Current-dependent linewidth of a spin-transfer nano-oscillator

Power spectra of a spin-transfer nano-oscillator, in which a reflection layer is included, are measured for current densities up to 1.2×107 A/cm2. A clear oscillation peak, which is slightly asymmetric with respect to the peak frequency, is observed at 5 GHz under the external field of 700 Oe. The large red shift of the peak frequency on increasing the current, which has been reported in nano-contact devices, is not observed. The relatively low oscillation frequencies as well as the absence of the large red shift indicate the reduction in the demagnetizing field in the free layer. The linewidth decreases with the current and is inversely proportional to the square of the precession amplitude of magnetization in accordance with the recent linear theories.