Critical behavior of strained epitaxial Gd films:In situac-susceptibility measurements in UHV

300-Å Gd films grown on W(110) under UHV conditions have been investigated Iin situR by ac-susceptibility measurements above and below ${\mathit{T}}_{\mathit{c}}$. A sharp peak of ${\mathrm{\chi }}_{\mathrm{ac}}$=230 is observed. To our knowledge, this is the largest and sharpest (2σ=1 K) cusp measured for a rare-earth ferromagnet. No domain-wall signal is superimposed upon the divergence of the paramagnetic susceptibility. At ${\mathit{T}}_{\mathit{c}}^{+}$ we find χ=${\mathrm{\chi }}_0$ ${\mathit{t}}^{\mathrm{-}\gamma }$ with γ=1.235(25). This is clear evidence for a three-dimensional Ising-like behavior above ${\mathit{T}}_{\mathit{c}}$. The sharpness and peak position of χ(T) change, depending on the annealing temperature. Simultaneously, a change in the LEED pattern is observed.