Abstract
We use spin-torque-driven ferromagnetic resonance to study the mode structure of spin waves in elliptically-shaped MgO-based magnetic tunnel junction (MTJ) nanopillars with different aspect ratios (AR) of a semiminor axes of $<50\text{ }\text{nm}$ and semimajor axes of $<100\text{ }\text{nm}$. We find that only one quasiuniform precession mode exists in the small AR samples, whereas, in the samples with large ARs, precession evolves into multiple modes, i.e., a quasiuniform mode and an additional nonuniform mode are simultaneously observed. The spatially nonuniform mode is a spin-wave mode localized in the edge regions, which can be well understood by considering the pinning conditions at the lateral boundaries. The appearance of the nonuniform modes results in the increase in the critical switching current density $({J}_{c})$ in MTJ nanopillars.
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