Magnetic thin films exhibit a close correlation between structural and magnetic properties that makes possible to tailor magnetism by means of a structural engineering, e.g. by a fine tune of the growth conditions. Recently, Fe100-xGax alloys have received great attention due to their unique magnetoelastic properties and their even higher versatility than other magnetostrictive alloys such as Terfenol because of their both higher mechanical performance and chemical stability [1-5]. The correlation between structural properties and magnetic anisotropy [6-8] affects to the magneto-elasticity of these compounds . When Ga content increases from 0 to about 20 at. %, there is a decrease of the anisotropy accompanied by an increase of the magnetostriction constant (λ), the first peak of magnetostriction. For higher Ga contents, there is a decrease of the magnetostriction followed by a second and higher maximum of λ when Ga is between 28 and 30 at. %, known as the second peak. Most of the works devoted to Fe100-xGax report on alloys with x ∼ 20 at. %, in spite of the higher λ for x ~ 30 at. %. A more profound analysis of this compositional range is much needed and therefore, we are investigating the impact of growth conditions on the structural and magnetic properties of Ga-rich Fe100-xGax thin films. We have deposited Fe100-xGax with x ~ 30 at.% by sputtering under two different regimes, ballistic and diffusive, in the oblique incidence. X-ray diffraction (XRD) measurements (Bragg-Brentano) have shown a lower structural quality for the diffusive flow. We have investigated the impact of the growth regime by means of X-ray absorption fine structure (XAFS) being obtained signs of its influence on the local atomic order (Fig 1). Full multiple scattering and finite differences calculations based on XAFS point out to a more relevant presence of disordered A2 phase and of orthorhombic Ga clusters on the diffusive Fe-Ga alloy. On the contrary, in the ballistic case a higher presence of D03/B2 phases is evidenced. The local structure first, as well as the long-range order, seems to determine the magnetic behavior of the layers (Fig.1). Whereas a clear in-plane magnetic anisotropy is observed in the ballistic film, the diffusive is magnetically isotropic. Therefore, our experimental results provide evidences of the correlation between the flow regime and structural and magnetic properties of a rather unexplored compositional region of Fe-Ga alloys . We have also analyzed the correlation between layer thickness and structural and magnetic properties on diffusive layers. Fe72Ga28 films with a thickness of 30 nm, denoted as thin, and 250 nm, denoted as thick, were sputtered. The layers were characterized by XRD showing as in the previous case, the Fe-bcc structure typically observed in these alloys (Fig. 2). The magnetic response at room temperature was determined by VSM and MOKE in order to determine bulk and surface magnetism. In case of the thick layer, it exhibits a clear bulk and surface uniaxial magnetic anisotropy in the reference direction taken as the sputtering beam direction in the oblique direction. For the thin layer, both the in-plane VSM and MOKE hysteresis loops show no preferential direction being therefore magnetically isotropic (Fig.2). Although in the literature it is often reported the decrease of uniaxial magnetic anisotropy with film thickness often due to an interfacial origin, our experimental results show the existence of new phenomena in Ga-rich Fe100-xGax films . We have performed linear dichroism XAFS (LD-XAFS) to determine the existence of structural anisotropy at shortrange. We have clearly observed a structural anisotropy with preferential alignment respect to the substrate in the thick film. In addition, we have found evidences of low symmetry structures other than bcc, which include highly distorted bcc, fcc or even fct. All these results suggest the distorted bcc phase tends to have the elongated axis aligned parallel to the film plane. Our experimental data implies a correlation between the development of an in-plane magnetic anisotropy and the formation of a structural anisotropy as the layer thickness increases, suggesting that the surface to bulk free energy ratio plays a role in the formation of oriented ordered phases in Ga-rich Fe100-xGax alloys. By means of first-principles theoretical calculations, we have analyzed the magnetic anisotropy of different structural phases (Fig.2). Calculations show the in-plane magnetic anisotropy depends on the structure of the layers, attaining larger values when the disordered (A2) and ordered (D03/B2) phases are segregated. We have obtained strong evidences of the correlation between the relative proportion of structural phases and the development of magnetic anisotropy, with the B2 phase being the main candidate for increasing the magnetic anisotropy at Ga concentrations above 20 at.% .  A. E. Clark, J. B. Restorff, M. Wun-Fogle, T. A. Lograsso, and D. L. Schlagel, IEEE Trans. Magn. 36, 3238 (2000).  A. E. Clark, M. Wun-fogle, T. A. Lograsso, and J. R. Cullen, IEEE Trans. Magn. 37, 2678 (2001).  O. Ikeda, R. Kainuma, I. Ohnuma, K. Fukamichi, and K. Ishida, J. Alloys Compnd. 347, 198 (2002).  Q. Xing, Y. Du, R. J. McQueeney, and T. A. Lograsso, Acta Materialia 56, 4536 (2008).  M. P. Ruffoni, S. Pascarelli, R. Grössinger, R. S. Turtelli, C. Bornio-Nunes, and R. F. Pettifer, Phys. Rev. Lett. 101, 147202 (2008).  S. Fin, R. Tomasello, D. Bisero, M. Marangolo, M. Sacchi, H. Popescu, M. Eddrief, C. Hepburn, G. Finocchio, M. Carpentieri, A. Rettori, M.G. Pini, S. Tacchi, Phys. Rev. B 92, 224411 (2015).  M. Barturen, J. Milano, M. Vasquez-Mansilla, C. Helman, M. A. Barral, A. M. Llois, M. Eddrief, and M. Marangolo, Phys. Rev. B 92, 054418 (2015).  A. Muñoz-Noval, A. Ordóñez-Fontes, R. Ranchal. Phys.Rev. B 93, 214408 (2016).  J. Cullen, P. Zhao, and M. Wuttig, J. Appl. Phys. 101, 123922 (2007).  A. Muñoz-Noval, S. Fin, E. Salas-Colera, C. de Dios, D. Bisero, and R. Ranchal. The role of surface to bulk ratio on the development of magnetic anisotropy in high Ga content Fe100-xGax thin films. Submitted.  A. Muñoz-Noval, S. Gallego, J.I. Cerdá, E. Salas, G. Herráiz, M. Tsvetanova, A. Ordóñez-Fontes, and R. Ranchal.Correlation between growth conditions, structure and in-plane magnetic anisotropy in Fe100-xGax (x ~ 30) films grown in the diffusive flow. submitted Fig. 1. (a) XANES spectra at the Ga K-edge for diffusive and ballistic layers and different theoretical XANES spectra. In-plane hysteresis loops at different angles(●) 0°, (Δ) 30°, (○) 60°, and (■) 90° for (b) ballistic Fe72Ga28, and (c) diffusive Fe68Ga32. Fig. 2. In-plane VSM and MOKE hysteresis loops. (a) 30 nm Fe72Ga28, (b) 250 nm Fe72Ga28 layer. Insets: XRD patterns. (c) Evolution of the MAE with Ga content for the D03, B2 and A2 phases. The D03 bcc phase corresponds to an undistorted bcc lattice with fixed (c/a)bcc=1.0. (d) The corresponding unit cell distortions.
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|Titolo:||Correlation between growth, structure and magnetic properties of Ga rich Fe(100-x)Gax alloys|
|Data di pubblicazione:||2017|
|Appare nelle tipologie:||04.3 Abstract (Riassunto) in convegno in Rivista/Volume|