xGax Thin Films Research Articles

MBE growth and abnormal magnetic behaviors of epitaxial Fe100−xGax on SrTiO3

Here, we present the magnetic behaviors of disordered bcc α-Fe phase Fe100−xGax (x = 10, 20, and 30) thin films grown on a SrTiO3 substrate using molecular beam epitaxy. With the Ga content increasing from 10% to 30%, the saturation magnetization decreased from 1575 to 991 emu/cm3 while the coercivity increased from 23 to 197 Oe. A two-step-hysteresis loop was observed in all films. Specifically, the hysteresis loop of the film with 10% Ga concentration reveals an obvious kink. Magneto-optical Kerr images confirmed the abnormal magnetic domain switching in the films, which is due to the appearance of perpendicular domains under external magnetic field.

Comparative investigation of linear and nonlinear optical properties of As–70 at% Te thin films: influence of Ga content

Thin films (~ 150 nm) of amorphous As30Te70−xGax (where x = 0, 1, 3, 6, and 10 at%) are prepared through thermal evaporation of As30Te70−xGax bulk samples on glass substrates. X-ray powder diffraction (XRD) analysis reveals the amorphous nature of the as-prepared As30Te70−xGax thin films. The influence of Ga content on the As30Te70−xGax thin films’ linear and nonlinear optical properties is determined based on the optical reflectance and transmittance spectra. The estimated (direct or indirect) optical bandgap decreases with an increase in Ga content up to 3 at% and then increases, whereas the Urbach energy exhibits an opposite trend. The linear and nonlinear refractive indices, extension coefficient, optical conductivity, electrical conductivity and nonlinear susceptibility, optical density, inter-band transition strength, etc., are found to be significantly influenced by Ga content and the energy of incident waves. The As30Te67Ga3 composition can be considered as a puzzling compound as most of the investigated parameters in As30Te70−xGax alloys demonstrate opposite behaviors around that composition. Moreover, the optical surface resistance and thermal emission of As30Te70−xGax thin films are estimated from the investigated optical parameters and it was found that they are dependent on Ga content. The obtained results enhanced basic understanding and showed that the As–Te–Ga system qualifies for various optoelectronic applications.

Physical and optical properties of thermally deposited Ge-S-Ga thin films

Optical properties of Ge20S80−xGax thin films, using well established Swanepoel's method, have been reported. Thin films of Ge20S80−xGax glassy alloy were deposited on glass substrates by thermal evaporation technique under high vacuum conditions (~10−5Torr). FE-SEM images show that the films are amorphous and surface smoothness improves with an increase in Ga concentration. Presence of excess number of lone pair's electrons (L) and increase in average coordination number ⟨Z⟩, show that the material under study is a good glass former. Refractive index (n1) is found to increase whereas the extinction coefficient (K) decreases with an increase in Ga content. Increase in refractive index is a consequence of increased polarization and density of the glassy alloy. Addition of Ga augments the non-linear response of GeS binary alloy. Obtained values of non-linear refractive index (nnl) are higher than the values reported in the literature for other chalcogenide glasses. Optical band gap decreases with an increase in Ga concentration. Decrease in cohesive energy and formation of enough localized states in the band gap is liable for the decrease in optical band gap. Correlation between the refractive index dispersion and the microstructure of films has been elucidated by the single effective oscillator model. Obtained optical properties are consistent with predicted physical properties of Ge20S80−xGax glassy alloy.

Structure, magnetic properties and electrical resistivity of Co2FeSi1−xGax Heusler alloy thin films

The influence of Ga on the structural, magnetic and half-metallic properties of Co2FeSi1−xGax (0≤x≤1) thin films grown on Si (100) substrates using ultra high vacuum magnetron sputtering has been systematically investigated. The linear increase in cubic lattice parameter from 5.63Å to 5.73Å and the Curie temperature (TC) from 854K to 941K with x varying from 0 to 1 indicate the progressive substitution of Ga for Si. The coercivity (Hc) was found to decrease from 26Oe (x=0) to 3Oe (x=1) at room temperature and is attributed to the decrease in magnetic anisotropy. The magnetic hysteresis loops measured from 300–873K revealed that the film where Ga completely replaces Si exhibit better stability in both saturation magnetization (Ms) and Hc with temperature. The increase in coercivity at higher temperatures is attributed to the film to substrate interaction. The measured Ms at 100K decreases from 5.01µB/f.u. (x=0) to 4.49µB/f.u. (x=1) and follows the trend of Slater-Pauling rule. The indirect evidence of half-metallic nature is examined from the temperature dependent electrical resistivity measurements.

Growth and characterization of Fe1−xGax thin films from citrate-based electrolytes

In this work we report on the growth of Fe1−xGax films by means of the electrodeposition technique in baths containing sodium citrate as an antioxidant agent. We have investigated the effect of the electrolyte composition in terms of its Ga content on the structural and magnetic properties. Our experimental results indicate that for the optimized bath composition, O-free Fe1−xGax alloys can be achieved for a potential of E = −1.4 V and magnetic stirring at 500 rpm. Both the composition and the coercivity of the samples deposited from this electrolyte can be tuned by means of the duration of the rest pulse at E = −1.12 V. However, when the composition of the bath is not optimized the films exhibit a large amount of O that cannot be dissolved during the rest pulse. Our experimental results show that O-free Fe1−xGax alloys with a Ga content of around 19 at% and optimum magnetic properties can be deposited on Au substrates when using an optimized electrolyte and long rest pulses.

Influence of Ga content on the structure and anomalous Hall effect of Fe1−xGax thin films on GaSb(100)

The Fe1−xGax thin films (x = 0.4, 0.5) have been grown on GaSb(100) substrate using molecular beam epitaxy. An epitaxial film with bcc α-Fe crystal structure (A2) is observed in Fe0.6Ga0.4 film, while an impure Fe3Ga phase with DO3 structure is appeared in Fe0.5Ga0.5 film. The saturated magnetizations at room temperature are observed to be 570 emu/cm3 and 180 emu/cm3 and the coercivities to be 170 and 364 Oe for Fe0.6Ga0.4 and Fe0.5Ga0.5, respectively. A hysteresis trend in Hall resistance vs. magnetic field is observed for Fe0.5Ga0.5 film. However, there is a weak hysteresis noticed in Fe0.4Ga0.6 thin film.

Single crystal Fe1−xGax thin films for monolithic microwave devices

Modern, high frequency, microwave devices for communications technologies can be made with thin ferromagnetic films with narrow microwave resonance linewidths. Recently, there has been interest in magnetostrictive materials where the material constants can change substantially with stresses and applied magnetic fields. We report the development of single crystal thin (20 nm thick) magnetostrictive films of Fe1−xGax (x = 0.20 FeGa(A), 0.23 FeGa(B), 0.28 FeGa(C) on GaAs(001) substrates and on their use in prototype microwave devices. These Galfenol films have a narrower linewidth than any previously reported similar thin films. We fabricate and characterize novel microstrip-based monolithic microwave devices using Galfenol thin films as an active element. We find a number of important features: (1) There is a large absorption (up to 30 dB/cm) at the resonance frequency. (2) The linewidth of the device is narrow ∼1.5 GHz. (3) The saturation magnetization of the samples decreases with the increase in Ga contents. (4) The cubic anisotropy is close to zero (∼0.06 kOe for FeGa(A)) and becomes negative for higher concentration of Ga content in the samples, and (5) the damping increases with increase in Ga concentration.

X-ray absorption spectroscopy and magnetic circular dichroism studies of L1-Mn-Ga thin films

Tetragonally distorted Mn3−xGax thin films with 0.1<x<2 show a strong perpendicular magnetic anisotropy and low magnetization and thus have the potential to serve as electrodes in spin transfer torque magnetic random access memory. Because a direct capping of these films with MgO is problematic due to oxide formation, we examined the influence of a CoFeB interlayer and of two different deposition methods for the MgO barrier on the formation of interfacial Mn-O for Mn62Ga38 by element specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). A highly textured L10 crystal structure of the Mn-Ga films was verified by X-ray diffraction measurements. For samples with e-beam evaporated MgO barrier no evidence for Mn-O was found whereas in samples with magnetron sputtered MgO, Mn-O was detected, even for the thickest interlayer thickness. Both XAS and XMCD measurements showed an increasing interfacial Mn-O amount with decreasing CoFeB interlayer thickness. Additional element specific full hysteresis loops determined an out-of-plane magnetization axis for the Mn and Co, respectively.

Controlled electrochemical deposition of magnetostrictive Fe1 − xGax alloys

In this study, using steady-state electrochemistry at a rotating disk electrode, a deposition mechanism for giant magnetostrictive Fe1−xGax alloys is proposed in which the formation of an adsorbed monovalent [Fe(I)]ads intermediate is determined to be the rate-determining step. In subsequent steps, this intermediate either gets reduced to iron or catalyzes the reduction of gallium by forming an adsorbed [Ga(III)–Fe(I)]ads intermediate. In line with the proposed mechanism, it was experimentally shown that the differences in the mass-transport rates of Fe(II) species determined the thin film composition. Therefore, this study has made possible a controllable and reproducible deposition of Fe1−xGax thin films with compositions in the entire range of interest (15%–30% Ga). As-grown Fe80Ga20 thin films were found to have magnetostriction constants of ~112ppm.

Ferrimagnetic ordering of single crystal Fe1−xGax thin films

Molecular beam epitaxy was used to deposit body centered cubic single crystal Fe1−xGax thin films on MgO(001) and ZnSe/GaAs(001) substrates well beyond the bulk stability concentration of about 28%. The crystal quality of the substrate surface and each deposited layer was monitored in situ by reflection high energy electron diffraction. The magnetization of the samples as a function of Ga is found to decrease more rapidly than a simple dilution effect, and element-specific x-ray magnetic circular dichroism ascribes this trend to a decrease in the Fe moment and an induced moment in the Ga that is antialigned to the Fe moment.

Magnetostrictive effect in single crystal Fe1−xGax thin films

The magnetic properties of single crystal Fe1−xGax thin films deposited on ZnSe/GaAs(001) and MgO(001) substrates by molecular beam epitaxy were investigated by vibrating sample magnetometry and angle dependent ferromagnetic resonance. Depositions on the ZnSe buffer layer feature a strong uniaxial anisotropy that scales with the thin film magnetostriction of the samples, while depositions on MgO(001) substrates result in a purely cubic anisotropy whose cubic anisotropy constant, K1, switches sign at a lower Ga concentration than is seen in bulk.

Properties of single crystal Fe1−xGax thin films

Molecular beam epitaxy was used to deposit single crystal thin film Fe1−xGax samples on ZnSe buffer layers grown on (001) and (110) single crystal GaAs substrates. The crystal quality of the GaAs surface and each deposited layer was monitored in situ by reflection high energy electron diffraction. The magnetic properties of the samples were characterized by vibrating sample magnetometry and ferromagnetic resonance (FMR). The FMR linewidth increases dramatically with Ga concentration while the cubic anisotropy term K1 switches sign.

Structural and electro‐optical characteristics of Ga‐doped ZnO films prepared by thermal oxidation of metallic Zn100–xGax films

AbstractGa‐doped ZnO thin films have been obtained by thermal oxidation of electron beam evaporated metallic Zn100–xGax thin films. The structural, optical, and electrical resistivity of the oxidized films depend on Ga content. The value of oxidation temperature shifts to higher temperatures with increasing Ga content. After oxidation, polycrystalline ZnO phase is formed and no Ga2O3 peaks are found. The Ga content is found to affect transmittance strongly and the optical bandgap slightly. The optical bandgap values of the oxidized Ga‐doped films are comparable at higher oxidation temperatures. Depending on the Ga content, the electrical resistivity increases with increasing oxidation temperature. At 550 °C all the Ga‐doped films have electrical resistivity values around 105 Ω cm. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase stability versus the lattice mismatch of (100)Co1−xGax thin films on (100)GaAs

Thin films of the intermetallic compound Co1−xGax, which has a broad homogeneity range and a continuously variable lattice constant, were grown on the (100)GaAs surface by molecular beam epitaxy. The stoichiometry of the films and the cleanliness of the substrates were determined in situ by Auger electron spectroscopy. The identity and orientation of the phases present in the films were determined ex situ by x-ray diffraction (XRD). The films were annealed to various temperatures in N2 and reexamined by XRD to detect any chemical interactions between the Co1−xGax and GaAs. Films of Co1−xGax deposited with a 2% lattice mismatch (x=0.5) reacted with the substrate to produce CoGa3 at 600 °C, whereas films with no detectable mismatch (x=0.61) did not react until they had been heated to 800 °C.