Microwave oscillations in Serially Coupled Spin Transfer Nano-Oscillators

Microwave oscillations in Serially Coupled Spin Transfer Nano-Oscillators
Spin transfer nano-oscillators; Magnetic tunnel junctions
Issue Date
56th Annual Conference on Magnetism and Magnetic Materials
The microwave oscillation from spin transfer nano-oscillators (STNOs) based on magnetic tunnel junctions has been successfully demonstrated. However, there are still two main drawbacks to implement the STNOs for real microwave applications, namely the weak output power and broad spectral linewidth of microwave signals. In order to address these issues, we have studied the interaction between two electrically connected STNOs and the consequence of coupling via self-emitted microwave current. The coupled STNOs consist of two elliptical nanopillar structures (100 x 60 nm2) having a layer stack of SiO2 substrate/buffer layer/Co60Fe20B20 (5)/MgO (1)/Co60Fe20B20 (2)/capping layer (in nm). The coupled STNOs have a tunnel magnetoresistance of 63 % and resistance of 1.23 kΩ in the parallel state. The microwave oscillation spectra have been investigated as a function of bias current and in-plane external magnetic field (Hext) at room temperature. In contrast to a single STNO which follows the Kittel's ferromagnetic resonance formula, the coupled STNOs produce strong microwave spectra (~40 nV/Hz1/2) showing an unconventional dependence on Hext. In order to enhance the coupling via strong self-emitted microwave current, we have increased the bias current up to 1.3 mA. The coupled STNOs show two individual oscillation peaks at a low bias current (< 0.8 mA), but two peaks are merged as the bias current is increased. The possibility of frequency and phase locking in our coupled STNOs will be discussed in comparison with the output power and linewidth of the ideally synchronized STNOs.
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