Enhanced ferromagnetic resonance linewidth of the free layer in perpendicular magnetic tunnel junctions

We report the frequency dependence of the ferromagnetic resonance linewidth of the free layer in magnetic tunnel junctions with all perpendicular–to–the–plane magnetized layers. While the magnetic–field–swept linewidth nominally shows a linear growth with frequency in agreement with Gilbert damping, an additional frequency–dependent linewidth broadening occurs that shows a strong asymmetry between the absorption spectra for increasing and decreasing external magnetic field. Inhomogeneous magnetic fields produced during reversal of the reference and pinned layer complex is demonstrated to be at the origin of the symmetry breaking and the linewidth enhancement. Consequentially, this linewidth enhancement provides indirect information on the magnetic coercivity of the reference and pinned layers. These results have important implications for the characterization of perpendicular magnetized magnetic random access memory bit cells.