Onset of ferromagnetism in Fe epitaxially grown on GaAs(001) (4x2) and (2x6)

Ultrathin Fe films were epitaxially grown at room temperature on GaAs(001) with either predominant (4 x 2) or (2 x 6) surface reconstruction. At nominal Fe coverages of t(Fe) greater than or equal to 2.8 monolayers (ML), a ferromagnetic state is observed below a certain critical temperature, T-C. Surprisingly, the magnetic phase transition at T-C appears even sharper than for Fe films on metallic single-crystal substrates, which were believed to be an excellent representation of two-dimensional (2D) ferromagnets. This may be due to the extremely short lateral length scale of film inhomogeneities. The critical exponent beta = 0.26 is close to the value expected for 2D XY systems of finite size. For t(Fe) = 3.6 ML, T-C is close to room temperature. T-C decreases steeply with decreasing Fe coverage, with an average slope of 270 K/ML. From a power law extrapolation, T-C seems to vanish at t(Fe) = 2.5 ML. The onset of ferromagnetism at t(Fe) = 2.5 ML is interpreted as a percolation phenomenon during the coalescence process of Fe islands. (C) 2001 American Institute of Physics.