Analysis of injection-locking of a standard Spin-Torque Oscillator at 2!

C. DIEUDONNÉ; M. TORTAROLO; E. MONTEBLANCO; M. ROMERA; M-C. CYRILLE; L. BUDAPREJBEANU; U. EBELS

Dresden

Workshop; Intermag 2014; 2014

IEEE

Spin-Torque Oscillators (STOs) are promising nano-scale radiofrequency oscillators (50-100 nm) that make use of the large-amplitude precession of the magnetization of a thin magnetic layer induced by Spin-Transfer-Torque (STT). Compared to the Voltage-Controlled Oscillators (VCO) used nowadays in RF chips, STOs have the advantage of being highly tunable with DC current because of their non-linear frequency-amplitude dependence. However, they do not meet yet the requirements in terms of linewidth and power to be competitive. To give an example, standard magnetic tunnel junction (MTJ) STOs with in-plane magnetization typically generate signals at frequencies from 5 to 10 GHz, with an output power between 1 and 10 nW and a linewidth between 10 and 100 MHz. In order to address issues of power and linewidth, the following approach was proposed: If the signal properties cannot be enhanced through engineering of the STO device (magnetic stack and nanofabrication), then one can rather synchronize an array of STOs . There are several physical mechanisms to couple STOs depending on the type of STO considered, e.g. through spin-wave or dipolar coupling. Electrical synchronization of N STOs in series based on the Kuramoto model has been described theoretically [1, 2], however, in experiments, synchronization of even 2 STOs electrically connected in series or in parallel has not been achieved. What has been done experimentally is injection-locking , in other terms, synchronization of an STO to an external AC current source [3, 4], leading to strong linewidth reduction and increase of the peak power density for current ratios IAC/IDC0.1-1.