Evidence for domain formation near the Curie temperature in ultrathin Ni/Cu (001) filmswith perpendicular anisotropy

Ni films, 8–10 monolayers thick on a Cu(001) substrate, are studied in UHV by the polar magneto-optic Kerr effect near the Curie temperature ${\mathrm{T}}_{\mathrm{C}}$. The films are magnetized normal to the surface. An unusual sharp drop of the remanent magnetization (${\mathrm{M}}_{\mathrm{r}}$) and a large difference of about 10 K are measured between the temperature ${\mathrm{T}}_{\mathrm{r}}$ at which ${\mathrm{M}}_{\mathrm{r}}$ and the temperature ${\mathrm{T}}_{\mathrm{H}}$ at which the magnetization M(H) in a static field of 20 G vanishes. We show that at ${\mathrm{T}}_{\mathrm{r}}$ the films break up into ferromagnetic domains with perpendicular magnetization components and that ${\mathrm{T}}_{\mathrm{C}}$ is larger than ${\mathrm{T}}_{\mathrm{r}}$. The true ${\mathrm{T}}_{\mathrm{C}}$ is deduced from field-dependent magnetization measurements.