Magnetic tunnel junction sensors with pTesla sensitivity

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 (MTJ) 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 (FG). 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 FG is also thoroughly discussed. Devices consisting of MgO-based MTJ with magnetoresistance levels of ~200 %, and incorporating thin film permanent magnets (CoCrPt) and CoZrNb flux guides, could reach sensitivities of ~2,000 %/mT at room temperature, in a non-shielded environment. The noise levels of the final device measured at 10 Hz yield 3.9 × 10⁻¹⁷ V²/Hz, leading to the lowest detectable field of ~49 pT/Hz^0.5. This value is half of the best value we obtained with MTJ-based devices, and represents a step further towards integration in a biomedical device for magnetocardiography.