Linearization of Magnetic Sensors With a Weakly Pinned Free-Layer MTJ Stack Using a Three-Step Annealing Process

In this paper, a three-step magnetic annealing process is used to linearize magnetic tunnel junctions (MTJs) with two exchange pinning layers: one used to strongly pin the reference synthetic antiferromagnet (SAF) and the other one used to weakly pin the magnetic sensing layer. With respect to a more conventional two-step annealing process, the advantage of the three annealing cycle is to provide independent control over the three relevant internal fields, which apart from the demagnetizing field, control the linear response of large area/low aspect ratio MTJs: the sensing layer soft pinning field (SL-H
_ex
), the sensing layer uniaxial anisotropy field (SL-H
_k
), and the reference layer strong pinning field (RL-H
_ex
). As a result of the three annealing process, a configuration, where SL-H
_k
and SL-H
_ex
point both in a direction orthogonal to RL-H
_ex
, can be achieved resulting in very good linearity and low coercivity without compromising the sensitivity of the devices, as desired for applications, such as magnetocardiography that targets the detection of magnetic fields in the pT/Hz
^0.5
range at room temperature and low frequencies within the 1/f noise.