Minority Surface States Research Articles

Surface spin polarization of the nonstoichiometric Heusler alloy Co2MnSi

Using a combined approach of spin-resolved photoemission spectroscopy, band structure and photoemission calculations we investigate the influence of bulk defects and surface states on the spin polarization of Co2Mn(alpha)Si thin films with bulk L21 order. We find that for Mn-poor alloys the spin polarization at EF is negative due to the presence of Co_Mn antisite and minority surface state contributions. In Mn-rich alloys, the suppression of Co(Mn) antisites leads to a positive spin polarization at the Fermi energy, and the influence of minority surface states on the photoelectron spin polarization is reduced.

D- andsp-like surface states on fcc Co(001) with distinct sensitivity to surface roughness

The sensitivity of electronic surface states to modifications of the surface topography has been investigated in regard to the orbital character of the respective states. The surface of face-centered-cubic Co(001) shows two characteristic electronic states: an occupied $d$-like minority surface state around the center of the surface Brillouin zone $\overline{\ensuremath{\Gamma}}$ and an unoccupied $sp$-like surface state with an exchange splitting of 0.56 eV around the zone boundary $\overline{\text{X}}$. These states were studied experimentally on a 15-monolayer Co film on Cu(001) by spin-resolved direct and inverse photoemission. In addition, the Co(001) surface was theoretically described by calculations of the Bloch spectral function and of the direct and inverse photoemission intensities within the one-step-model approach. Different surface topographies from atomically smooth to very rough, as proved by scanning tunneling microscopy, were achieved by choosing varied growth and annealing temperatures for the preparation of the Co films. Both types of surface states are highly sensitive to the film roughness, yet in a distinctly different way. The less localized $sp$-like wave function causes a much higher sensitivity of the unoccupied surface state to lateral inhomogeneities than the more localized $d$-type wave function of the occupied surface state.

Surface electronic structure of ferromagnetic Fe(001)

A thorough investigation of the surface electronic structure of ferromagnetic Fe(100) films, epitaxially grown on single-crystal W(100), has been conducted using spin- and angle-resolved photoemission combined with state-of-the-art density-functional theory slab computations. The dispersion of the surface emission close to the Fermi level has been assessed quantitatively. The experimental results are in a good agreement with the calculations and, in particular, the presence of a minority surface state with d{sub xz+yz} character along the {Gamma}{bar X} high-symmetry direction is unambiguously established. Additionally, the calculations predict the existence of a different unoccupied surface state localized at {Gamma}. The existence of the related minority interface resonance near the Fermi edge and outside of the surface-Brillouin-zone center {Gamma} is believed to control the tunneling magnetoresistance in Fe/MgO/Fe(001) for very thin MgO spacers thus our results serve as indirect confirmation to these predictions.

Surface electronic structure of fcc Co films: a combined spin-resolved one- and two-photon-photoemission study

The surface electronic structure of face-centred-cubic cobalt films on Cu(0 0 1) was studied by spin-resolved one- and two-photon photoemission. A minority surface state in a Δ1-symmetry gap of the minority band-structure was identified at about 0.45 eV below the Fermi energy. This state causes a resonance-like enhancement in the population of the minority image-potential surface state in the two-photon-photoemission experiment excited by p-polarized light. Additionally, it appears as a surface-sensitive spectral feature in normal photoemission (PE) and its existence is confirmed by calculations within the one-step model of PE. The majority counterpart of the surface state is theoretically expected at a binding energy of about 1.95 eV. Due to lifetime broadening and bulk transitions in this energy range, the majority state does not appear as a pronounced feature in the spectra. Bulk-derived states close to the Fermi level exhibit shifts to higher binding energy with increasing film thickness, while the minority surface state does not change its energy as a function of Co coverage. These results provide a basis for the interpretation of time-resolved measurements concerning ultrafast magnetization dynamics, which rely on a detailed knowledge of the surface electronic structure of ultrathin films.

Electronic transport through Fe/MgO/Fe(1 0 0) tunnel junctions

The transport properties of Fe/MgO/Fe(1 0 0) tunnel junctions are studied from first principles at finite bias. For parallel alignment of the Fe leads and voltages below 25 mV the current is carried by both majority and minority spins, with the largest contribution coming from the minority. Their transmission is dominated by a sharp resonance through surface states, whose contribution to the current decreases rapidly as the bias increases. The same minority surface state leads to resonant transmission also for the antiparallel alignment of the Fe leads. However, in contrast to the parallel case the resonance is weakly dependent on the bias, leading to a large magnetoresistance at small voltages, which rapidly decreases with increasing voltage and approximately saturates at about 25 mV.

Static and dynamic aspects of spin tunnelling in crystalline magnetic tunnel junctions

Single-crystal magnetic tunnel junctions employing bcc (100) Fe electrodes and MgO(100)insulating barrier are elaborated by molecular beam epitaxy. The magneto-transportproperties are investigated in two extreme regimes. First, for extremely small MgOthickness, we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads toantiferromagnetic interactions, mediated by the tunnelling of the minority spin interfacialresonance state. Second, for large MgO barrier thickness, the tunnel transport validatesspecific spin filtering effects in terms of symmetry of the electronic Bloch function andsymmetry-dependent wavefunction attenuation in the single-crystal barrier. Within thisframework, we present giant tunnel magnetoresistive effects at room temperature(125–160%). Moreover, we illustrate that the interfacial chemical and electronic structureplays a crucial role in the spin filtering. We point out imperfect filtering effects and astrong implication of the minority surface state of Fe on the low voltage variation oftunnel magnetoresistance. The insertion of carbon impurities at the Fe/MgOinterface changes radically the voltage response of the tunnel magnetoresistance andactivates a resonant tunnelling mechanism via the interfacial resonance state.

Strain induced alteration of the gadolinium surface state

The electronic structure of strained and unstrained Gd(0001) has been studied with photoemission, inverse photoemission, and spin-polarized photoemission. A shift of the occupied majority and unoccupied minority surface states has been observed as a result of the strain, consistent with the phase accumulation model. There is a strain induced shift of the minority spin surface state across the Fermi level.

Magnetic surface states on Fe(001).

The direct observation of magnetic surface states on Fe(001) is reported. A spin-polarized angle-resolved photoemission study identifies a minority surface resonance at {bar {Gamma}} (2.40-eV binding energy) and a minority surface state at {ital {bar X}} (2.25-eV binding energy) in the surface Brillouin zone. The majority counterpart of the minority surface resonance is not observed. Good agreement with 7- and 13-layer full-potential linearized-augmented-plane-wave calulations is found for the minority {bar {Gamma}} to {ital {bar X}} surface band. In addition, the 13-layer slab calculation is found to more correctly model the decay of the surface resonance into the bulk in contrast to the 7-layer slab where size effects play a role.

Sulfur-induced occupation of Delta -bar Fe minority surface states in c(2 x 2) S/Fe(001).

The poisoning of iron's catalytic value in the synthesis of hydrocarbons through contamination by small amounts of sulfur is a well-known, though poorly understood, phenomenon. Using results from our recent first-principles, full-potential linearized augmented-plane-wave study of $c(2\ifmmode\times\else\texttimes\fi{}2)$ sulfur chemisorbed on magnetic Fe(001), we review a number of important adsorption-induced modifications of electronic and magnetic structure, which probably have bearing on this poisoning phenomenon. These include a significant reduction in the density of states (DOS) at ${E}_{F}$, and a narrowing and sharpening of various surface resonance states above ${E}_{F}$. The most prominent of these features involves a striking level repulsion of minority surface states (SS's), all along the $\overline{\ensuremath{\Delta}}$ line of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface Brillouin zone, which is largely responsible for the decline in the DOS at ${E}_{F}$. At $\overline{X}$, this level repulsion leads to a large (\ensuremath{\sim} 1 eV) upward shift of one of the two degenerate $d(xz,yz)$ SS's, located 0.2 eV above ${E}_{F}$, accompanied by a downward shift of the remaining SS, which becomes occupied and remains highly localized all along $\overline{\ensuremath{\Delta}}$. We examine the origins of this effect and suggest various spin-polarized photoemission and inverse-photoemission experiments, which could provide independent confirmation of our prediction of this dramatic effect.

First-principles study of the electronic and magnetic structure of c(2 x 2) sulfur chemisorbed above Fe(001).

We have performed the first, first-principles study of the adsorption of sulfur above a magnetic, Fe surface. Our results, derived from the all-electron, film full-potential linearized augmented-plane-wave method applied to a seven-layer Fe film with and without c(2\ifmmode\times\else\texttimes\fi{}2) layers of S positioned next to the two surface Fe [Fe(S)] layers, include determinations of the equilibrium sulfur height (${H}_{\mathrm{eq}}$) and vibrational frequency, as well as the associated electronic and magnetic structures. We find excellent agreement between our calculated value (1.12 A\r{}) of ${H}_{\mathrm{eq}}$ with the earlier result [1.09(5) A\r{}] derived by Legg, Jona, Jepsen, and Marcus from a dynamical low-energy electron diffraction intensity analysis. The adsorption induces antibonding minority surface states immediately above and below ${E}_{F}$ which play an important role both in reducing the magnetic moment of the Fe(S) atom (by \ensuremath{\sim}20%) and in the rather small calculated increase (0.85 eV) in work function. These states should be clearly resolvable in both integrated and angle-resolved, spin-polarized photoemission and inverse-photoemission experiments. We present additional predictions, including the adsorption-induced changes in the hyperfine fields and in the angle-resolved, spin-polarized surface state spectra, and relate our findings to questions associated with the sulfur-induced poisoning of an iron catalyst.

All-electron study of c(2×2) S chemisorbed above magnetic Fe(001)

We have performed a first-principles study of the adsorption of sulfur above a magnetic Fe surface. Our results, derived from the all-electron, film full-potential augmented-plane-wave method applied to a seven-layer Fe film with and without c(2×2) overlayers of S, include determinations of the equilibrium sulfur height (heq) and vibrational frequency, as well as the associated electronic and magnetic structures. We find excellent agreement between our calculated value (1.12 Å) of heq with the earlier result (1.09±0.05 Å) derived by Legg et al. from a dynamical low-energy electron diffraction intensity analysis. The adsorption induces antibonding minority surface states immediately above and below EF which play an important role both in reducing the magnetic moment of the surface Fe atom (by ∼20%) and in the resulting calculated increase (0.86 eV) in work function. These states should be clearly resolvable in both integrated and angle-resolved spin-polarized photoemission and inverse-photoemission experiments. We present additional predictions, including the adsorption-induced changes in the hyperfine fields.