Dimensional crossover in ultrathin Ni(111) films on W(110)

IIn situR magnetic resonance measurements of ultrathin Ni(111) on W(110) prepared in ultrahigh vacuum have been carried out to determine the thickness dependence of the critical exponent \ensuremath{\beta} and the Curie temperature ${\mathit{T}}_{\mathit{c}}$. We find that \ensuremath{\beta} changes drastically from 0.30 to 0.13 between 7 and 5 monolayers. We attribute this behavior to a crossover from a three- to a two-dimensional magnetic behavior. The observed ${\mathit{T}}_{\mathit{c}}$'s and linewidths also exhibit the effect of the dimensional crossover. Ni films \ensuremath{\le}4 layers thick show a truly two-dimensional magnetic phase transition with \ensuremath{\beta}=0.13.