Description
- Abstract:
- We systematically studied the spin-dependent transport and electrical noise characteristics of MgO-based magnetic tunnel junctions (MTJs). Utilizing the coherent tunneling effect in the MgO tunnel barrier, we have successfully fabricated MTJ devices with large tunneling magnetoresistance (TMR) ratios. To explore the magnetization properties of these devices at a fundamental level, we first investigated the magnetization dynamics of sputtered CoFeB thin films used as the free layers in MgO-based MTJ stacks by a broad-band FMR spectrometer. Then we presented a new method for estimating the magnetic anisotropy dispersion in MTJ arrays in serial configurations, using the simulated field sensitivity maps. Based on a revised Stoner-Wohlfarth model, we were able to assign dispersion parameters to an actual MTJ array with arbitrary magnetic attributes such as TMR ratio, coercivity and hysteresis loops. As a result of our work, the field sensitivity of an MTJ array was found to be inversely correlated to its anisotropy dispersion and magnetic coercivity.<br/> <br/> In the second part, we focused on the noise properties of MTJ systems. At low frequencies, the flicker noise was measured in two distinct MTJ arrays: MTJ Wheatstone bridges and MTJ discrete resistors, each in their own serial arrays. For MTJ bridges, the statistical dispersion in device resistance and normalized voltage noise were attributed to the greater-than-expected magnetization noise, whereas magnetic coupling among tightly-packed MTJ elements was found to amplify the magnetic fluctuations in the MTJ discrete resistors. Finally, high frequency shot noise measurements were carried out to study the spin-dependent charge transport in MTJ systems. The normalized shot noise, or Fano factor, exhibited a sinusoidal-like variation with a continuous change in the relative orientation between the magnetization vectors of MTJ free and reference layers. We explained the noise behavior with a model of sequential tunneling in the spin-blockade regime, in which the faster transport of majority spin electrons in tunnel barriers is modulated by the slower tunneling of minority spin electrons.
- Notes:
- Thesis (Ph.D. -- Brown University (2012)
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Citation
Zhang, Wenzhe,
"Noise and Spin-dependent Transport in MgO-based Magnetic Tunnel Junctions"
(2012).
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z0QF8R5F
