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 nomin
al Fe coverages of t(Fe) greater than or equal to 2.8 monolayers (ML), a fe
rromagnetic state is observed below a certain critical temperature, T-C. Su
rprisingly, 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 m
ay be due to the extremely short lateral length scale of film inhomogeneiti
es. 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 temper
ature. T-C decreases steeply with decreasing Fe coverage, with an average s
lope 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.