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 Fe100-xGax (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 D03 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.This work has been financially supported through project MAT2015-66888-C3-3-R (MINECO/FEDER) and RTI2018-097895-B-C43 of the Spanish Ministry of Economy and Competitiveness and through PR26/16-3B-2 of Santander and Universidad Complutense de Madrid. We thank "CAI Difracción de rayos-X" of UCM for the X-ray diffractometry measurements and the "Instituto de Sistemas Optoelectrónicos y Microtecnología" (ISOM) for using its facilities. We also want to thank BM25-Spline, the Spanish CRG at ESRF for providing beamtime

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

Carbon nanotube fibers with martensite and austenite Fe residual catalyst: room temperature ferromagnetism and implications for CVD growth

We report on the room temperature ferromagnetic properties of continuous macroscopic fibers made up of carbon nanotubes grown by floating catalyst chemical vapor deposition. Their ferromagnetic behavior originates from the presence of residual catalyst nanoparticles: martensite with 0.77 wt% C content and FCC Fe. The first is intrinsically ferromagnetic, but the latter only due to severe lattice distortion as a consequence of C supersaturation. The stabilization of martensite and austenite occurs mainly because of the small diameter of the nanoparticles, in the range of 4-20 nm. This is smaller than the embryonic nucleus of the relevant equilibrium phases, but also implies that large C concentrations can build up in FCC Fe before C can be segregated as a stable graphitic nucleus. The room temperature remanence ranges from 10% to 25% and the coercivity from 55 to 300 Oe, depending on the choice of promoter for fiber synthesis (S or Se). Superparamagnetic behavior is only observed in S-grown samples on account of the smaller diameter of residual catalyst particles. The results of this work provide an explanation for the widespread observation of magnetic properties in oxide-free CNT samples produced by catalytic growth under a wide range of synthesis conditions

Co-doped (La,Sr)TiO3-d: a high-Curie temperature diluted magnetic system with large spin-polarization

We report on tunneling magnetoresistance (TMR) experiments that demonstrate the existence of a significant spin polarization in Co-doped (La,Sr)TiO3-d (Co-LSTO), a ferromagnetic diluted magnetic oxide system (DMOS) with high Curie temperature. These TMR experiments have been performed on magnetic tunnel junctions associating Co-LSTO and Co electrodes. Extensive structural analysis of Co-LSTO combining high-resolution transmission electron microscopy and Auger electron spectroscopy excluded the presence of Co clusters in the Co-LSTO layer and thus, the measured ferromagnetism and high spin polarization are intrinsic properties of this DMOS. Our results argue for the DMOS approach with complex oxide materials in spintronics

Correlation between local structure and magnetic behavior in co-sputtered TbxFe73Ga27−x (7 ≤ x ≤ 11) thin films

We report on the evolution of the microstructure of Tb-Fe-Ga films deposited by co-sputtering from Tb33Fe67 and Fe72Ga28 targets. The sputtering power was fixed (90 W) in the Fe72Ga28 whereas it was increased from 50 to 90 W in the Tb33Fe67 target resulting on TbxFe73Ga27−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-L3 edges. The increase of Tb in the alloy promotes the phase segregation that produces a larger amount of the TbFe2 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.This work has been financially supported through projects MAT2015-66888-C3-3-R and PIE-2010-OE-013-200014 of the Spanish Ministry of Economy and Competitiveness (MINECO/ FEDER) and through the project PR26/16-3B-2 of Santander and Universidad Complutense de Madrid. We thank “Instituto de Sistemas Optoelectronicos y Microtecnología ” (ISOM) for using its facilities. We also want to thank ESRF and BM25-Spline, the Spanish CRG at ESRF, for providing beamtime

Full oxide heterostructure combining a high-Tc diluted ferromagnet with a high-mobility conductor

We report on the growth of heterostructures composed of layers of the high-Curie temperature ferromagnet Co-doped (La,Sr)TiO3 (Co-LSTO) with high-mobility SrTiO3 (STO) substrates processed at low oxygen pressure. While perpendicular spin-dependent transport measurements in STO//Co-LSTO/LAO/Co tunnel junctions demonstrate the existence of a large spin polarization in Co-LSTO, planar magnetotransport experiments on STO//Co-LSTO samples evidence electronic mobilities as high as 10000 cm2/Vs at T = 10 K. At high enough applied fields and low enough temperatures (H < 60 kOe, T < 4 K) Shubnikov-de Haas oscillations are also observed. We present an extensive analysis of these quantum oscillations and relate them with the electronic properties of STO, for which we find large scattering rates up to ~ 10 ps. Thus, this work opens up the possibility to inject a spin-polarized current from a high-Curie temperature diluted oxide into an isostructural system with high-mobility and a large spin diffusion length.Comment: to appear in Phys. Rev.

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

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

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

We report on the evolution of uncapped FeGa 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 FeO. 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 FeO. 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 FeO is detected.This work has been financially supported through the projects MAT2015-66888-C3-3-R of the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) and PR26/16-3B-2 of Santander and Universidad Complutense de Madrid