Description
- Abstract:
- This project's primary objective is to apply a quantum device called the magnetic tunnel junction (MTJ) for ultrasensitive magnetic field sensing. Towards this goal, we have systematically improved the MTJ system using novel physical ideas. Firstly, by optimizing the magnetron sputtering conditions of the magnetic flux concentrator (MFC), we managed to enhance the field detectability of MTJ to unprecedented 30 pT/√Hz at 10 kHz. As another way to boost sensitivity, we have also tuned the magnetic anisotropy in the MTJ free layer, where an interfacial perpendicular magnetic anisotropy (PMA) is induced. By tuning PMA with temperature, a compensation between PMA and demagnetization energy enhances the MTJ sensitivity to 1.2 %/Oe. Besides, an interesting voltage-dependent random telegraph noise is discussed. As the MTJ sensitivity is greatly enhanced by MFC and PMA, the intrinsic hysteresis still prevents MTJ from detecting small fields. Therefore we have attempted to eliminate the hysteresis at a fundamental level using a superparamagnetic free layer. We experimentally observed a transition from ferromagnetic to superparamagnetic states by raising the temperature, and the sensor in the latter state is non-hysteretic with good sensitivity. We have also resorted to the vortex magnetic states, where a topologically protected vortex magnetization in the MTJ free layer can evolve without hysteresis. The generation of vortex magnetization was revealed using the MUMAX simulation package first. We then experimentally fabricated vortex MTJ sensors, with outstanding sensing performances such as a sensitivity of 0.3 %/Oe, a thermal stability of 400 ppm/K, and an extensive dynamic range of 200 Oe. In brief, we have used MFC and PMA as methods to raise the sensitivity of MTJ. We have taken advantage of the superparamagnetic effect and vortex magnetic states to eliminate ferromagnetic hysteresis. From these results, we have generalized the scaling relation between the magnetic sensitivity and noise of MTJs in the ferromagnetic, superparamagnetic and vortex states, to propose that hysteresis elimination is indispensable for a favorable linear scaling. To demonstrate the application prospect for MTJ sensors with our improvements, we have developed a magnetic gradiometer prototype and applied the device in the wellbore crack detection. The gradiometer can detect defects with a large stand-off distance of 22 mm.
- Notes:
- Thesis (Ph. D.)--Brown University, 2021
Citation
He, Guanyang,
"Ultrasensitive and Hysteresis-free Magnetic Sensing using Magnetic Tunnel Junctions"
(2021).
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://repository.library.brown.edu/studio/item/bdr:9rsekz9t/
