Magnetic hyperfine fields at uncovered ultrathin Ni films on Cu(100) substrates and at single-crystal Ni surfaces

Magnetic and electric hyperfine fields were measured at 111 In( 111 Cd) probe atoms in the topmost atomic layer of Ni(111) and Ni(100) single-crystal surfaces as well as in ultrathin Ni films epitaxially grown on Cu(100), utilizing the perturbed γγ-angular correlation (PAC) method. The behaviour of the magnetic hyperfine fields as a function of temperature were studied for both Ni surfaces as well as for different Ni(100) films with thicknesses ranging from 2 up to 10 monolayers. It was found that strength and orientation of the magnetic hyperfine fields are quite different for these systems. The critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness, whereas at the surfaces no change in the Curie temperatures was detected.