Tunable linear magnetoresistance in MgO magnetic tunnel junction sensors using two pinned CoFeB electrodes

Abstract

MgO-barrier magnetic tunnel junction sensors with both CoFeB layers pinned by IrMn have been fabricated, which show a tunneling magnetoresistance (TMR) of up to 255% at room temperature. The perpendicular configuration for magnetic field sensing is set using a two-step field annealing process. The linear TMR field range and sensitivity are tuned by inserting an ultrathin Ru layer between the upper IrMn and the top-pinned CoFeB layer. The field sensitivity reaches 26%/mT, while the noise detectivity is about 90 nT/Hz at 10 Hz for a 0.3 nm Ru insertion layer. The bias dependence of the noise suggests that this is a useful design for sensor applications.

Linearization strategies for high sensitivity magnetoresistive sensors

“…2.3) or iv. use of exchange biasing on the sensing layer, which upon annealing can be set orthogonal to the reference layer [62,82,83] (Fig. 12e, Sect.…”

“…The exchange bias vanishes above the blocking temperature (T b ), being close to the Neel temperature (T N ) for thick AFM films with large grain sizes, while for thin films T b << T N due to finite size effects [97]. Therefore, the T b value is not characteristic of the material but depends on the AFM thickness [62,97] Through consecutive annealing steps under orthogonal in-plane magnetic fields at different temperatures, the crossed configuration between the magnetization of reference and sensing layers is then defined [62,82,83,98]. The first annealing, performed at higher temperature, sets both AFM fixed layers magnetizations in the same direction, while the second annealing step at a lower temperature sets the soft pinned sensing layer magnetization at a perpendicular direction to the bottom one (Fig.…”

“…As its thickness increases, the linear operating range decreases [62,83], allowing a sensitivity improvement. The sensing layer is thereby weakly pinned when compared with the reference one.…”

“…CoFeB/MgO/CoFeB-MTJ data obtained at operation point. Data marked with i from references [119,123,124] and ii is unpublished data from INESC-MN; * from reference [90]; + from reference [85]; & from reference [42]; α from reference [118]; β from reference [125]; δ is unpublished data from INESC-MN; σ from reference [41]; γ from reference [83]. Comparison between single sensors (solid shapes) and sensors with integrated MFCs (half-right shapes).…”

“…Comparison between single sensors (solid shapes) and sensors with integrated MFCs (half-right shapes). Square shapes represent data for micrometric structures from reference [41], either individual circular sensors (radius of 10, 28 and 36 µm) or series of 952 square sensors (50 × 50 µm 2 each); pentagonal shape from reference [83] measured at 10 Hz; left-triangle shape from reference [125]; right triangle shape from reference [118] whereas star shape represents data for nanometric circular MTJ pillars (radius 200 nm) from reference [42]. Simple sensing layer data from reference [90] (upward triangle) and [85] (downward triangle), and SPM-like sensing layer results from reference [92].…”