Brillouin study of long-wavelength spin waves in quasimonatomic Co films with uniaxial perpendicular magnetic anisotropy

We have observed spin-wave Brillouin light scattering from ultrathin Co/Au/Cu(111) films with Co thicknesses $t_{\mathrm{Co}}$ down to 1 monolayer (ML) and with a 1-ML Au interlayer. The detection of a well-defined spin-wave spectrum and the field dependence of its frequency show directly long-range collective and ferromagnetic ordering in these films at room temperature. From the field dependence of the spin-wave frequency, we derive uniaxial perpendicular magnetic anisotropy constants as a function of $t_{\mathrm{Co}}$ with various overlayer materials, including Cu, Pd, and Au. With a Cu overlayer, we observe that the first-order perpendicular anisotropy $K_u^{\mathrm{}\left(1\right)\mathrm{}}$ obeys well a linear relation between $K_u^{\mathrm{}\left(1\right)\mathrm{}}{t}_{\mathrm{Co}}$ and $t_{\mathrm{Co}}$ for $t_{\mathrm{Co}}\geqq 1.5\mathrm{ML},$ which indicates a constant contribution of the interface anisotropy of 0.16 ${\mathrm{m}\mathrm{J}/\mathrm{m}}^2$ in addition to the volume anisotropy of 0.73 ${\mathrm{M}\mathrm{J}/\mathrm{m}}^3$. With an Au or a Pd overlayer, we find that both the interface and volume anisotropies are significantly larger than those with the Cu overlayer. We quantify magnetic inhomogeneities from the field dependence of the spectrum width. With the Au or Pd overlayer, $K_u^{\mathrm{}\left(1\right)\mathrm{}}$ shows a steep decrease with decreasing $t_{\mathrm{Co}}$ for $t_{\mathrm{Co}}<3.0\mathrm{}\mathrm{ML},$ which agrees well with a significant increase in the structure-related magnetic inhomogeneity. We show directly that long-ranged ferromagnetic ordering exists, with the perpendicular anisotropy, in our quasimonatomic Co films thinner than 1.5 ML. $K_u^{\mathrm{}\left(1\right)\mathrm{}}$ for each overlayer tends to be zero at 1 ML of Co, accompanied by heavy damping of the spin wave. In addition, we find the second-order perpendicular anisotropy is still maintained with a comparable value to $K_u^{\mathrm{}\left(1\right)\mathrm{}}$ in such quasimonatomic Co films, indicating significant deformation of the uniaxial anisotropy.