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In this work we are going to present the ability to tune the linear range of magnetic sensors based on magnetic tunnel junctions (MTJs) from 20 Oe to 400 Oe. Controlling the linear range we control the sensitivity, the lower the linear range is the higher is the sensitivity of the sensor but in a smaller field range. To control the linear range it is fundamental to tune all the thicknesses of MTJ stack and to control all its anisotropies.
The MTJ stack under study is composed of: buffer / 20 PtMn / 2 CoFe30 / 0.7 Ru / 2.6 CoFe40B20 / MgO / 2 CoFe40B20 / 0.21 Ta / tNiFe / tCoFe / tRu / tIrMn / Top Contact. The free layer is softly pinned to an anti-ferromagnet (AFM) that allows to have the pinned layer and the free layer anisotropies independent to each other. To have a good linearity we need to have the pinned and the free layers to be perpendicular. By changing any of the free layer and the top AFM thicknesses we are able to change in a control way the linear range of the sensors. [1]
We achieve a linear response with the magnetic field performing two annealing in the double pinned MTJs, one to define the direction of the bottom AFM and a second one perpendicular to the first to rotate the top AFM direction. The signal is not completely centred in zero field because the free layer crystalline anisotropy is along the first annealing, perpendicular to the top AFM.
In order to improve this we develop a three-step annealing process instead of only two annealing that controls the three anisotropies present in the sample (bottom and top AFM and crystalline anisotropy). [2]
[1] R. Ferreira, E. Paz, P. P. Freitas, J. Wang, and S. Xue, IEEE Trans. Magn., vol. 48, no. 11, pp. 3719–3722, Nov. 2012.
[2] E. Paz, R. Ferreira, and P. P. Freitas, IEEE Trans. Magn., vol. 52, no. 7, p. 4002104, 2016.
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