DC Bias Reversal Behavior of Spin-Torque Ferromagnetic Resonance Spectra in CoFeB/MgO/CoFeB Perpendicular Magnetic Tunnel Junction

Spin-torque ferromagnetic resonance (ST-FMR) spectra of nano-scaled CoFeB/MgO/CoFeB full perpendicular magnetic tunnel junctions (p-MTJs) were investigated, especially in detail at low dc-bias voltage region. Usually in in-plane magnetized MTJs (i-MTJs), the ST-FMR spectrum line shape reverses its symmetry as switching dc-bias voltage polarities; however, it is found that in the p-MTJs the line shape reversal behaves differently, that not only the spectrum shows anti-symmetric line shape at zero dc bias but also the dependence of reversal symmetry on dc bias is broken. Based on the framework of homodyne-detected ST-FMR, we extracted the parameters characterizing the spectra and discussed the possible factors resulting these differences.