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Optimizing Magnetic Nanostructures and Microstructures for Ultrahigh-Sensitivity Magnetoresistive Sensors

Yi Yang, University of Nebraska - Lincoln


In this dissertation, the magnetic nanostructures and microstructures of magnetic tunnel junction (MTJ) based magnetoresistive sensors are optimized for ultrahigh sensitivity of magnetoresistive sensors at room temperature. Firstly, it is demonstrated that even with the total magnetic thickness being several micrometers, laminated magnetic films can exhibit significantly reduced coercivity and saturation field compared to single-layer films with the same total magnetic thickness. Secondly, an integrated structure that connects micro magnetic flux concentrators (MFC) and MTJs to reduce loss of magnetic flux density is designed and studied, where laminated micro MFCs are utilized. In this novel structure, the laminated micro MFCs exhibit a gain of about 24. With external MFCs of centimeter scale added, the double staged MFCs show a very high gain of about 2760. Moreover, a world-leading sensitivity as high as 4.71×10^3 %/Oe (4.71×10^4 %/mT) is demonstrated at room temperature. Thirdly, an optimized structure that combines a new layer structure of MTJ, where free layers are slightly pinned by antiferromagnetic layers in a direction perpendicular to the sensing direction, with the novel integrated structure is studied for improved overall performance of magnetic tunnel junction based magnetoresistive sensors. It is demonstrated that in the optimized structure, the magnetic anisotropy of the free layer can be tuned. The fine-tuned magnetoresistive sensor exhibits reduced hysteresis, which results in improved reversibility and linearity, and a sensitivity as high as 3.82×10^3 %/Oe (3.82×10^4 %/mT). In addition, a simple and practical ac modulation method to reduce the hysteresis in magnetic tunnel junction based magnetoresistive sensors is demonstrated. It is shown that with a suitable ac modulation magnetic field that is applied along the sensing direction, the hysteresis in the sensors can be reduced to a negligible level. The progress achieved in this dissertation paves the way for the development of room-temperature ultrahigh-sensitivity magnetoresistive sensors for femtotesla magnetic field measurement.

Subject Area

Condensed matter physics|Materials science|Electromagnetics

Recommended Citation

Yang, Yi, "Optimizing Magnetic Nanostructures and Microstructures for Ultrahigh-Sensitivity Magnetoresistive Sensors" (2021). ETD collection for University of Nebraska-Lincoln. AAI28865305.