Influence of the Stress Current on Spin-Torque-Oscillator With NCMR-SV Devices

We investigated the microwave oscillations induced by spin transfer torque with a current-perpendicular-to-plane (CPP) nanocontacts spin-valve (NCSV) structure under dc current. First, we carefully studied the influence of the stress current, at which a synthetic-antiferromagnet (SAF)-spin-flop might take place with Joule heating under the applied field for measurement, on the SAF magnetization state for CPP-NCSV with high magnetoresistance (MR) ratio of approximately 20% to determine the applied current densities of the devices for microwave oscillation measurement, and then its microwave oscillation characteristics were clarified. The clear decrease in the MR ratio due to Joule heating was observed only in NCSV devices with high MR ratios. Therefore, the critical current density of the device for the microwave oscillation decreases with an increase in the MR ratio, which improves the oscillation efficiency. It suggests that the current is more effectively confined at NCs compared with the low MR devices. As a result, we succeeded in observing excellent oscillation characteristics, i.e., a large oscillation power of 64 nW and a narrow line width of 6.5 MHz at approximately 1.1 GHz in an NCSV device.