Voltage-controlled switching of CoFeB/MgO magnetic tunnel junctions has been analyzed using micromagnetic simulation that includes thermal fluctuations and surface roughness. It is shown that the large samples typically studied experimentally switch by domain wall motion, nucleated at points where the CoFeB is slightly thicker. This is a consequence of small thickness variations producing large percentage changes in the anisotropy owing to the near balance of interface anisotropy and shape anisotropy in these films. It is also found that these films are likely never saturated at the experimental fields employed owing to the importance of thermal fluctuations in the near 2-D geometry.
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