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Ferromagnetic, pure ZnO films were grown on Al2O3 substrates at various nitrogen pressures (0.01–1.0 mbar) and investigated with x-ray diffraction (XRD) and x-ray absorption spectroscopy. According to XRD data, the crystalline films were composed of crystallites of approximately 50 nm in size, oriented with respect to the substrates, and the lattice spacings show slight deviations with respect to single-crystalline ZnO of wurtzite structure. The parameters determined by XRD agree with those determined by extended x-ray absorption fine structure, except for the sample grown at the lowest N2 pressure of 0.01 mbar, which was attributed to deviations from the ZnO single crystals. The results for the ZnO films grown at 0.1 to 1.0 mbar partial N2 pressure indicate wurtzite unit cells compressed along the c axis. The x-ray absorption near-edge structure (XANES) spectra exhibited a strong dependence on the x-ray polarization and on nitrogen partial pressure, which was explained by the increase in the concentration of defects with nitrogen partial pressure and by interface or grain boundary effects. First-principles calculations using multiple-scattering formalism suggested that the XANES spectra changes were because of increasing Zn vacancy concentration with increasing nitrogen pressure. The results indicated that Zn vacancy defects play a significant role in the ferromagnetism of these films.

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Magnetic dichroism with unpolarized x rays in angle-resolved photoemission has been systematically investigated for the 2p core levels of Fe, Co, and Ni. In contrast to Fe 2p, we find the dichroism in the Ni 2p spectra to be mainly governed by electronic correlations. The fine structure observed in the Co 2p spectrum indicates a distinct influence of many-body interactions also in this material. ͓S0163-1829͑96͒50446-7͔Electronic interactions govern many phenomena in solidstate physics. Ferromagnetism, for instance, is a direct consequence of spin-dependent electronic correlations. Besides their influence in the ground state, correlations are very important for understanding electron spectroscopy results. These techniques always probe the many-electron system, even if the results may often be interpreted in the singleparticle limit. A famous example for the inadequacy of the single-particle picture is photoemission from Ni. Although a density functional approach can reproduce the average exchange splitting found in the experiment by a proper choice of the exchange correlation potential, 1 it fails to explain the photoemission satellite structures found, 2 yet another manifestation of electronic correlations. Their interpretation requires suitable many-electron formalisms, for example, a Hubbard-type approach as has been successfully applied to Ni. 3 A narrow d band in Ni-as compared to Fe and Co-is held responsible for the formation of correlation-induced spectral features. This bandwidth argument leads to an exciting aspect if surfaces and thin films are considered. It is well-known that the smaller number of nearest-neighbor atoms at the surface causes a dehybridization of the wave functions and thereby a band narrowing. A similar effect happens in monolayer films as a consequence of the reduced dimensionality, as long as hybridization with the substrate is negligible. As pointed out recently by Chen, 4,5 many-particle effects may thus be enhanced in systems of reduced dimensions.Magnetodichroic phenomena in angle-resolved photoemission appear for various experimental situations. 6-9 Previous studies have centered on the role of experimental geometry, light polarization, and excitation energy. The question of how the presence of electronic correlations will affect the magnetodichroic spectra remained unanswered. In this paper we demonstrate the strong influence of spindependent electronic correlations on the magnetic linear dichroism in 2 p photoemission. In order to make contact to explicitly spin-resolved results, the dichroism was measured with unpolarized x rays. On the one hand, the Fe 2 p spectra are similar to those observed earlier with circularly polarized light 6 and can be understood within a single-particle picture. On the other hand, we find the highest magnetodichroic signal for Ni 2p in the 6 eV satellite. In this case the magnetic dichroism is clearly dominated by correlation effects. The fine structure in the Co 2 p spectra from a 5 monolayer ͑ML͒ thick film shows evidence for a many-body satell...

The electronic structure of the central iron ion of perfluorinated iron phthalocyanine (FePcF 16 ) in thin films has been studied on Cu(111) and Ag(111) using polarization dependent X-ray absorption spectroscopy (XAS). The data are compared to FePc on Ag(111). Ligand field parameters have been computed, and multiplet calculations (CTM4XAS) were carried out to simulate XAS spectra. The planar molecules are preferentially oriented lying flat on the substrate surface during the growth of the 1−4 nm thick films. A clear polarization dependence of the Fe L edge absorption spectra is observed, arising from transitions into orbitals with in-plane and out-of-plane character. The shape of the spectra for three to four monolayers of FePcF 16 on Cu(111) is comparable to that of the thin films of FePc on Ag(111). However, a drastic change of the XAS peak shape is observed for thicker FePcF 16 films on both Ag(111) and Cu(111), although the molecular orientation is very similar to coverages consisting of a few monolayers. Since in both cases the film thickness is distinctly beyond the monolayer regime, interface interactions can be ruled out as a possible origin of this behavior. Rather, the different XAS peak shapes seem to indicate that the multiplicity may depend on the detailed arrangement of the FePcF 16 molecules. The large flexibility of the ground state of Fe could be of high interest for spintronic applications.

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We present a first-principles study of electronic and magnetic properties of thin Co films on a BaTiO3(0 0 1) single crystal. The crystalline structure of 1-3 monolayer thick Co films was determined and served as input for calculations of the electronic and magnetic properties of the films. The estimation of exchange constants indicates that the Co films are ferromagnetic with a high critical temperature, which depends on the film thickness and the interface geometry. In addition, we calculated x-ray absorption spectra, related magnetic circular dichroism (XMCD) and linear dichroism (XLD) of the Co L 2, 3 edges as a function of Co film thickness and ferroelectric polarization of BaTiO3. We found characteristic features, which depend strongly on the magnetic properties and the structure of the film. While there is only a weak dependence of XMCD spectra on the ferroelectric polarization, the XLD of the films is much more sensitive to the polarization switching, which could possibly be observed experimentally.

This work focuses on the generation of ferromagnetism at the surface of anatase TiO 2 films by low-energy ion irradiation. Controlled Ar +-ion irradiation resulted in a thin (∼10) nm ferromagnetic surface layer. The intrinsic origin and robustness of the magnetic order has been characterized by x-ray magnetic circular dichroism at room temperature revealing that a Ti band is spin-polarized. These results, together with density functional theory calculations, indicate that Ti vacancy-interstitial pairs are responsible for the magnetic order. Superconducting quantum interference device measurements show the existence of a perpendicular magnetic anisotropy and a low remanent magnetization. Magnetic force microscopy reveals that this low remanence is due to oppositely aligned magnetic domains with magnetization vectors normal to the main surface. The weak domain-wall pinning, the magnetic anisotropy, together with the simplicity of the preparation method, open up interesting possibilities for future applications. As an example, single domain patterns of ∼1 μm width and several μm length can be easily prepared.