Ferromagnetic resonance studies of CoFeB-MgO

There has been much interest in ferromagnetic magnetic tunnel junctions (MTJs) as a potential candidate for spin transfer torque memories. Many parameters are important in order to optimize the spin transfer torque effect to minimize the critical switching current density (J$_{c})$ without compromising an energy barrier (E$_{B})$ between stable states. CoFeB/MgO systems have many desirable properties including high spin polarization and, thereby high tunnel magnetoresistance. Recently, Ikeda et al. reported that Fe-rich CoFeB/MgO MTJs can induce perpendicular anisotropy that is strong enough to overcome the in-plane shape anisotropy, demonstrating CoFeB-based perpendicular MTJs [1]. In this work, we have performed FMR studies as a function of alloy composition, layer thickness, pre and post annealing of CoFeB/MgO systems. Coplanar waveguide method VNA FMR experiments were performed [2]. From the FMR resonance frequency and linewidth we were able to extract the Gilbert damping as well as the effective magnetization. Experimental details as well as results will be presented. \\[4pt] [1] S. Ikeda et al, \textit{Nature Materials}~\textbf{9}, 721 - 724 (2010)\\[0pt] [2] J.M. Beaujour et al, Eur. Phys. J. B \textbf{59}, 475--483 (2007)