Magnetism and structure of CoxFe(1-x)Al Heusler alloys synthesized from multilayers of Co and FeAl

Full text: Heusler alloys like CoxFe(1-x)Al (CAF) are promising materials for a variety of spintronic applications like, for example, spin transfer-torque magnetic RAM (STT-MRAM) devices [1]. We have synthesized thin CoxFe(1-x)Al (CAF) alloy films with different thickness (33 nm< d <220 nm) on tantalum buffered SiO2/Si substrates at room temperature (RT) and at 620 K by alternating sputtering multilayers from Co and Fe0.5Al0.5 targets and subsequently annealing in-situ these multilayers at higher temperatures in ultrahigh vacuum. X-ray diffraction techniques (XRD) and Energy-dispersive X-ray spectroscopy (EDS) showed that the structural properties and composition of the synthesized alloys are sensitive to growth temperature, thickness, annealing conditions and number of annealed multilayers (ML). The XRD demonstrate that under appropriate conditions, films with B2 chemical ordering were obtained. Magneto-optic Kerr effect (MOKE) and Mossbauer Spectroscopy were used to investigate the magnetic properties of the samples which are ferromagnetic for synthesized films with thickness under 100 nm and present a dominant uniaxial in-plane anisotropy which is independent of chemical order. We report a uniaxial anisotropy that is less pronounced for thicker films, pointing to a surface related origin of uniaxial anisotropy in polycrystalline CAF alloys synthesized from multilayers over Si substrates. [1] Z. Wen et al, Tunnel Magnetoresistance and Spin-Transfer-Torque Switching in Polycrystalline Co₂FeAl Full-Heusler-Apply Magnetic Tunnel Junctions on Amorphous Si/SiO2 Substrates, Phys. Rev. Applied 2, 024009 (2014). 1920. (author)