Dongqi Li

Initial and final state contributions of the core level shifts for Gd(0001)

Abstract The initial state and final state contributions to the core level shift are separated by comparing the shifts of occupied and unoccupied Gd 4f levels, which exhibit the initial state shift with equal signs and the final state shift with opposite signs. For clean Gd(0001) we find a surface shift of −0.4 eV for both the unoccupied and occupied 4f level. On the other hand, with 3–4 L of oxygen the unoccupied surface 4f level is shifted by + 1.0 eV with respect to the bulk, and the occupied 4f by −1.0 eV. Therefore, the initial state effect dominates at the clean surface, while the final state effect plays a significant role at the oxidized surface. The difference is explained by metallic versus dielectric screening.

Magnetic and structural instabilities of ferromagnetic and antiferromagnetic Fe/Cu(100)

Fe wedges epitaxially grown on Cu(100) have been employed to investigate the interplay between magnetic and structural instabilities. 2–4 monolayer (ML) clean Fe films grown at room temperature are ferromagnetic with perpendicular easy axes. bcc Fe films≳11 ML thick are ferromagnetic with in‐plane easy axes. Most importantly, 6–11 ML fcc Fe films are antiferromagnetic and have a ferromagnetic surface. Films grown below 200 K and annealed to room temperature do not exhibit the antiferromagnetic phase, but remain ferromagnetic and undergo a spin‐reorientation transition from perpendicular to in plane at ∼6 ML. A new phase diagram for Fe/Cu(100) is proposed as a function of thickness and growth temperature. In addition, an impurity‐stabilized layer‐by‐layer growth that persists to 30–40 ML Fe is also reported.

Magnetic and structural instabilities of ultrathin Fe(100) wedges (invited)

An overview is provided of recent efforts to explore magnetic and related structural issues for ultrathin Fe films grown epitaxially as wedge structures onto Ag(100) and Cu(100). Experiments were carried out utilizing the surface magneto‐optic Kerr effect. Ordinary bcc Fe is lattice matched to the primitive unit cell of the Ag(100) surface. Fe wedges on Ag(100) can be fabricated whose thick end has in‐plane magnetic easy axes due to the shape anisotropy, and whose thin end has perpendicular easy axes due to the surface magnetic anisotropy. A spin‐reorientation transition can thus be studied in the center of the wedge where the competing anisotropies cancel. The goal is to test the Mermin–Wagner theorem which states that long‐range order is lost at finite temperatures in an isotropic two‐dimensional Heisenberg system. Fe wedges on Cu(100) can be studied in like manner, but the lattice matching permits fcc and tetragonally distorted fcc phases to provide structural complexity in addition to the interplay of...

Superparamagnetic behavior of ultrathin Fe films grown on Al2O3(0001) substrates

We have studied superparamagnetic behavior of ultrathin Fe films grown on an Al2O3(0001) substrate at various growth temperatures. It is demonstrated that 1-nm-thick Fe films are in the superparamagnetic state, and the blocking temperature is strongly dependent on the growth temperature. The blocking temperature has a minimum value of 30 K for a growth temperature of 473 K, while it is ∼70 K at other growth temperatures. In order to clarify the behavior, we consider the Fe growth mechanism and the magnetic interactions between Fe particles. Fe grows as three-dimensional islands at all temperatures studied and forms particles. The volume of the particles is observed via atomic force microscopy to increase with increasing growth temperature. In the case of growth at 323 and 373 K, Fe forms small particles that are close together and that interact with each other. For growth at 673 and 773 K, Fe forms relatively large particles and the magnetic properties are dominated by the individual particles.

Angle-resolved photoemission study of oxygen chemisorption on Gd(0001)

The initial stages of oxidation and the electronic structure of chemisorbed oxygen on Gd(0001) thin films have been studied by angle-resolved photoelectron spectroscopy. Dissociative chemisorption of oxygen occurs with initial exposures and is followed by formation of a more complex surface oxide of Gd. The symmetry of oxygen-induced states has been identified. Initial oxygen chemisorption occurs through bonding to the Gd(0001) surface via the px,y derived oxygen states. The surface work function decreases with initial oxygen adsorption. The electronic structure induced by oxygen and the work function change indicate that the chemisorbed oxygen layer is below the plane of the Gd(0001) surface layer.

Resonant photoemission studies of the thickness dependence of the unoccupied Gd 5d bands

Ultrathin Gd films have been studied with constant initial‐state spectroscopy (CIS) of photoemission utilizing linearly polarized light of synchrotron radiation. The photoemission cross section of the 5d surface state near EF shows strong photon energy dependence, i.e., sharp peaks in CIS spectra near the Gd 5p1/2 absorption edge. These peaks originate from resonant photoemission processes involving the occupied 5p and unoccupied 5d levels of the surface atoms. The symmetry of the unoccupied states were determined experimentally by changing the polarization of light, where p‐polarized light excites only to the unoccupied dz2 or dxz,yz states and s‐polarized light to all the d states including dx2−y2 and dxy states. Based on the symmetry assignments, the lower bound of the exchange splitting of the Gd 5dz2 surface state can be abstracted, which increases when the thickness of Gd film decreases and reach maximum at the monolayer limit.

Structural and magnetic studies of fcc Fe films with self-organized lateral modulation on striped Cu(110)–O(2×1) substrates

Face-centered cubic (fcc) Fe wedges of 0–12 ML were grown by means of molecular beam epitaxy onto a novel substrate: flat Cu(110) with an oxygen-induced, long-range ordered striped phase, and studied in situ with medium energy electron diffraction (MEED) and the surface magneto-optical Kerr effect (SMOKE). In contrast to Fe growth on either clean or oxygen-saturated Cu(110), the films on the striped substrates retain a layer-by-layer growth mode up to 6–7 ML and are fcc at least up to 12 ML. In addition, satellite peaks were observed on both sides of the MEED (0, 0) streak, indicating a long-range-ordered lateral modulation of the Fe surface. We postulate that the Fe films grow conformally onto the original striped substrate. SMOKE studies show that these fcc Fe wedges are ferromagnetic with an easy axis along the original stripes for Fe thickness >4 ML and a remanant magnetization that increases linearly with thickness beyond 4 ML.

Magnetic phase transition and anisotropy of ultrathin Fe films grown on inclined Al2O3(0001) substrates

We investigated the magnetic properties of ultrathin Fe films grown on inclined Al2O3(0001) substrates at various growth temperatures. We report the evolution of the magnetism with Fe thickness tFe, growth temperature, and the effect of the inclination of the substrate orientation on the magnetic anisotropy. The films are superparamagnetic (tFe≈5 monolayer, ML), ferromagnetic (tFe>15 ML), or coexistent (tFe≈10 ML). The effect of inclination of the substrate is small in the superparamagnetic region and substantial in the ferromagnetic region. Fe thin films grown on the inclined substrate have a uniaxial magnetic anisotropy with the magnetic easy axis parallel to the step edge. This uniaxial magnetic anisotropy might be derived from the effective demagnetizing field due to the magnetic charge distribution at the corrugated surface. The strength of the uniaxial magnetic anisotropy decreases as the growth temperature increases. The dependence of the uniaxial magnetic anisotropy on growth temperature is caused...

MAGNETIC PROPERTIES OF ULTRATHIN FE FILMS GROWN ON STEPPED W(001) AND PD(001) SUBSTRATES

In both Fe/W(001) and Fe/Pd(001) systems, the atomic steps induce an in-plane uniaxial magnetic anisotropy with the easy magnetization axis perpendicular to the step edges. The strength of the step-induced anisotropy was found to have a power law dependence on the step density: a quadratic dependence in the Fe/W system but a linear dependence in the Fe/Pd system. In addition, the Curie temperature is found to be higher on the stepped surface in the Fe/Pd system as compared to the flat surface. The enhancement of the Curie temperature is attributed to the step-induced Pd moments which is supported by the increased surface magneto-optic Kerr effect signal on the stepped surface. No such enhancement of either Curie temperature or magnetic moment was observed in the Fe/W system.

Magnetic domains and magnetostatic interactions of self-assembled Co dots

Submicron quasihexagonal Co dots and dot chains have been grown epitaxially on Ru(0001) substrate with molecular beam epitaxy and investigated with magnetic force microscopy. All the dots exhibit in-plane single domains. The magnetization of the dots in a chain prefers to align along the chain direction due to inter-dot magnetostatic interactions. Micromagnetic calculations suggest that the observed single domains are metastable energetically and both vortex and single domain can exist in zero field. The inter-dot magnetostatic coupling, modeled with a dot pair, introduces a uniaxial anisotropy with easy axis along the pair/chain direction. The induced anisotropy field decreases with increasing inter-dot distance and approaches the dipolar limit.