Quasi-two-dimensional ferromagnetism in polycrystalline Fe

Ultrathin polycrystalline Fe films of an average thickness 5 Å are prepared on a Ta substrate by ion-beam sputter deposition in ultrahigh vacuum. The Ta itself is deposited on top of a three-dimensional (3D) ferromagnet, and it acts as a nonmagnetic metallic spacer layer through which exchange fields ${\mathit{H}}_{\mathrm{ex}}$ of various strengths are transferred from the magnetic substrate into the overlayer ultrathin film. The temperature dependence of the magnetization M(T) of the overlayer film is obtained from measurement of the spin polarization of the low-energy cascade electrons. M(T) depends critically on the thickness x of the Ta nonmagnetic spacer layer. The magnetic transition of the overlayer film is increasingly smeared out as ${\mathit{H}}_{\mathrm{ex}}$ increases, yet the transition temperature ${\mathit{T}}_{\mathit{C}}$ is clearly defined down to x=3 Å. In the context of crossover from 3D to 2D magnetism, ${\mathit{T}}_{\mathit{C}}$ increases with decreasing x in a manner consistent with the theory of phase transitions in quasi-2D systems. Adsorption of residual gas molecules such as CO changes the shape of M(T) but does not affect ${\mathit{T}}_{\mathit{C}}$. The nature of the changes suggests that adsorbed residual gas molecules act as surfactants that spread the film slightly on the substrate, thereby converting it from the superparamagnetic to the ferromagnetic state.