We investigate free-layer size D dependence of effective anisotropy field in nanoscale CoFeB/MgO magnetic tunnel junctions by homodynedetected ferromagnetic resonance. The effective anisotropy field HeffK monotonically increases with decreasing D for a device with the referencelayer size much larger than the free-layer size. In contrast, HeffK does not increase in a monotonic manner for a device with the reference-layer size comparable to the free-layer size. We reveal that the difference can be explained by the variation of the anisotropy field in the vicinity of the device edge.
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