Two coexisting spin torque oscillation (STO) modes in dual magnetic tunnel junctions with MgO barriers are observed by measuring the field-dependent power spectra. The frequency-field relation of the main STO mode is found to follow the Kittel equation at both positive and negative fields applied along the easy axis, which is consistent with spin torque oscillation of the magnetic free layer between the two MgO barriers. The second oscillation mode shows a different frequency-field relation for positive and negative applied fields, which is tentatively understood by considering the oscillation of the weakly-pinned top layer.
Bibliographical noteFunding Information:
This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0819885. We acknowledge Dr. Jaesang Oh, Professor Anand Gopinath and Professor Rhonda Franklin for technical assistance with the microwave measurements and Professor Ilya Krivorotov from UC Irvine for useful discussions.