Effect of MgO Cap Layer on Gilbert Damping of FeB Electrode Layer in MgO-Based Magnetic Tunnel Junctions

We investigated the Gilbert damping of thin films with a MgO-barrier/Fe 80 B 20 /MgO-cap/Ta structure, in which the Fe 80 B 20 layer corresponds to a free layer of a perpendicularly magnetized magnetic tunnel junction, by using ferromagnetic resonance (FMR). We systematically varied the thickness of the MgO cap layer and estimated the damping constant (α) by measuring the dependence of the FMR spectrum on the magnetic field angle. The MgO cap layer was found to reduce α to about 0.005, which is less than half the value without the cap. The significant reduction can be explained by the suppression of spin pumping by the dielectric MgO cap layer.