C. L. Fu

Strongly enhanced 2D magnetism at surfaces and interfaces (invited)

The study of magnetism in low‐dimensional systems has entered a new phase thanks to (i) the advent of sophisticated synthesis and characterization techniques and (ii) the development of highly precise theoretical methods. We describe recent developments and applications of an all‐electron total energy local spin density approach for determining the structural, electronic, and magnetic properties of surfaces, overlayers and interfaces, and sandwiches. Particular emphasis is placed, and results are given, on these structures involving transition metals (V, Cr, and Fe) on noble metals (Cu, Ag, and Au), simple metals (Al), and a nonmagnetic transition metal (W). Magnetic hyperfine fields are given for some Fe systems since conversion electron Mossbauer spectroscopy now permits detailed layer‐by‐layer tests of the theoretical predictions.

Electronic structure and surface magnetism of fcc Co(001)

Abstract A full potential linearized augmented plane wave (FLAPW) all-electron local spin density calculation of the electronic and magnetic properties of both five and nine layer fcc Co(001) ferromagnetic films is reported. The surface magnetic moment of 1.85μ B is 13% larger than that of the bulk value as a result of the narrowing of the 3d band at the surface. The effects of the surface is found to be short-ranged, with changes in charge and spin densities localized mostly to the surface layer. The sub-surface Co atom layers have magnetic moments equal to 1.64μ B , i.e., a value very close to that of bulk fcc Co, indicating a short range effect of the surface on the magnetism (i.e., one atomic layer screening length). A contact magnetic hyperfine field calculation indicates that the core electron contribution is, as usual, precisely proportional to the magnetic moment, while the valence electron contribution is rather sensitive to the atomic environment. The total energy results yield a surface energy for the fcc Co(001) surface equal to 4.1 J/m 2 .

Local spin density total energy study of surface magnetism: V (100)

Abstract Results of self-consistent all-electron local (spin) density functional studies of the electronic and magnetic properties of vanadium (100) 1-, 3-, 5- and 7-layers films are reported using our full-potential linearized augmented plane wave (FLAPW) method. The calculated work function, 4.2 eV, agrees very well with the experimental value of 4.12 eV. From both Stoner factor analyses and spin-polarized total energy calculations, it is concluded that V(100) undergoes a ferromagnetic phase transition only for the monolayer system. The magnetic moment is found to be 3.09μ B per atom of this monolayer film and to have a total energy 57 mRy below that of the paramagnetic structure. For multilayer V(001) systems, the sharp surface density-of-states peak which is characteristic of the occurrence of surface magnetism in the 3d transition metals is located 0.3 eV above the Fermi level. As a result, the paramagnetic state is stable. In addition, no enhancement of the exchange-correlation integral is found for the surface atoms compared with the bulk value. The lower energy of the paramagnetic structure is further supported by total energy investigations of the multilayer relaxation of V(100) — the calculated interlayer spacings for the paramagnetic surface with a 9% contraction of the topmost interlayer spacing and a 1% expansion of the second interlayer spacing with respect to its bulk value are in good agreement with LEED measurements. It is suggested that the surface magnetism of V(100) may be associated with surface oxygen or caused by impurity induced surface reconstructions.

Giant two-dimensional ferromagnetic moments on metallic ovelayers and interfaces

Abstract Giant two-dimensional ferromagnetic moments are predicted for monolayers of 3d bcc transition metals as overlayers or sandwiched with noble or simple metals. The isolation of the interface states is the main mechanism behind these phenomena. Comparisons are made with results for transition metal-transition metal interfaces.

Magnetism and the electric and magnetic hyperfine interactions at transition metal surfaces: Fe(110)

Abstract A highly precise local spin density (LSD) study of Fe(110) surface (investigated by Korecki and Gradmann using in situ conversion-electron Mossbauer spectroscopy, CEMS) predicts a 20% enhancement of the surface magnetic moment over bulk; the absence of a Friedel oscillation of the moment into the bulk region but the presence of an oscillatory behavior for the magnetic hyperfine field, H c ; a magnetically induced contribution to the isomer shift; and a large electric field gradient only at the surface layer.

Electronic structure and magnetism of metastable bcc Co(001)

Abstract The electronic structure and magnetism of thin films of metastable bcc phase Co(001) consisting of one-, five- and nine-layers is determined by means of all-electron local spin density full potential linearized augmented plane wave (FLAPW) calculations at the lattice constant found for bcc Co stabilized on a GaAs substrate by Prinz. Band structure, surface states, density of states, charge and spin densities and contact hyperfine fields are presented. The center layer of the many-layer films yields a spin magnetic moment of 1.76μB which agrees well with results of bulk calculations for bcc Co. The surface layer has a moment (1.94μB) which is only 10% greater than the bulk value - in sharp contrast to the enhancement found for other transition metal surfaces [35% for Fe(001), 20% for Ni(001) and over 300% for Cr(001)]. Unlike these other transition metal surfaces, even the first layer below the surface layer in Co has the bulk magnetic moment - which indicates that the spin density, like the charge density, shows very short range screening of the surface-vacuum interface.

Monolayer magnetism: Electronic and magnetic properties of Fe/Au(001)

Abstract A highly precise all-electron local spin density FLAPW calculation has determined the electronic and magnetic properties of an Fe monolayer adsorbed on an Au(001) substrate. The magnetic moment of the Fe atom is found to be 2.97μ B , i.e., enhanced by 30% from the bulk value and very close to that determined previously for the surface layer of Fe(001) itself (2.96μ B ). As a result of Au d-band hybridization with the Fe d-band, Au interface atoms have a small moment of +0.03μ B . Although the Fe magnetic moment is strongly enhanced , the contact hyperfine filed at the Fe nucleus is substantially reduced to -213 kG, a result of the large positive conduction electron contribution which is rather sensitive to the atomic environment.

Structural and magnetic properties of Au/Pd/Au sandwiches: a total energy all-electron approach

The possibility of magnetism induced in Pd when sandwiched between Au layers, first proposed and studied experimentally by Brodsky and Freeman, is investigated by means of highly precise all‐electron total energy local spin‐density calculations employing the thin‐film full‐potential linearized augmented plane wave (FLAPW) method. Total energy local density calculations are employed on a Au/Pd/Au sandwich with three Pd layers to obtain the structural properties and five Pd layers to obtain the magnetic properties of an Au/Pd/Au sandwich. Detailed results are obtained with spin polarized calculations for the Pd lattice constant stretched taking into account (i) the misfit between the lattice constants of Au and Pd and (ii) proximity effects in a self‐consistent way. The magnetic moment of the center Pd layer is found to be 0.02μB which is consistent with the experiment of Brodsky. A negligible magnetic moment for the interface Pd layers is obtained.The possibility of magnetism induced in Pd when sandwiched between Au layers, first proposed and studied experimentally by Brodsky and Freeman, is investigated by means of highly precise all‐electron total energy local spin‐density calculations employing the thin‐film full‐potential linearized augmented plane wave (FLAPW) method. Total energy local density calculations are employed on a Au/Pd/Au sandwich with three Pd layers to obtain the structural properties and five Pd layers to obtain the magnetic properties of an Au/Pd/Au sandwich. Detailed results are obtained with spin polarized calculations for the Pd lattice constant stretched taking into account (i) the misfit between the lattice constants of Au and Pd and (ii) proximity effects in a self‐consistent way. The magnetic moment of the center Pd layer is found to be 0.02μB which is consistent with the experiment of Brodsky. A negligible magnetic moment for the interface Pd layers is obtained.

Electronic structure, charge transfer excitations, and high‐temperature superconducting oxides (invited)

We present high‐precision results on the electronic band structure and properties of YBa2Cu3O7−δ, YB2Cu3O6, GdBa2Cu3O7−δ, and La2−xMxCuO4 as obtained from highly precise state‐of‐the‐art local density calculations. The results obtained demonstrate the close relation of the band structure to the structural arrangements of the constituent atoms and provide an integrated chemical and physical picture of the interactions and their possible relation to superconductivity. The ionic character of the Y is proven by similar detailed highly precise local density calculations for high TC GdBa2Cu3O7, and explains the coexistence of magnetism and superconductivity in the high TC rare‐earth superconductors. Surprising features are the low density of states (DOS) at EF, especially for δ≥0.1 which is lower per Cu atom than that in La2−xSrxCuO4—in agreement with experiment and a relatively large magnetic Stoner factor. Strong indications are demonstrated for the inadequacy of a conventional phonon mechanism for obtaining ...