Thermal magnetic noise in tunneling readers

We present modeling results, using the stochastic Landau-Lifshitz-Gilbert approach, of thermal magnetic noise in TMR sensors of dimensions suitable for 100-150 Gbit/in². In particular, we have investigated the effect of aspect ratio and permanent-magnet bias on the thermally induced magnetization fluctuations as well as the induced noise voltage spectral density. We show that there can be a lower frequency resonance due to the motion in the reference layer of the tunneling magnetoresistance reader, uncorrelated with the motion of the free layer, which gives rise to a structure in the noise voltage spectral density. We also present measured noise spectra which exhibit similar characteristics with a well defined resonance peak and a smaller peak at a lower frequency, which we attribute to resonance motion in the reference layer. The motions of the free and reference layers are greatly influenced by the strength of the bias field from the permanent magnets. For example, too low bias field may give rise to increased low-frequency noise due to switching behavior in the pinned and reference layers.