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 TC. The films are magnetized normal to the surface. An unusual sharp drop of the remanent magnetization (Mᵣ) and a large difference of about 10 K are measured between the temperature Tᵣ at which Mᵣ and the temperature TH at which the magnetization M(H) in a static field of 20 G vanishes. We show that at Tᵣ the films break up into ferromagnetic domains with perpendicular magnetization components and that TC is larger than Tᵣ. The true TC is deduced from field-dependent magnetization measurements.