Magnetic and structural anisotropic properties of magnetostrictive Fe-Ga flake particles and their epoxy-bonded composites

Materials with large magnetostriction are widely used in sensors, actuators, microelectromechanical systems, and energy harvesters. Binary Fe-Ga alloys (Galfenol) are the most promising rare-earth-free candidates combining numerous advantages such as low saturation magnetic field $(\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{Oe})$, excellent ductility, and low cost, while further improving their performance is imperative for practical applications. Using density functional theory calculation, we report the results of the effect of substituting small amounts of additional elements $X$ (e.g., $X=\mathrm{Ag}$ or Cu) on magnetostriction of Fe-Ga alloys, and we find that it may double the magnetostriction with a substitutional percentage of only $1.6%$. Moreover, adsorbents with high chemical activity (e.g., O or Os atoms) may affect the surface energy of different face orientations of Fe-Ga alloys, indicating that proper surface treatments are necessary to tune the alignment of Fe-Ga grains to achieve better performance. These results may be helpful to further optimize the magnetostrictive properties of Fe-Ga alloys for device applications.

https://link.aps.org/accepted/10.1103/PhysRevB.99.205125

First-principles investigation of the effect of substitution and surface adsorption on the magnetostrictive performance of Fe-Ga alloys

Developing protocols for making thin sheet FeGa (Galfenol) with abnormally grown Goss or Cube grains, which provide maximum magnetostriction, is challenging because the mechanisms that regulate grain boundary mobility and texture development in these alloys are not yet understood. Grain boundary energy models do not account for forces caused by the control of surface energy from atmospheric annealing conditions. By characterizing the surface energy of specific Galfenol grains, we can develop a more accurate thermodynamic framework for modeling abnormal grain growth and texture development. To non‐destructively measure surface energy of specific crystal orientations and overcome passivation layer difficulties in previous studies, a two‐liquid‐phase contact angle method is utilized. A single‐crystal (1 0 0) Fe82Ga18 is used as a proof of concept for its isotropic surface crystal orientation. The resultant contact angle data shows a high dependence on the use of Ar‐plasma surface preparation to remove native oxides exposing a true interaction between a sessile drop and the Galfenol surface. Experimentally measured surface energy values are in agreement with density functional theory simulations. Surface texture and composition are confirmed using EBSD and XPS measurements. This non‐destructive technique paves the way towards studying surface energies of bare metal surfaces.

Non‐Destructive Surface Energy Measurements on (1 0 0) Galfenol

Electrically controlled magnetism is highly desirable for future magnetic memories and spintronic devices. In this work, magnetoelectric (ME) coupling at the interfaces in the Fe3Ga/BaTiO3/Fe3Ga heterostructure is investigated by using first-principles calculations. It is found that the ME coupling in the Fe3Ga/BaTiO3/Fe3Ga heterostructure originates from interfacial bonding, which results in a changed magnetization of interfacial atoms when the ferroelectric layer reverses its polarization direction. The results of electronic density of states indicate that the induced magnetization arises from the changed hybridization state among the d-orbitals of interfacial Fe and Ti atoms. The ME coefficient of the Fe3Ga/BaTiO3 bilayer structure is estimated to be 4.13 × 10−7 μB m/V from the change of magnetic moment induced by polarization reversal, providing a theoretical guidance on further development of multiferroic heterostructures for potential multifunctional device applications.

First-principles study of interfacial magnetoelectric coupling in Fe3Ga/BaTiO3/Fe3Ga heterostructure

Magnetic properties of Fe100−xGax (x = 15 and 30) epitaxial thin films on GaAs(001) substrates were studied experimentally and theoretically. The samples grown by molecular beam epitaxy under certain conditions adopt body-centered structures with tetragonal distortion along the film normal with a c/a ratio varying from 1.014 to 1.036. The density functional theory (DFT) calculations were performed for different (c/a) ratio values and Ga contents. DFT results showed that Fe atoms placed at the first neighborhood of Ga positions present a much stronger distortion in the local exchange-correlation field than second neighbor Fe atoms. The formation of non-bonding Fe 3d states in the minority spin sub-band and the predominance of hybridization of Ga 4p states with Fe 4p states due to Ga content cause this magnetic behavior. The in-plane magnetic anisotropies are explained in terms of the exchange-correlation field isosurfaces. As a result of that, the global magnetic anisotropy can be described as cubic magnetocrystalline mixed with sixfold anisotropy.

Interplay between magnetic moment and magnetocrystalline anisotropy in tetragonally distorted galfenol films

Orientation dependences of surface morphologies and energies of iron-gallium alloys

We investigated the surface energies of several low-index surfaces of the D03-type FeGa alloys (Galfenol), using density functional theory (DFT) simulations and contact angle measurements. DFT calculations predict that (1) the Ga-covered (110) surface of Galfenol is more stable in the Ga-rich condition, while Ga-covered (001) surface of Galfenol become more favorable in Ga-poor condition; and (2) a full Ga overlayer tends to form on top of Galfenol surfaces regardless their orientation, both in agreement with the experimental observation. We also studied Ga segregation in the bcc Fe matrix in order to explore the possibility of Ga precipitation away from Fe. It was found that the Fe–Ga separation is unlikely to occur since Ga diffusion toward the surface is effectively self-stopped once the Ga overlayers form on the facets.

/pdf/orientation-dependences-of-surface-morphologies-and-energies-3aal3cdf8j.pdf

A method of increasing the band gap of iron pyrite by alloying with oxygen is disclosed. According to one embodiment, a method comprises alloying iron pyrite (FeS2) with oxygen to form an iron pyrite and oxygen alloy (FeS2−xOx). The iron pyrite and oxygen alloy (FeS2−xOx) has a band gap greater than iron pyrite (FeS2).

Method of increasing the band gap of iron pyrite by alloying with oxygen

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Orientation dependences of surface morphologies and energies of iron-gallium alloys

Orientation dependences of surface morphologies and energies of iron-gallium alloys

Method of increasing the band gap of iron pyrite by alloying with oxygen