Spin reorientation transition in Co/Au multilayers

Wereport a study about the spin reorientation transition (SRT) fromperpendicular easy axis to in-plane easy axis of magnetization in Co/Au multilayers. A series of multilayers of Si/Au(100 Å)/{[Co(tCo)/Au(20 Å)]20}/Au(50 Å) family were studied, with Co layer thickness varying between 6 Å to 30 Å. The thickness of the Au layer was chosen large enough in order to minimize the interlayer exchange coupling between Co layers. In such thick Au-layer samples the magnetic properties aremainly the result of competition between interlayer magnetostatic coupling due to stray field, perpendicular magnetic anisotropy and shape anisotropy. The effective anisotropy constant Keff and the second order anisotropy K2 were deduced from the fit of the resonant magnetic field obtained from out of plane dependence Ferromagnetic Resonance (FMR) experiments. To study the SRT, we have plotted the phase diagram between Keff and K₂. The results showthat SRT occurs through the metastable regionwith K₂≤-1/2 Keff, (Keff N 0). It is interesting to note that FMR shows the coexistence of twomodeswith different anisotropy for small Co thickness,while for thick Co layers themodes have the sameanisotropy.Moreover, in thick Co layer samples, volume and surface spin wave resonance (SWR) modes were also excited by the microwave field, around the perpendicular FMR geometry, giving a clear evidence of a magnetic coupling between the Co layers.