results can be understood considering the grain distribution. Therefore, the different physical origins for anisotropy and magnetoelasticity open up the possibility to independently tune these two characteristics in Ga-rich FeGa films

Little-Parks effect governed by magnetic nanostructures with out-of-plane magnetization

Little-Parks effect names the oscillations in the superconducting critical temperature as a function of the magnetic field. This effect is related to the geometry of the sample. In this work, we show that this effect can be enhanced and manipulated by the inclusion of magnetic nanostructures with perpendicular magnetization. These magnetic nanodots generate stray fields with enough strength to produce superconducting vortex-antivortex pairs. So that, the L-P effect deviation from the usual geometrical constrictions is due to the interplay between local magnetic stray fields and superconducting vortices. Moreover, we compare our results with a low-stray field sample (i.e. with the dots in magnetic vortex state) showing how the enhancement of the L-P effect can be explained by an increment of the effective size of the nanodots

Novel perovskite materials for thermal water splitting at moderate temperature

Materials with the formula Sr_2CoNb_1-xTi_xO_(6-delta) (x=1.00, 0.70; delta=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N_2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N_2), involving splitting of water at a temperature as low as 700 ºC, produces hydrogen. Both compounds displayed outstanding H_2 production in the first thermochemical cycle, the Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr_2CoTiO_(6-delta), two phases with different oxygen contents segregated, whereas in Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 mu molH_2/g_(perovskite) after eight thermochemical cycles at 700 ºC, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices

Correlation between local structure and magnetic behavior in co-sputtered Tb_xFe_(73)Ga_(27-x) (7 ≤ x ≤11) thin films

We report on the evolution of the microstructure of Tb-Fe-Ga films deposited by co-sputtering from Tb_(33)Fe_(67) and Fe_(72)Ga_(28) targets. The sputtering power was fixed (90 W) in the Fe_(72)Ga_(28) whereas it was increased from 50 to 90 W in the Tb_(33)Fe_(67) target resulting on Tb_xFe_(73)Ga_(27-x) layers with 7 ≤ x ≤ 11. The local structure was determined by means of x-ray absorption fine structure spectroscopy at Fe-K, Ga-K and Tb-L_(3) edges. The increase of Tb in the alloy promotes the phase segregation that produces a larger amount of the TbFe_2 structural phase. The structural results have been correlated with the magnetic characterization that shows the enhancement of the out-of-plane component of the magnetization

Engineering of silicon surfaces at the micro- and nanoscales for cell adhesion and migration control

The engineering of surface patterns is a powerful tool for analyzing cellular communication factors involved in the processes of adhesion, migration, and expansion, which can have a notable impact on therapeutic applications including tissue engineering. In this regard, the main objective of this research was to fabricate patterned and textured surfaces at micron- and nanoscale levels, respectively, with very different chemical and topographic characteristics to control cell–substrate interactions. For this task, one-dimensional (1-D) and two-dimensional (2-D) patterns combining silicon and nanostructured porous silicon were engineered by ion beam irradiation and subsequent electrochemical etch. The experimental results show that under the influence of chemical and morphological stimuli, human mesenchymal stem cells polarize and move directionally toward or away from the particular stimulus. Furthermore, a computational model was developed aiming at understanding cell behavior by reproducing the surface distribution and migration of human mesenchymal stem cells observed experimentally