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

300-{} Gd films grown on W(110) under UHV conditions have been investigated Iin situR by ac-susceptibility measurements above and below Tc. A sharp peak of χac=230 is observed. To our knowledge, this is the largest and sharpest (2{σ}=1 K) cusp measured for a rare-earth ferromagnet. No domain-wall signal is superimposed upon the divergence of the paramagnetic susceptibility. At Tc⁺ we find {χ}=χ₀{t}^{{{-}}{γ}}with γ=1.235(25). This is clear evidence for a three-dimensional Ising-like behavior above{T}c$. The sharpness and peak position of {χ}(T) change, depending on the annealing temperature. Simultaneously, a change in the LEED pattern is observed.