Ultrasensitive magnetic field sensors at low frequencies are necessary for several biomedical applications. Suitable
devices can be achieved by using large area magnetic tunnel junction sensors combined with permanent magnets to
stabilize the magnetic configuration of the free layer and improve linearity. However, further increase in sensitivity and
consequently detectivity are achieved by incorporating also soft ferromagnetic flux guides. A detailed study of tunnel
junction sensors with variable areas and aspect ratios is presented in this work. In addition, the effect in the sensors
transfer curve, namely in their coercivity and sensitivity, as a consequence of the incorporation of permanent magnets
and flux guides is also thoroughly discussed. Using sensors with a tunnel magnetoresistance of ~200 %, incorporating
both permanent magnets and flux guides sensitivities of 220-260 %/mT were obtained for high aspect ratio sensors,
increasing to values larger than ~2000%/mT for large areas and low aspect ratio sensors. Measured noise levels of the
final device at 10 Hz yield 3.9×10-17 V2/Hz, leading to an improved lowest detectable field of ~ 94 pT/ Hz0.5.
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