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Non-white frequency noise in spin torque oscillators and its effect on spectral linewidth



Mark W. Keller, Matthew R. Pufall, William H. Rippard, Thomas J. Silva


We measure the power spectral density of frequency fluctuations from the output voltage waveform of nanocontact spin torque oscillators over timescales up to 50 ms. We facilitate digitization of the waveform by using a mixer to convert oscillator signals up to 40 GHz into a band near 70 MHz. We analyze the waveform using both zero crossing timestamps and a sliding Fourier transform, discuss the different limitations and advantages of these two methods, and combine them to obtain a final frequency noise spectrum spanning more than five decades of Fourier frequency f. For devices having a free layer consisting of either a single Ni80Fe20 layer or a Co/Ni multilayer, and measured at a variety of applied magnetic fields and bias currents, we find a frequency noise spectrum that is white at large f and varies as 1/f at small f. The crossover frequency ranges from {approximately equal}104 Hz to {approximately equal}106 Hz and the 1/f component is stronger in the multilayer devices. Through actual and simulated spectrum analyzer measurements, we show that the non-white frequency noise causes the width of the oscillator's spectral line to increase weakly with measurement time. Our results indicate that the long term stability of spin torque oscillators cannot be accurately predicted from models based on thermal (white) noise sources.
Physical Review B


spin torque, phase noise, 1/f noise


Keller, M. , Pufall, M. , Rippard, W. and Silva, T. (2010), Non-white frequency noise in spin torque oscillators and its effect on spectral linewidth, Physical Review B, [online], (Accessed May 24, 2024)


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Created August 1, 2010, Updated February 19, 2017