Frequency-tunable perpendicular spin torque oscillator

Abstract

We present an experimental study on the frequency tunability of a perpendicular spin transfer torque oscillator with a perpendicularly magnetized (Co/Pd)5 reference electrode and a Co/(Pd/Co)4 composite oscillating layer of a giant magnetoresistive stack. A current-perpendicular-to-plane magnetoresistance ratio of 2.55% is obtained for the stack. Spin transfer torque induced steady oscillation with a narrow line-width has been obtained with direct current injection. As the injected dc current level is varied from 4.5 to 8.5 mA under a small constant applied field, the oscillation frequency increases from 5 to 6.2 GHz and back to 5.8 GHz. The oscillation with the narrowest linewidth occurs at the injection current magnitude at which the highest oscillation amplitude is also obtained. Micromagnetic modeling suggests that a nanomagnet above its critical single domain size is subjected to higher order spin wave generation at large current density. The spatial inhomogenity created across the nanomagnet can account for the drop in its oscillation frequency at relatively large injection currents.