Spin-polarized Photoemission Study Research Articles

Potassium adsorption and an unoccupied surface state on Fe(001)

Spin-polarized photoemission studies of potassium adsorption on an Fe(001) surface provide evidence of a minority-spin surface state sitting immediately above the Fermi level on the clean surface. This state of symmetry is associated with the surface state predicted in first-principles calculations and the state utilized in chemically selective scanning tunnelling microscopy studies of the Fe(001) surface.

Spin polarized photoemission studies of the 3s core level in ferromagnetic systems (abstract)

Spin polarized core level photoemision studies are capable of providing local site specific magnetic information. 3s core level photoemission spectra are characterized by a main peak and satellite whose separation reflects the exchange interaction between the core level and the net spin in the valence bands. However, there is currently considerable disagreement as to whether the splitting between these two peaks may be taken as a measure of the local moment. An alternative approach is to examine the intensities of the different peaks in the spectra. Indeed analysis of the multiplet intensities in the final state suggests that it should be possible to obtain a reasonable measure of the local magnetic moment through measurement of the spin dependent intensities in the multiplet structure. In detailed spin polarized photoemission studies of the 3s core level of Fe and Co films grown on Ag(001) and Cu(001) substrates, respectively, we find that this is indeed the case. This suggests that studies of the spin polarization of the 3s core level photoemission may provide an important new technique for the measurement of magnetic moments in thin films. Lineshape fitting reveals new information about the lifetime of the core holes. Previous studies have suggested that a majority spin core hole will be shorter lived than a minority spin core hole because of the higher density of majority spin electrons in the valence bands. However, our studies indicate that the lifetime is more complicated and that it shows a strong dependence on the LS in the final state.

Spin polarized photoemission studies of the 3s core levels in Fe and Co

Spin polarized photoemission is used to study the 3s core levels of Fe and Co. The exchange splitting between the two spin components in the main 3s3p 63d n configuration is found to be larger in Co than Fe. The satellites corresponding to emission from the 3s 23p 43d n+1 configuration are identified in both metals. The application of a sum rule to obtain the spin moment from the complete spectrum including all configurations in investigated.

Spin-orbit coupling, exchange interaction, and hybridization in the photoexcitation of the Ni 3p core level.

A spin-polarized photoemission study of the Ni 3p core level reveals the spin polarization of the main line and the satellites. A detailed comparison with calculation confirms earlier assignments that the satellites have ${\mathit{d}}^{9}$ character. However, the measurement also reveals the role of spin-orbit and exchange interactions in determining the spin polarization of the main peak, which is shown to be composed of both ${\mathit{d}}^{9}$ and ${\mathit{d}}^{10}$ character. The study shows that the spin polarization in the leading edge of the main line reflects its ${\mathit{d}}^{9}$ character as a result of the hybridization between the different configurations.

Spin polarized photoemission studies of magnetic quantum well states

Quantum well states with discrete binding energies dependent on the thickness of the film are observed in copper films deposited on a Co(001) substrate. They are found to be spin polarized, preferentially with minority spin. These states pass up to and through the Fermi level with a frequency identical to the long period of oscillation in the associated magnetic multilayers. In the ‘pre-asymptotic limit’ the dispersion of these states away from the center of the zone is described by enhanced effective masses. This has implications for theories of the oscillatory exchange coupling that invoke the bulk Fermi surface.

Spin-polarized photoemission studies of the exchange splitting of the Gd 5d electrons near the Curie temperature.

Spin-polarized photoemission was employed to investigate the temperature dependence of the exchange splitting of Gd(0001) for both the surface state near the Fermi level ${\mathit{E}}_{\mathit{F}}$ and the 5d bulk bands. The bulk bands at 1--2 eV below ${\mathit{E}}_{\mathit{F}}$ show Stoner-like behavior where two peaks with opposite spin polarization shift toward each other as the temperature increases. In contrast, the temperature dependence of the surface state indicates spin-mixing behavior due to fluctuating local moments. These differences are attributed to the degree of itinerancy of the 5d electrons in the two cases.

Spin-polarized photoemission study of the Fe 3s multiplet.

The results of a spin-polarized photoemission study of the Fe 3s level clearly reveal the multiplet structure characterizing the final states of the excitation process. The energy separation between the high-spin and low-spin ionic states of 3.5 eV is smaller than has previously been measured, to our knowledge. The two spin components of the high-spin ionic state are clearly resolved and shown to be separated in binding energy by 0.9 eV. Line-shape fitting provides evidence that the individual peak widths reflect the total spin of the ionic final state rather than the spin of the core hole.

Spin Polarized Photoemission Studies of Quantum well States in Thin Films

ABSTRACTSpin polarized photoemission is used to study the electronic structure of noble metals deposited on ferromagnetic substrates. Studies of Ag deposited on an Fe (001) substrate reveal a series of minority spin interface or quantum well states with binding energies dependent on the thickness of the silver. Similar behavior is observed for Cu films deposited on a fee Co (001) substrate. Tight-binding Modeling reproduces many of the observations and shows that hybridization of the sp-bands with the noble metal d-bands cannot be ignored.

Picosecond spin-polarized photoemission studies of magnetic materials (invited) (abstract)

With pulsed lasers as light sources, a new and important element is added to photoemission: time resolution. In order to make use of the high light intensities available, the experiment should measure the spin polarization of the total photoyield since this quantity is insensitive to space-charge effects. The following examples are discussed: (1) For amorphous GdTbFe, a magneto-optical storage material, it is shown that the speed of thermomagnetic writing critically depends on whether the initial temperature of the sample is above or below the compensation temperature. (2) The response of the magnetic order to pico- and nanosecond heating pulses has been measured in iron and other materials. It is found that the spin polarization of the electrons emitted from Fe by 30-ps pulses does not depend on the pulse energy even if melting occurs. Accordingly, ferromagnetic relaxation in Fe takes place on a time scale which is considerably longer than 30 ps. (3) Using optical spin orientation, the dynamics of the band bending induced by high-intensity laser pulses has been studied in a p-GaAs/Cs Schottky barrier. The buildup time for the photovoltage created by the electron-hole pairs excited in the band-bending region is of the order of several hundred picoseconds.

Spin-polarized photoemission studies of the adsorption of O and S on Fe(001).

Spin-resolved angle-resolved photoemission spectra have been measured for the adsorption systems Fe(001)-p(1\ifmmode\times\else\texttimes\fi{}1)O and Fe(001)-c(2\ifmmode\times\else\texttimes\fi{}2)S. Adsorbate-induced features in the spectra display exchange splittings (\ensuremath{\Delta}E) and linewidths which vary with ${\mathit{k}}_{\mathrm{?}}$, with orbital symmetry (${\mathit{p}}_{\mathit{x}}$ or ${\mathit{p}}_{\mathit{z}}$), and with spin character. For adsorbate ${\mathit{p}}_{\mathit{z}}$ orbitals, the measured \ensuremath{\Delta}E is larger at the center of the surface Brillouin zone (1.3 eV for O, 0.5 eV for S) than at the zone boundary (0.25 and 0.2 eV, respectively). The results are compared with the predictions of tight-binding simulations and first-principles calculations.

Magnetism and Chemisorption

Spin polarized photoemission studies of different adsorbates on an Fe (001) substrate show the interaction between the adsorbate and the substrate d bands. Studies of the adsorption of oxygen reveal exchange split adsorbate bands indicative of both a magnetic moment on the adsorbate atom and also a strong adsorbate p z substrate d z2 bond. The oxygen p z bands show a large variation in exchange splitting across the surface zone, an observation that is not predicted in first principles calculations of this system. Spin Polarised Auger Electron Spectroscopy studies of sulfur adsorbed on the same surface provide some indication of the magnetic moment on the adsorbate. It is found to be of the order of 0.1 μB. Studies of carbon monoxide adsorption in two different configurations, vertical and tilted, reveal little or no exchange splitting in the molecular 5σ orbital suggesting that the interaction of this orbital with the substrate d bands is much weaker.

Interaction of carbon monoxide with Fe(001).

New insight into the bonding of carbon monoxide with transition-metal-surfaces is provided by spin-polarized photoemission studies of CO on Fe(001). The interaction of the molecular 5\ensuremath{\sigma} levels with the substrate d bands is found to be very weak for both tilted and vertical CO species. The former shows clear evidence for a \ensuremath{\pi}-d band interaction with discrete spin-polarized features in the occupied D band; the latter shows only small changes. Inverse photoemission studies (non-spin-polarized) indicate that the unoccupied 2\ensuremath{\pi} level is closer to the Fermi level for the tilted CO species.

Spin-polarized photoemission study of epitaxial Fe(001) films on Ag(001).

The electronic and magnetic character of epitaxial Fe films on Ag(001) has been studied as a function of Fe coverage by spin- and angle-resolved photoemission with synchrotron radiation. At coverages well below a monolayer, the spectra exhibit a local spin-split electronic state. Although spectra for films in the monolayer coverage range display electronic structure in close agreement with calculated monolayer film critical-point energies, no spin polarization is observed up to 2.5 monolayers coverage. Thicker films approach the spin-split electronic structure and spin polarization of bulk Fe(001).

Spin-polarized photoemission study on the temperature dependence of the exchange splitting of Ni

Using spin-polarized photoemission with high energy- and angle resolution (ΔE=100 meV, ΔΘ=±3°) we have investigated the temperature dependence of the exchange splitting of Ni in the temperature range 0.5≦T/Tc≦0.94. At room temperature we findΔex=0.18 eV for the exchange splitting of theS4 band at theX point of the Brillouin zone. With increasing temperature the total (spin-averaged) energy distribution shows a narrowing and merges into one peak. The spin-resolved energy distribution curves approach each other and are strongly broadened. A discussion of the data within current theories of itinerant electron magnetism is given. The spectra indicate that neither the pure Stoner model nor the prediction of local band theory, assuming a temperature independent exchange splitting are justified for Ni. We conclude that the exchange splitting decreases with increasing temperature and that transverse as well as longitudinal spin fluctuations are responsible for the broadening of the spin-resoived energy distribution curves.

Surface magnetization and hydrogen chemisorption on Gd: A spin-polarized photoemission study

The spin polarization of photoelectrons emitted from polycrystalline Gd decreases linearly with temperature between 20 and 300 K. Submonolayer coverages of hydrogen, chemisorbed at 20 K, drastically reduce the observed spin polarization. This indicates a very short probing depth of spin-polarized photoemission, which arises from strong spin-exchange scattering. Hydrogen chemisorption is found to induce a canted or disordered spin structure at the Gd surface with a much lower ordering temperature than that of the clean Gd surface.