Abstract
Amplitude fluctuations in an output signal of a tunnel-type spin-torque oscillator are examined by a time-domain measurement using a real-time oscilloscope (40 GS/s). Unsteady intermittent oscillation just above the threshold changes gradually to steady oscillation with increasing current. The amplitude fluctuation in the steady oscillation state is well described by a recent nonlinear theory, which is essentially based on a macrospin model under the thermal fluctuation, indicating that the fluctuation is attributed to the thermal magnetization fluctuation rather than to (nonthermal) mode instabilities.