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Arizona State University, USA

The growth, structure, and magnetism of Fe films on the W(111) surface were investigated using low energy electron microscopy (LEEM) and diffraction (LEED), spin polarized LEEM (SPLEEM), and work function measurements. In contrast to an earlier report that Fe grows with fcc structure following initial pseudomorpic layer growth, we observe no evidence of the formation of fcc Fe over the entire thickness range studied, up to 18 monolayers (ML). Observations are instead consistent with the formation of a well-ordered, laterally (tensile) strained bcc Fe layer that gradually relaxes vertically and develops increasing disorder with increasing thickness. Ferromagnetic order appears at 6 ML, but surprisingly vanishes at 8 ML, and reappears just as suddenly at 9 ML during Fe deposition at room temperature. Ferromagnetism between 6 and 8 ML also vanishes at only 5 deg above room temperature. The magnetization direction of a monodomain structure remains constant before and after the ferromagnetic thickness gap at 8–9 ML until the formation of a multidomain structure at about 12 ML. Variations of exchange asymmetry in spin-polarized elastic electron scattering are also observed with increasing film thickness, particularly above 12 ML, that indicate changes in the spin-polarized electron band structure above the vacuum level. The evolution of magnetism and exchange asymmetry with increasing thickness and the appearance of the ferromagnetic gap are attributed to structural and morphological changes in the strained Fe layer, which eventually lead to a relaxed although highly disordered bcc Fe layer.

The growth morphology, structure, and magnetism of ultrathin Co films on the W(111) surface are studied with conventional and spin polarized low energy electron microscopy and diffraction and first principles total energy calculations. Pseudo-layer-by-layer growth of thick pseudomorphic (ps) Co films is observed at 380 K, while Stranski-Krastanov growth and transformation to a (6×6) closed-packed structure are observed at higher temperatures. On the basis of the calculations, only one ps Co monolayer is expected to be thermodynamically stable against three-dimensional island formation. Calculations also indicate that the magnetic moment on Co atoms at the interface is reduced due to strong hybridization with the substrate, but remains non-zero even for the single ps monolayer. Metastable ps Co/W(111) is found experimentally to be ferromagnetically ordered at 380 K when film thickness exceeds 7.6 ps ML. Although only in-plane magnetization is observed, substrate atomic steps have a strong influence on the magnetization easy axis and magnetic domain structure, which suggests that the in-plane magnetocrystalline anisotropy is weak.

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Effective potentials for exchange and correlation are derived by drastically simplifying the Hartree-Fock equations for the system (atom + free electron) and applying some ideas from the theory of the free electron gas. The resulting uncoupled differential equations for the free electron partial waves are solved numerically. Total and differential elastic scattering cross sections are computed for H, He, Ne, Ar, Kr, Zn, Li, Na, K, N, and O. Good agreement is found in general with other semi-empirical or more elaborate calculations and with experiment. (Author)