Fig. 1: Spin-torque oscillator synchronization with dc bias.
a, b STO measurement set-up using dc bias (Idc) and spectrum analyzer for the parallel (a) and series (b) connection, respectively. Idc is supplied by the current source and the bias tee isolates the dc bias and the microwave output signal from the STOs. The rf output is then fed into the spectrum analyzer. The device stack is also shown with the numbers in brackets representing the individual layer thickness in nm. c, d Power spectral density measured in the spectrum analyzer by increasing the dc bias for the parallel (c) and series (d) connections, respectively. The magnetic field was adjusted with a different dc bias to obtain the single spectral peak at 2.4 GHz. For the condition of synchronization of four oscillators at 2.4 GHz with Idc,sync = 3 mA (1.2 mA) for the parallel (series) configuration, the in-plane magnetic field of 180 Oe (140 Oe) is applied along the major axis of the STOs.
