Magnetic properties and structural characterization of CoxFe(1−x)Al Heusler alloys synthesized from Co/FeAl multilayers

Thin CoxFe(1−x)Al (CFA) alloy films with varying thicknesses (34 nm < d < 219 nm) were prepared on Ta/SiO2/Si substrates at room temperature (RT) and at 620 K by alternating sputtering layers from Co and Fe0.5Al0.5 targets, followed by in situ annealing of the sputtered multilayers at higher temperatures in ultrahigh vacuum. The films were synthesized by magnetron sputtering in a ultra-high vacuum chamber and the deposition was carried out at room temperature (RT) and at 620 K. X-ray diffraction (XRD), energy-dispersive x-ray spectroscopy, magneto-optical Kerr effect investigations, and Mössbauer spectroscopy revealed that the structural, magnetic properties, and composition of the synthesized alloys are highly sensitive to growth temperature, film thickness, annealing conditions, and the number of multilayers (ML) annealed. The XRD measurements demonstrated that films with B2 chemical ordering were obtained for stacks with total thickness smaller than 100 nm and under optimal growth and annealing conditions. The magnetic properties of the samples were investigated using magneto-optic Kerr effect and Mössbauer spectroscopy, showing that after appropriate annealing, films exhibit ferromagnetic character for synthesized films with thickness below 100 nm. Polycrystalline CFA alloys synthesized from Co/FeAl multilayers over Ta/SiO2/Si substrates exhibit dominant uniaxial in-plane anisotropy (UMA), which is independent of chemical order and growth temperature. The UMA is less pronounced for thicker films, suggesting that surface effects are related to the uniaxial magnetic anisotropy observed and that either the number of interfaces, grain size, or grain boundaries have a greater influence than thickness for obtaining the B2 phase in Co/FeAl stacks by annealing.