Growth and magnetic properties of ${\mathrm{Co}}_x{\mathrm{Ni}}_{1-x}$ and ${\mathrm{Fe}}_x{\mathrm{Ni}}_{1-x}$ ultrathin films on Cu(100)

We have grown ultrathin Co${}_x$Ni${}_{1-x}$ and Fe${}_x$Ni${}_{1-x}$ films on Cu(100) with varying stochiometry $x$. We find that these alloy films grow in a fcc phase on Cu(100). With the surface magneto-optic Kerr effect we measured the variation of the Curie temperature $T_C$ as a function of the film thickness $n$ in monolayers. Fitting an empirical scaling curve to our results we are able to extrapolate the value $T_C(n=\infty )$ for samples with different stochiometry. We use this framework in order to determine $T_C(n=\infty )$ for Fe${}_x$Ni${}_{1-x}$ alloy films, in particular for concentrations close to 65% Fe content. Bulk Fe${}_{65}$Ni${}_{35}$ shows a collapse of magnetic long-range order and a fcc-to-bcc structural transition, which is the so-called Invar effect. In ultrathin Fe${}_{65}$Ni${}_{35}$ films, we observe a “quenching” of the Invar effect, because growth on a Cu(100) substrate forces the film to adopt the Cu lattice spacing thereby suppressing the structural relaxation.