Finite-Size Effects and Anisotropic Behavior in Thin Film Ferromagnetism.

Magnetic behavior of thin films were studied using the surface magneto-optic Kerr effect under different conditions by varying film thickness, substrate temperature, surface morphology, crystal symmetry and film-substrate combination. These studies provide a comprehensive understanding on thin-film magnetism. High quality thin films of Co, Ni and their alloys epitaxially grown on Cu(001) have been extensively studied and exhibit a variety of new phenomena: (i) the Curie temperature T_{c} decreases monotonically as a function of film thickness, and the thickness dependence of T_{c} is found to be well-described by a modified finite-size scaling relation over a wide thickness range down to the monolayer limit; (ii) as film thickness decreases, the magnetic behavior undergoes a dimensionality crossover from three dimensions (3D) to two dimensions (2D) at a finite thickness; (iii) in 2D regime, these films exhibit finite-size XY magnetic behavior, which can be considered as an experimental realization of the Kosterlitz-Thouless-Berezinskii phase transition. A drastic enhancement of perpendicular anisotropy induced by transition metal overlayers has been observed in ultrathin Co(111) films. This phenomenon raises interesting questions pertinent to the basic mechanisms of magnetic anisotropy on one hand, and prompts technological applications on the other. Another new phenomenon, a "spin reorientation transition" has been observed in the alloy films of Fe and Pd on Cu(001) on the scales of both thickness and temperature. The magnetization easy axis is perpendicular to the film plane at small thickness and switches to in-plane at large thickness. Such behavior is consistent with the conventional competing shape vs. surface anisotropy picture. However, there is an opposite spin reorientation from in-plane to perpendicular with increasing thickness in the Ni films on Cu(001), showing the complexity of the anisotropic behavior of the magnetic thin films.