Bias voltage dependence of the total magnetic field in CoFeB magnetic tunnel junctions

We report experimental thermal-noise spectrum-based ferromagnetic resonance (T-FMR) measurements on CoFeB magnetic tunnel junctions in magnetic fields perpendicular to the film plane. The junctions tested have lateral sizes of $45 \times 80$ nm$^2$. In a simple model a dc junction bias voltage should affect both the slope and the intercept of the T-FMR frequency's dependence on applied magnetic field. The intercept would vary linearly with changes in bias voltage due to an electric field-induced change in uniaxial anisotropy [1]. The slope would have a quadratic dependence on changes in bias voltage based on the existence of a perpendicular spin-torque as discussed by Sankey {\it et al.} [2]. In this experiment we attempt to de-construct the contribution from these two mechanisms. This is done by a careful analysis of the magnetic field dependence of the T-FMR spectra [3]. \\[4pt] [1] Suzuki et al, Appl. Phys. Lett. {\bf 96}, 022506 (2010). \newline [2] Sankey et al, Nature Physics {\bf 4}, 67 (2008). \newline [3] Mascaro et al, Intermag/MMM paper FB-11.