Solid-State synthesis, structural and magnetic characterization of ferromagnetic phases in 24Ga/76Fe(0 0 1), 40Ga/60Fe(0 0 1) and 60Ga/40Fe(0 0 1) bilayers

https://doi.org/10.1016/j.jmmm.2022.169709

The A2, B2, D03 phases and dispersive nanoprecipitates play key roles in the nature of super-functional properties, such as the large and sensitive magnetostriction, in Fe100-xGax alloys. However, the temperature conditions for the occurrence of the chemical interaction between Fe and Ga, leading to the synthesis of these phases and nanoprecipitates, remain completely unexplored. Herein we first report results of the start of the chemical interaction at the Ga/Fe(0 0 1) interface and the structural and magnetic phase transformations in 24Ga/76Fe(0 0 1), 40Ga/60Fe(0 0 1) and 60Ga/40Fe(0 0 1) bilayers from room temperature to 800 °C. For all bilayers the magnetic ordered D03 phase is the first phase which is formed at the Ga/Fe(0 0 1) interface at ∼ 375 °C. When the annealing temperature is increased above 580 °C in 24Ga/76Fe(0 0 1) samples the epitaxial D03(0 0 1) layer begins to develop into the epitaxial magnetic ordered B2(0 0 1) layer, which remains after annealing to 800 °C. In 40Ga/60Fe(0 0 1) samples the epitaxial D03(0 0 1) layer evolves into the epitaxial B2(0 0 1) layer and a new epitaxial layer with interplanar spacing 0.1484 nm (L60ʹ), which has close reflections with L60 nanoprecipitates. The L60ʹ phase exhibits unique magnetic properties, including a large magnetic fourfold anisotropy constant of ∼ 7·105 erg/cm3 and 8-fold anisotropy. The solid-state reactions in 60Ga/40Fe(0 0 1) samples start with the synthesis of D03 nanograins embedded in amorphous phases, whose peaks are centered on D03(0 0 4) and Fe(0 0 2) reflections. Above 700 °C the amorphous phases partially crystallize and the epitaxial magnetic L60ʹ and ordered B2(0 0 1) phases emerge. Dewetting and unidentified secondary precipitations are observed in all samples and their influence on the magnetic properties is discussed. Our results demonstrate not only the complex nature of initial stage Fe100-xGax alloy synthesis, but also predict the low-temperature transformation at ∼ 375 °C in the Fe - Ga phase diagram.