Frequency transition of spin-torque oscillator under the magnetic-field pulse in nanosecond range

We report a time domain study of the frequency transition of spin-torque oscillator (STO) under the magnetic-field pulse in nanosecond range. We fabricated the pillar-structured STO devices consisting of MgO-based tunnel junctions with CoFeB free layers. Single-shot waveforms of the STO were obtained using a real-time oscilloscope (40 GS/sec). First, we measured current dependence of the waveform to investigate the time-domain stability of the oscillation. With the increase in the dc current applied to the STO, the oscillation state changed continuously in the following order: thermal fluctuation, intermittent unsteady oscillation, steady oscillation, and chaotic oscillation. Next, we measured the response of the STO to the magnetic-field pulse with a rise time of 0.5 ns, a duration time of 10 ns, and an amplitude of 60 Oe. In this measurement, the oscillation state was kept in the above-mentioned steady state with the frequency similar to 3.5 GHz and the spectral linewidth similar to 50 MHz. In the presence of the magnetic-field pulse, the oscillation frequency shifted around 150 MHz and the frequency transition occurred in less than 2 ns. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3549592]