Thermal and spin-transfer-torque excitation of precessional modes in magnetic tunnel junction nanopillars with symmetric interfaces and a thick free layer

Abstract

The microwave emission of magnetic tunnel junction nanopillars with symmetric CoFeB/MgO/CoFeB interfaces and a 9-nm-thick free layer was studied as a magnetic field was applied in plane at different angles ${\ensuremath{\phi}}_{H}$ with respect to the easy axis. As ${\ensuremath{\phi}}_{H}$ was increased, a more complicated mode spectrum containing modes of larger amplitude was observed. The character of the different modes was deduced by screening many devices fabricated from the same wafer, observing the variation of frequency both between different junctions and with ${\ensuremath{\phi}}_{H}$, and through comparison with micromagnetic simulation. Within selected junctions at certain ${\ensuremath{\phi}}_{H}$ values, the dependence of the frequency, amplitude, and linewidth of the lowest-frequency edge-localized mode upon the applied field and current can be interpreted in terms of combined thermal and spin-transfer-torque (STT) excitation. Fitting to a simple analytical model yielded values for the in-plane and out-of-plane spin STT. The out-of-plane STT was found to contain terms with both a linear and a quadratic dependence on the bias current. The linear term was often found to dominate, as expected for a tunnel junction with asymmetric rather than symmetric interfaces, suggesting that the fuller structure of the free and reference layers should be taken into account. The amplitude of the in-plane STT was somewhat larger than expected, suggesting that the dominant edge mode occupies only part of the area of the free layer but is subject to a proportionately larger fraction of the torque exerted upon the free layer as a whole. This might be expected when the resonant modes of the free layer interact and the lowest-frequency mode dominates.