Spin-Transfer Torque-Induced Dynamics of CoFe/Pd Superlattice-Based Nano-Oscillators in Perpendicular Magnetic Field

Spin-transfer torque provides a mechanism that can be employed to control magnetization on the nano-scale [1,2]. This scheme is used in spin-torque nano-oscillators (STNOs) where it is used to excite steady state precession of the magnetization, leading to microwave emission. Typical STNOs are nanopillars comprising two in-plane magnetized layers [3], one of which has a fixed magnetization while the other one is susceptible to spin-transfer torque. In such configurations, spin-torque generally leads to small angle or clam shell precession of the free layer around an in-plane easy axis of magnetization.
In this work, we investigate a different type of oscillator: The device consists of an in-plane magnetized CoFe layer and an out-of-plane magnetized [CoFe/Pd] superlattice. While the in-plane magnetized layer is confined to about 70 nm, the perpendicular layer is extended. In contrast to common STNOs, our structure is designed as to enable spin-torque-induced excitation in each of the layers. In the experiment, the spin-torque-excited magnetization dynamics is investigated with respect to the sample current and the applied out-of-plane magnetic field. For both current polarities, dynamic states are observed. Considering both layers separately, the in-plane magnetized layer contributes to the dynamics in either case of the current sign, however, the spin-torque asymmetry [4] allows excitation of the perpendicular layer only for one polarity. Since the observable signal is due to the relative motion of the layers’ magnetizations, the output frequency is significantly reduced compared to the frequency of each individually precessing layer. The concept of using the frequency difference between two dynamic layers coupled by spin-torque might be particularly useful for customizing STNOs for microwave generation.

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