Anisotropy Of Ultrathin Films Research Articles

Magnetism and Magnetic Interactions at Transition Metal Surfaces and Interfaces

ABSTRACTIn the exciting field of low dimensional magnetic systems including surfaces, interfaces and thin-films, local spin density (LSD) functional ab initio electronic structure calculations have played a key role by not only providing a clearer understanding of the experimental observations but also predicting new systems with desired properties. Our extensive calculated results demonstrate that: (i) Magnetic clean surfaces or interfaces with inert substrates undergo strong magnetic moment enhancements; (ii) the strong interaction with nonmagnetic transition metals diminishes (completely in some cases) the ferromagnetism and usually stabilizes the antiferromag-netic configuration. By including spin-orbit coupling as a perturbation, (i) reliable results for the Magneto-crystalline anisotropy of ultra-thin films can be obtained using the state-tracking procedure, although the anisotropy energy is a few tenths of a MeV; (ii) spectra of the Magneto-optical Kerr effects and magnetic circular dichroism in the soft x-ray region can be determined.

Influence of roughness on magnetic surface anisotropy in ultrathin films

A magnetostatic model for shape anisotropy of an ultrathin film with surface roughness is presented. Apart from the usual volume anisotropy, an additional perpendicular surface anisotropy results from the surface roughness. For a continuous film, the shape anisotropy follows the famous 1/t behavior. By considering the correlation between the surfaces, the perpendicular surface anisotropy can be further increased. When the film thickness is less than a certain value, the magnetic layer may become a discontinuous or island-like structure and the shape anisotropy is proportionally lower with decreasing film thickness. With a nonzero quality factor (Q=Kc/2πMs2), a marked kink in the dependence Kefft on the film thickness gradually appears with increasing roughness. Observed results agree qualitatively with predictions based on the present model.

Perpendicular anisotropy of ferromagnetic ultrathin films from anisotropic exchange interactions.

We calculate the perpendicular anisotropy in ultrathin films using a phenomenological anisotropic exchange interaction. We find that the experimental observations of the thickness and temperature dependence of the perpendicular anisotropy can be explained by this heuristic approach.

ANISOTROPIES IN ULTRATHIN FILMS OF IRON GROWN ON SILVER

Ferromagnetic resonance and Brillouin light scattering have been used to measure the uniaxial magnetic anisotropies in ultrathin films of Fe(001) grown epitaxially on Ag(001) substrates. Brillouin light scattering has also been used to obtain magnetic surface anisotropies for bulk iron. Surface anisotropies obtained for the bulk (001) surface are compared with the uniaxial magnetic anisotropies obtained using thin films, and with the theoretical predictions of Gay and Richter. The bulk surface anisotropies are in good agreement with those measured on thin films and also with the theoretical calculations based upon the reduced symmetry at a surface.

Large surface anisotropies in ultrathin films of bcc and fcc Fe(001) (invited)

Large uniaxial anisotropies associated with interfaces are observed for ultrathin films (3-28 ML) of bcc Fe(001) grown epitaxially on Ag(001) single-crystal substrates and for epitaxial sandwiches of fcc Fe(001) grown with three layers of Fe using Cu as substrate and coverlayers. The uniaxial anisotropy is well described by a pseudosurface anisotropy term as theoretically predicted, yet that theory also predicts large in-plane anisotropies that are not observed. Adequate treatment of spin-orbit coupling in magnetic theories remains a challenge. Comparisons of ultrathin films of bcc Fe(001) on Ag(001) with different coverlayers of Ag or Au show subtle differences in magnetic behavior as studied by ferromagnetic resonance (FMR) and Brillouin light scattering (BLS). The FMR measurements were carried out at 9.6, 36.6, and 73 GHz microwave frequencies. The BLS measurements were performed using a six-pass Fabry–Perot interferometer. The power of the techniques of molecular-beam epitaxy (MBE) for producing well-characterized interfaces is stressed. Growths at 140 K are compared with those at 300 K to show the limited role of interdiffusion. Oscillations in the intensity of reflection high-energy electron diffraction (RHEED) are exploited in the characterization of growth. Comparison of the properties of films grown on perfect iron whisker surfaces with results for mosaic single-crystal substrates show that, while the former are much to be preferred for growth studies, the latter are really adequate for magnetic studies.