VP16-09 High-Sensitivity Magnetoresistive Sensor with Optimized Orthogonal Exchange Bias for Low-Field Measurement
Creators: Yi Yang
Suvechhya Lamichhane
Chun-Yen Chen
Andrei Sokolov
Xiaolu Yin
Yen-Fu Liu
Jhen-Yong Hong
Liang-Juan Chang
and Sy-Hwang Liou
Publisher: name: IEEE
Year: 2025

Magnetoresistive sensors based on magnetic tunnel junction (MTJ), among different types of magnetic sensors, have exhibited extraordinary performance, especially high achievable sensitivity at room temperature [1,2,3]. Other than high sensitivity, in some applications where low magnetic fields are measured, improved reversibility and linearity of the response of magnetoresistive sensors to external magnetic fields are also needed. To achieve good reversibility and linearity, as studied in some other experiments [4], a potentially effective technique is setting exchange bias that is perpendicular to the sensing direction. In this paper, a high-sensitivity magnetoresistive sensor that combines optimized orthogonal exchange bias with the novel magnetic flux guiding structure that was discussed in our previous papers [2,3] is studied for improved overall performance. The MTJ stack is shown in Fig. 1, where a Co70.5Fe4.5Si15B10 layer that is about 50 nm thick is coupled to the free layer (3.4-nm Co20Fe60B20) through a 0.5-nm Ru layer to reduce the magnetic hysteresis of the free layer and guide magnetic flux, where the Co70.5Fe4.5Si15B10 compound is an amorphous soft magnetic material [5]. This Co70.5Fe4.5Si15B10 layer is adjacent to an antiferromagnetic Ir20Mn80 layer, so the Co70.5Fe4.5Si15B10 layer and free layer are expected to be pinned by the antiferromagnetic layer after field annealing. The MTJ devices were annealed multiple times in different conditions to set orthogonal exchange bias for the reference layer and free layer. With external magnetic flux concentrators added to the sensor to form double staged magnetic flux concentrators, the output voltage was measured in the field range from about -0.043 Oe to about +0.043 Oe, as shown in Fig. 2. With double staged magnetic flux concentrators, the sensor exhibits a sensitivity of 3.82×103 %/Oe (3.82×104 %/mT). The studied magnetoresistive sensor has great potential for application in low-field measurement at room temperature.References: [1] C. Zheng et al., IEEE Transactions on Magnetics, vol. 55, no. 4, pp. 1-30 (2019) [2] Y. Yang, A. Sokolov, X. Yin, J. Hua, Y.-F. Liu, S. Lamichhane, and S.-H. Liou, IEEE Transactions on Magnetics, vol. 59, no. 1, pp. 1-7 (2023) [3] S. Lamichhane, Y. Yang, A. Sokolov, X. Yin, Y.-F. Liu, and S.-H. Liou, Appl. Phys. Lett. 124, 212401 (2024) [4] R. Ferreira, E. Paz, P. P. Freitas, J. Wang, and S. Xue, IEEE Transactions on Magnetics, vol. 48, no. 11, pp. 3719-3722 (2012) [5] D. Kato, M. Oogane, K. Fujiwara, T. Nishikawa, H. Naganuma, and Y. Ando, Appl. Phys. Express 6, 103004 (2013)
Magnetoresistive sensors based on magnetic tunnel junction (MTJ), among different types of magnetic sensors, have exhibited extraordinary performance, especially high achievable sensitivity at room temperature [1,2,3]. Other than high sensitivity, in some applications where low magnetic fields are measured, improved reversibility and linearity of the response of magnetoresistive sensors to external magnetic fields are also needed. To achieve good reversibility and linearity, as studied in some other experiments [4], a potentially effective technique is setting exchange bias that is perpendicular to the sensing direction. In this paper, a high-sensitivity magnetoresistive sensor that combines optimized orthogonal exchange bias with the novel magnetic flux guiding structure that was discussed in our previous papers [2,3] is studied for improved overall performance. The MTJ stack is shown in Fig. 1, where a Co70.5Fe4.5Si15B10 layer that is about 50 nm thick is coupled to the free layer (3.4-nm Co20Fe60B20) through a 0.5-nm Ru layer to reduce the magnetic hysteresis of the free layer and guide magnetic flux, where the Co70.5Fe4.5Si15B10 compound is an amorphous soft magnetic material [5]. This Co70.5Fe4.5Si15B10 layer is adjacent to an antiferromagnetic Ir20Mn80 layer, so the Co70.5Fe4.5Si15B10 layer and free layer are expected to be pinned by the antiferromagnetic layer after field annealing. The MTJ devices were annealed multiple times in different conditions to set orthogonal exchange bias for the reference layer and free layer. With external magnetic flux concentrators added to the sensor to form double staged magnetic flux concentrators, the output voltage was measured in the field range from about -0.043 Oe to about +0.043 Oe, as shown in Fig. 2. With double staged magnetic flux concentrators, the sensor exhibits a sensitivity of 3.82×103 %/Oe (3.82×104 %/mT). The studied magnetoresistive sensor has great potential for application in low-field measurement at room temperature.References: [1] C. Zheng et al., IEEE Transactions on Magnetics, vol. 55, no. 4, pp. 1-30 (2019) [2] Y. Yang, A. Sokolov, X. Yin, J. Hua, Y.-F. Liu, S. Lamichhane, and S.-H. Liou, IEEE Transactions on Magnetics, vol. 59, no. 1, pp. 1-7 (2023) [3] S. Lamichhane, Y. Yang, A. Sokolov, X. Yin, Y.-F. Liu, and S.-H. Liou, Appl. Phys. Lett. 124, 212401 (2024) [4] R. Ferreira, E. Paz, P. P. Freitas, J. Wang, and S. Xue, IEEE Transactions on Magnetics, vol. 48, no. 11, pp. 3719-3722 (2012) [5] D. Kato, M. Oogane, K. Fujiwara, T. Nishikawa, H. Naganuma, and Y. Ando, Appl. Phys. Express 6, 103004 (2013)