Spin-Torque Oscillation Modes of a Composite Synthetic Antiferromagnetic Free Layer in Dual Magnetic Tunnel Junctions

The spin-torque oscillation modes of a composite synthetic antiferromagnetic free layer in dual magnetic tunnel junctions with MgO barriers are studied experimentally by analyzing the field- and current-dependent power spectra. Two oscillation modes are observed under different magnetic fields and currents due to the unique layer structure. With increasing field, the first mode shows a frequency reduction at low magnetic fields followed by a frequency increase at high magnetic fields, while the second mode reveals a frequency increase at both low and high magnetic fields. The current-dependent power spectra of these two modes are also different. With increasing current, the frequency of the first mode drops at low magnetic fields and rises at high magnetic fields, while the other decreases at low and high magnetic fields. These two modes could be tentatively understood by considering the exchange coupling of the synthetic antiferromagnetic free layer.