Nanoscale magnetic tunnel junction sensing devices with soft pinned sensing layer and low aspect ratio

Highly sensitive nanosensors with high spatial resolution provide the necessary features for high-accuracy imaging of isolated magnetic nanoparticles or mapping of magnetic fields. Here, we fabricated nanosensor devices based on MgO-magnetic tunnel junctions with soft pinned sensing layer. The exchange interaction at the free-layer is tuned to yield distinct linear operation ranges for the nanosensors. Circular (diameter D = 120-500 nm) and elliptical pillars with low aspect ratio (120 nm × 130 nm-120 nm × 200 nm) displaying a linear non-hysteretic transfer curves with tunnel magnetoresistance values up to 143% were obtained. A noticeable improvement in the sensitivity for circular structures from an average value of ∼ 1 %/mT up to ∼ 2 %/mT is observed with the use of a CoFe/CoFeB/Ta/NiFe/MnIr free-layer. The sensitivity values are almost independent on the size for circular devices, consistent with a linear operation range dominated by the exchange field strength. For elliptical devices, a high sensitivity is also observed, although displaying a dependence on the size, due to a competition with the demagnetizing field. The low-frequency noise features were also addressed revealing a detectivity in the tens of μT/surd Hz with Hooge parameters within 1- 3 × 10^-9 μm² in the linear range. Nevertheless, such high sensitivity values are a major improvement in comparison with those reported previously for nanometric sensors, and extremely competitive with values reported for micrometric spin-valve sensors, with the advantage of providing a reduced device footprint suitable for highly resolved measurements.