Determination of coupling state within a spin-torque oscillator using injection locking

Abstract Dynamic interactions between magnetizations enable complex operations in spintronic devices, such as coupled oscillation in spin-torque oscillators (STOs). Here, we report the experimental determination of coupling states in an STO using an analysis method based on injection locking. The STO is developed for assisted magnetic recording in hard disk drives and exhibits coupled oscillation of two magnetic layers and oscillation of a single magnetic layer depending on bias current polarities, with the former operation improving recording performance. While conventional spectrum measurements yield similar peaks for both oscillations, our method clearly distinguishes between the coupled and single oscillation modes through resistance measurements because of the insensitivity of the coupled oscillation to injection locking, as supported by simulations. Our results demonstrate that the injection locking method provides a sensitive probe for examining coupled oscillation states and is widely applicable to the development of spintronic devices functionalized through dynamic coupling.