DOI: 10.20350/digitalcsic/17107
title: Enhanced magnetoelastic stress in disordered iron-gallium alloy thin films revealed by direct measurement

description: [Description of methods used for collection/generation of data] RHEED images (files starting with Fig1b) were collected using a digital camera UK1117m equipped with a Sony ICX415AL sensor. The pixel type is gray with a pixel width of 8 bits and a variable exposure time and gain was used. Film thickness was obtained with a Discover D8 diffractometer with Cu Ka1 radiation (files starting with Fig1c). Bragg-Bentano scans were conducted with a Rigaku X-ray diffraction at Cu Ka radiation. (files starting with Fig1d). Composition was determined using energy-dispersive X-ray spectroscopy (EDX) using a INSPECT-F50 SEM (https://lma.unizar.es/sem-inspect/) and Wavelength Dispersive Spectroscopy (WDS) in an electron probe micro-analyzer (EPMA) instrument (JXA-iHP200F at the Universidad of Zaragoza). Synchrotron X-ray diffraction measurements were conducted at the BM25 beamline of the ESRF synchrotron at room temperature. A monochromatic beam with a wavelength of 0.7 Å was used. (files starting with Fig2). The reference grid is obtained using the MgO reflections of the monocrystalline substrate used to grow the films. Several non-collinear reflections are requested to calculate the matrix elements. Synchrotron XAS measurements were conducted at the Fe and Ga K-edges at beamline I18 of the Diamond UK’s National Light Source at beamline I18 of the Diamond UK’s National Light Source. Magnetization measurements were obtained using nanoMOKE3 magnetometer using laser beam with p-polarization and red (650 nm wavelength) light. High-field lineal correction is applied. Cantilever beam measurements. The magnetoelastic stress was obtained using a cantilever beam method. The sample is clamped by one end, and a constant magnetic field of 100 mT is rotated. The magnetic field is generated by a homemade dipole mounted on a motorized rotatory stage(Standa 8MR190-90-4247). Several turns are performed to correct drifts and other errors. The capacitor is built with the sample that covers a second smaller electrode. The capacitance is measured using an Andeen Hagerling 2500A capacitance bridge at 1kHz. The difference in capacitance with the magnetic field along the beam and forming 90 degrees is used to obtain the magnetoelastic stress. That value is multiplied by the factor indicated in the Appendix. Files starting with Fig6 include the capacitance recollected, Cap, the capacitance with a correction (using a 4-degree polynomial) to eliminate drift, deltaC, the magnetoelastic stress, ME. and the angular position, angle, starting with H along the beam length.
description: [Methods for processing the data] XAS data. The experimental spectra were fitted by theoretical EXAFS signals calculated by ab initio theoretical phase and amplitudes generated for a cluster of 78 atoms with a radius of 6.1 Å-1, by the TKAtoms code. The central absorber was a Ga or Fe atom with only one kind of Ga (Fe) scatterer. The alloying FeGa effect was obtained by combining the pure Ga-Fe(Ga) and Fe-Fe(Ga)systems at the Ga and Fe K-edge, respectively. The 8.01 Feff code generated the theoretical phase-shifts and amplitudes taking into account beam polarization. The ATHENA and ARTEMIS packages were used for extraction and best-fit procedure of the EXAFS spectra. The best-fit analysis for the two polarizations was performed simultaneously in q-space, in the range 2 - 11 Å-1, by Fourier Filtering of the first peak contribution of the FT spectra that is due to the first and second coordination shells. The contributions of next neighbor (NN) and next-nearest-neighbor (NNN) atoms are merged into a single FT peak due to the proximity of the interatomic distances values (2.48 Å and 2.87 Å respectively in pure bcc Fe). The best-fit parameters were the origin of the photoelectron energy E0, the Ga population in NN and NNN environments, the interatomic distances of first and second coordination shells, and the Debye-Waller factors. The FDMNES code is used to perform \textit{ab initio} simulations of the XANES spectra.
description: The large magnetostriction in FeGa alloys is relevant for manifold applications, but for thin films, it can play a prominent role in controlling the strength of the magnetic anisotropy. Bulk samples show values depending on the extensive preparation procedure compendium, which is limited in its temperature range for high-quality thin-film synthesis. Here, we present a study of the magnetoelastic coupling coefficients B1 and B2 in epitaxial FeGa thin films below 50 nm deposited on the MgO(001) surface at 150 ºC by the cantilever method. Series of films with 22, 28, and 33 at. % Ga do not show thickness-dependent variations for B1 and B2, but -B1 for the 22 at. % Ga composition is 10 MPa, roughly 2 times the bulk value and smaller than the bulk-like value of -B1=12.1 MPa obtained for a film with 17 at. % Ga. This enhancement is correlated with the A2 crystal structure for the film rather than the coexistence with D03 or other ordered nanometric precipitates proposed for bulk samples. Synchrotron diffraction excludes the formation of long-range L60, or D03 precipitates in samples with (001)A2 peaks at concentrations around 25 at. % Ga, which implies partial chemical disorder. The analysis of extended x-ray absorption fine structure measurements points to a D03 local order with a residual number of Ga-Ga pairs. Considering that the substrate quenches the movable strain in the A2 phase described in dual-phase structures, our results point to the important role of the electronic structure of the iron atoms modified by the presence of Ga in the alloy. This effect enlarges B1 in films with the A2 phase, stabilized using epitaxial growth.