Morphology and magnetic properties of submonolayer Gd films

We report on scanning tunneling microscopy (STM) measurements on ultrathin Gd films in the thickness range $0.5<~d_{\mathrm{Gd}}<~10\mathrm{ML}$ [1 monolayer (ML)=2.89 Å] grown on a Y (0001) buffer layer at substrate temperatures $T_s=473\mathrm{K}$ and $T_s=573\mathrm{K}$. The magnetic properties were investigated on these films capped with a 100-Å Y layer with the transverse magneto-optical Kerr effect. A ferromagnetic signal was found down to the sub-ML regime $d_{\mathrm{Gd}}\approx 0.5\mathrm{ML}$ for $T_s=473\mathrm{K}$. In this regime, the STM images show a lateral growth of ML thick Gd islands of hexagonal shape. The depression of the Curie temperature $T_C$ follows a power law $\Delta T_C\propto d_{\mathrm{Gd}}^{-\lambda }$ with an exponent $\lambda \approx$1 up to $d_{\mathrm{Gd}}\approx 3–5\mathrm{ML}$, where $\lambda$ changes to $\lambda =1.6\mathrm{}$. This transition is also observed in the same thickness range for the exponent $\beta$ of the magnetization $M$.