Ferromagnetic resonance in tunnel junctions: Mag-noise and complex impedance analysis.

Ferromagnetic resonance (FMR) in the Co[SUB80]Fe[SUB20]/Ni[SUB80]Fe[SUB20] free (sense) layer of micron-sized, magnetic tunnel junction (MTJ) devices is studied by two different techniques. The first method employs the MTJ device's magnetoresistance, and measures thermally excited magnetization fluctuations (mag-noise) as a resistance noise spectrum. Correlations between magnetoresistance and FMR frequency versus in-plane magnetic field have been observed for different devices. The FMR frequencies agreed well with calculations when including substantial out-of-plane magnetic anisotropy fields. The second method detects the change in the complex impedance of the MTJ before and after applying a dc current, as measured by a vector network analyzer. The latter method seems to be very sensitive to internal nonuniform states of the free layer magnetization. [ABSTRACT FROM AUTHOR]