Operational Amplifier Circuits for Magnetic Sensing Using Perpendicular Low-Coercivity Nanoscale Spin Transfer Torque Magnetic Tunnel Junctions

This article presents the use of nanoscale perpendicular spin transfer torque magnetic tunnel junctions (STT-MTJs) as magnetic sensing units in which the conditioning electronics consists of simple circuits based on operational amplifier (OpAmp). This approach allows low-power (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> W), nanoscale (50–100 nm), and high-frequency (100–1000 kHz) magnetic measurements, with, in addition, presenting CMOS compatibility for future mass production. Noise levels as low as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2~\mu $ </tex-math></inline-formula> T/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\surd $ </tex-math></inline-formula> Hz are achieved over a dynamic range of up to tens of milliteslas. The different circuits are demonstrated through theoretical modeling and confirmed with experimental measurements on fabricated devices, validating the working principle with sensing elements among the smallest ever reported to our knowledge. Further improvements are, however, required, in both the proposed electronics and stacks of the junctions, to lower the noise and reach the sub-microtesla noise level. Hence, this technology could open the way to new metrological possibilities, unaddressed with existing technologies, including photolithographic mask alignment or high-frequency current measurements, in the case of industrial applications, as well as measurements of magnetic beads and ferromagnetic particle detection in life sciences.