The noise spectral density amplitude Sδa is constant up to a relaxation frequency fp, and then decreases as 1/f². Due to the non-linearity of the system, the spectral density of phase S changes as 1/f² before fp and after falls off as 1/f4. Up to 1 GHz, the measurement results overlap perfectly with those from micro-magnetic simulations assuming the existence of a white noise from the effective field acting on the magnetization. After 1 GHz it is the measurement instrument noise that dominates.
In collaboration with LETI and Hitachi, Spintec has recently developed a technique to extract the non-linear parameters characteristic of the frequency line width of spintronic oscillators.
Spintronic oscillators are based on the magnetization precession of magnetic structures, resulting in an oscillation of the device’s electrical resistance. In these oscillators, the amplitude and the phase of the oscillations have a non-linear dependence. This characteristic is especially interesting, because it allows tuning the oscillation frequency by simply changing the applied DC current on the device. However, it also brings a degradation of the signal quality (wider line widths), due to the signal amplitude and phase fluctuations. It is therefore important to understand the origin of these fluctuations.
The fluctuations, i.e., noise, were measured on devices provided by Hitachi. Signal variations were recorded over time, and applying a Hilbert transform to the data allows for the separation of amplitude and phase. These results are then compared, using numerical simulation, to different theoretical models corresponding to possible noise sources. The figure shows how the results relate to a model, assuming that the effective magnetic field creates fluctuations that are independent of the frequency, white noise, just like thermal noise. These results allow for a better understanding of the mechanisms behind the line width of spintronic oscillators and open the way for further performance improvements.
Further reading: M. Quinsat et al., App. Phys. Lett. 97 (2010) 182507
Maj : 18/02/2014 (959)