Investigation on the structural and magnetic properties of sputtered TbFe2/Fe3Ga heterostructures

We have analyzed the structural and magnetic properties of as-grown and annealed [TbFe2/Fe3Ga]n heterostructures grown by sputtering. Evidence of the bcc structure in the Fe3Ga layers has been found. The diffraction peak related to this structure shifts to high angles with the annealing temperature. Also, we have observed a change in the microstructure of the Tb-Fe layers when the thickness layer is reduced in the as-grown heterostructures. Moreover, the Tb content is lower than 33% of the TbFe2 Laves phase and it depends on the layer thickness. The thermal treatments promote the increase of the Tb content, but only in the heterostructures with thick layers. The strong lattice mismatch between the Tb-Fe and Fe-Ga layers seems to prevent a complete Tb diffusion upon the annealing process. Thus, the crystallization of the TbFe2 Laves phase is inhibited in the heterostructures with thin layers, although our experimental results indicate the presence of potential magnetostrictive TbFeGa alloy

Evolution of the structural and magnetic properties of sputtered Tb_xFe_73Ga_27-x (7 at.% ≤ x ≤ 11 at.%) thin films upon the increase of Tb content.

Tb_xFe_73Ga_27-x (7 at.% ≤ x ≤ 11 at.%) ternary alloys have been obtained by cosputtering from Tb_33Fe_67 and Fe_72Ga_28 targets. In contrast with other Tb-Fe-Ga compounds that consist of just one structural phase, the diffraction pattern of the Tb_7Fe_73Ga_20 shows the presence of two different phases related to binary Tb -Fe and Fe-Ga alloys. This microstructure evolves as the Tb content is increased, and for a Tb of 11 at.% X-ray diffractometry only evidences the presence of a phase close to the TbFe_2. Although none of the studied samples show perpendicular magnetic anisotropy, there is a significant component of the magnetization perpendicular to the sample plane. The increase of the Tb content on the compounds from 7 at.% to 11 at.% enhances this component most probably due to the shift of the microstructure towards one similar to the TbFe_2

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

Strategies to unblock the n-GaAs surface when electrodepositing Bi from acidic solutions

Bismuth ultra-thin films grown on n-GaAs electrodes via electrodeposition are porous due to a blockade of the electrode surface caused by adsorbed hydrogen when using acidic electrolytes. In this study, we discuss the existence of two sources of hydrogen adsorption and e propose different routes to unblock the n-GaAs surface in order to improve Bi films compactness. Firstly, we demonstrate that increasing the electrolyte temperature provides ompact yet polycrystalline Bi films. Cyclic voltammetry scans indicate that this low crystal quality might be a result of the incorporation ofBi hydroxides within the Bi film as a result of the temperature increase. Secondly, we have illuminated the semiconductor surface to take advantage of photogenerated holes. These photocarriers oxidize the adsorbed hydrogen nblocking the surface, but also create pits at the substrate surface that degrade the Bi/GaAs interface and prevent an epitaxial growth. Finally, we show that performing a cyclic voltammetry scan before electrodeposition enables the growth of compact Bi ultra-thin films of high crystallinity on semiconductor substrates with a doping level low enough to perform transport measurements

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

Synthetic domain walls in (TbFeGa/TbFe)_2 multilayers

Here we report the possibility of creating synthetic domain walls in nominal (Fe72Ga28/Tb33Fe67)_2 multilayers. The magnetization as a function of the temperature reveals the absence of Compensation temperature in the samples which can be understood considering an interdiffusion process that results in the formation of TbFeGa alloys at the nominal FeGa layers. Therefore, samples actually comprise TbFeGa and TbFe layers. The hysteresis loops exhibit magnetization steps related to the nucleation of domain walls formed because of the competition between different interactions: (i) the antiferromagnetic exchange coupling between the heavy rare earth (Tb) and the transition magnetic metal (Fe) inside each layer, (ii) the antiferromagnetic coupling between Tb and Fe at the interfaces, and (iii) the Zeeman energy. In good agreement with the experimental values, the nucleation field of the domain walls has been theoretically calculated taking into account the tilt of the magnetization with respect to the sample plane. Our experimental results indicate that by a proper thickness adjustment, it can be tuned, both the value of the nucleation field and the layer in which this firstly occurs. Experimental values for the exchange bias field have also been calculated

Electrodeposition of Bi thin films on n-GaAs(111)B. II. Correlation between the nucleation process and the structural and electrical properties

The surface morphology and the crystal structure of 40 nm thin Bi films electrodeposited on GaAs(111)B at different growth overpotentials have been studied by means of atomic force microscopy and X-ray diffraction, respectively. The Bi/GaAs interface has also been electrically characterized by means of current-voltage curves that have been analyzed with the thermionic-field emission theory. Taking into account the results presented in Part I, we can conclude that the structural and electrical properties of the Bi layers are correlated with the nucleation process and, therefore, with the energy band diagram of the semiconductor-electrolyte interface. We have found that surface morphology is directly dependent on the amount of protons adsorbed on the GaAs surface, whereas the crystal quality and the interfacial properties also depend on the nucleation mechanism (instantaneous or progressive)

Use of light for the electrochemical deposition of Bi on n-GaAs substrates

In this work, we have explored the possibility of using light to remove the adsorbed hydrogen layer that blocks the GaAs surface when electrodepositing Bi thin films on lower-doped n-GaAs(111)B substrates. A light pulse of a few seconds applied under open-circuit (zero-current) conditions before starting the electrodepositionof Bi in darkness has a small effect on the structural, morphological and interfacial electrical properties of the Bi film in comparison to layers deposited without the use of light. The potentiostatic curves recorded during the Bi nucleation show that the light pulse does not remove the adsorbed hydrogen layer but modifies the n-GaAs surface, inhibiting the reduction of Bi(III) ions. The atomic force microscopy analysis of the n-GaAs surface corroborates that the light degrades the surface by inducing photo-oxidation reactions, phenomenon that is correlated to the photocorrosion of the substrate. To maintain the electrical neutrality during photocorrosion, proton reduction and electroless deposition of Bi occur in parallel to the photo-oxidations. The simultaneity of these processes and the inhibition of Bi(III) ions to get reduced on those areas of the n-GaAs surface chemically altered enables the electroless deposition of unconnected Bi flakes with morphological, structural and interfacial electrical properties close to the state of the art of Bi thin films. Only the out of plane crystal quality of the Bi flakes show a small detriment whereas the Schottky barrier height slightly increases

Morphological, structural and magnetic evolution of sputtered Fe70Ga30 thin films upon annealing in oxygen atmosphere

We report on the evolution of uncapped Fe_(70)Ga_(30) layers deposited by sputtering and post-growth annealed in oxygen atmosphere in a temperature range from 500 °C to 800 °C. We have investigated the morphology, structure and magnetic properties of films with a thickness of 200 nm deposited on Mo buffer layers on glass substrates. X-ray diffractometry shows a decrease of the lattice parameter up to 600 °C whereas a further increase of the temperature up to 800 °C promotes the transformation to Fe_2O_3. We have observed by x-ray absorption fine structure the partial oxidation of Ga and the formation of Ga aggregates at 600 °C. These aggregates form Ga-rich bubbles that can be observed on the sample surface from which Ga evaporates leaving a Ga-poor layer that is later oxidized into Fe_2O_3. The thermal treatment on oxygen atmosphere has also a clear impact on the magnetic properties of the layers. The uniaxial in-plane magnetic anisotropy of the as-grown film evolves to magnetic isotropy when annealed at 600 °C probably due to the segregation and formation of Ga-rich areas. After Ga evaporates from the sample, Fe is fully oxidized and only a weak ferromagnetism related to Fe_2O_3 is detected

Mixed effects of the atomic arrangement and surface chemistry on the electrodeposition of Bi thin films on n-GaAs substrates

We have studied the electrodeposition of Bi thin films on two GaAs orientations with different atomic arrangement and chemical composition, (110) and (111)B. The electrochemical properties of each substrate have been analyzed by means of cyclic voltammetries and current transients. Then, X-ray diffraction has been used to determine the crystal structure and quality of the Bi films, and atomic force microscopy images have provided information about the surface morphology. Finally, the Bi/GaAs interface has been electrically characterized by means of capacitance-voltage and current-voltage curves. In this study, we have been able to discriminate between the effect of surface chemistry and the arrangement of surface atoms. The former has a direct effect on the reduction process of Bi(III) ions and on the electrical properties of the Bi/GaAs interface, whereas the atoms arrangement at the substrate surface determines the texture and morphology of the Bi films