Tunnel Magnetoresistance and Spin-Transfer-Torque Switching in Polycrystalline Co2FeAl Full-Heusler-Alloy Magnetic Tunnel Junctions on Amorphous Si/SiO2 Substrates

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 (J(c0)) is determined to be 8.2 x 10(6) A/cm(2), which is lower than 2.9 x 10(7) A/cm(2), the value for epitaxial CFA MTJs [Appl. Phys. Lett. 100, 182403 (2012)]. We find that the Gilbert damping constant (a) 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 J(c0) for the polycrystalline MTJ is mainly attributed to the low alpha of the CFA layer compared with the value in the epitaxial one (approximately 0.04).