F. Delgado, K. Lopez, R. Ferreira, J. Fernandez-Rossier
In this work we consider two intrinsic sources of magnetic noise in ultra-sensitive magnetic field sensors based on MgO magnetic tunnel junctions (MTJ). The first source is associated to the $^{25}$Mg nuclear spins ($I=5/2$, 10% natural abundance) while the second comes from S=1 Mg vacancies. We show that the integrated noise scales inversely proportional to the radius $R$ of the detector. For detectors of 0.1 $\mu$m radius, we find typical values of 0.1 nT/$\sqrt{{\rm Hz}}$ peaked at frequencies around 0.5MHz for the nuclear spin noise, and assuming a vacancy fraction of 0.0075%, 1 nT/$\sqrt{{\rm Hz}}$ in the 2GHz range for the Mg vacancies. Interestingly, the noise spectrum under a finite magnetic field gradient may provide spatial information about the spins in the MgO layer.
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http://arxiv.org/abs/1211.5574
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