We study polycrystalline B2-type Co2FeAl (CFA) full-Heusler-alloy-based magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate. Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a flat surface are achieved by using a MgO buffer layer. A tunnel magnetoresistance ratio up to 175% is obtained for a MTJ with a CFA/MgO/CoFe structure on a 7.5-nm-thick MgO buffer. Spin-transfer-torque-induced magnetization switching is achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a switching layer. By using a thermal activation model, the intrinsic critical current density (Jc0) is determined to be 8.2×106 A/cm2, which is lower than 2.9×107 A/cm2, the value for epitaxial CFA MTJs [Appl. Phys. Lett. 100, 182403 (2012)10.1063/1.4710521APPLAB0003-6951]. We find that the Gilbert damping constant (α) evaluated by using ferromagnetic resonance measurements for the polycrystalline CFA film is approximately 0.015 and is almost independent of the CFA thickness (2-18 nm). The low Jc0 for the polycrystalline MTJ is mainly attributed to the low α of the CFA layer compared with the value in the epitaxial one (approximately 0.04).
ASJC Scopus subject areas
- Physics and Astronomy(all)