Magnetoresistive sensors have been enthusiastically selected for applications requiring magnetic field detection with sensors combining small footprint (1), highly linear response curves with low coercivity and low field detectivities. From the viable alternatives (2), MgO-based magnetic tunnel junctions (TMR) offer very competitive performance values, as sensitivity as large as 300 %/mT are reported (3). The optimization of the sensor response includes using soft magnetic free layers, based on CoFeB and NiFe alloys (4).
Here we report the performance of TMR sensors including CoFeBTa and CoFeBSi soft magnetic films as free layers (FL). Magnetic tunnel junction stacks based on (Ta 5/Ru 10)x3/Ta 5/Ru 5/MnIr 8/CoFe 2.2/Ru 0.65/CoFeB 2/MgO/CoFeB/FL2/cap (nm) (fig. 1) were grown on 200 mm wafers using a combination of ion beam and magnetron sputtering deposition from the 14 targets available in the machine. The TMR stacks were magnetically characterized by vibrating sample magnetometry and CIPT, while microfabricated devices (rectangular shaped pillars of varying areas and aspect ratios) were evaluated for transfer curve sensitivity, linear range and noise.
The magneto-crystalline anisotropy, Hk, was assessed for CoFeBTa and CoFeBSi films, with values of 2.1 and 5.3 mT measured for 4 nm films integrated in the TMR stacks. Although these values are larger than for single NiFe FL (1.7 mT)(fig. 2), the global performance of the CoFeBTa/CoFeBSi-based sensors is improved, with magnetoresistance of 210 %, when comparing with 190 % (CoFeB) or 150 % (NiFe). Field sensitivities up to 80 %/mT are obtained, with similar hysteresis (Hc) as for CoFeB or NiFe stacks.
The low frequency noise (1/f dominated) characteristic was evaluated for the microfabricated devices. As an example, sensors with FL2 = CoFeBTa (10 TMR elements connected in series) provided Hooge factor αH = 2.4x10-10 μm2 and detectivity levels of D = 4.5 nT/√Hz at 10 Hz. These values are aligned with previously reported values with similar materials (4).
Acknowledgments: FCT grant UI/BD/151461/2021
(1) M. Silva, D.C. Leitão, S. Cardoso and P. P. Freitas, IEEE Trans. Magn. 53, 7762720 (2017)
(2) C. Zheng et al., IEEE Trans. Magn. 55-4, pp. 1-30 (2019)
(3) TMR9001, MultiDimension Technology, Datasheet available online: http://www.dowaytech.com/en/1866.html (accessed on 24 June 2022)
(4) M. Rasly et al., J. Phys. D: Appl. Phys. 54, 095002 (2021)