Spin-polarized Photoelectron Diffraction Research Articles

Multiple Scattering Theory of Spin-Polarized X-ray Photoelectron Diffraction

Spin-polarized X-ray photoelectron diffraction (SPXPD) spectra are useful to study magnetic structures of solid surfaces. In this work two different SPXPD modes are studied on the basis of a relativistic quantum electrodynamic (QED) theory which provides a unified picture of the SPXPD analyses. An important advantage of this theory is to allow us to handle many-body relativistic XPS theory, which can be a basis of SPXPD analyses. One mode is to utilize the spin polarization caused by multiplet splittings as observed in O2 molecules and Gd metals. We demonstrate how to extract local magnetic order in surface O2 . The second SPXPD mode is to use circularly polarized X-rays to discriminate the spin polarization with aid of spin-orbit coupling. [DOI: 10.1380/ejssnt.2005.353]

SPIN ARRANGEMENT OF THE Mn/Fe(001) SYSTEM INVESTIGATED BY SPIN-POLARIZED PHOTOELECTRON DIFFRACTION

We demonstrate the application of the advanced spin-polarized photoelectron diffraction (SPPD) method, the photoelectron diffraction measurement of specific core levels in different spins, to the investigation of the spin arrangement of magnetic overlayers on magnetic thin films. Comparing the experimental SPPD result of the Mn 3p core level with that of calculation, we have determined the possible spin arrangements of the Mn overlayer (~1 ML) on the Fe(001) thin film which was epitaxially grown on Ag(001) substrate. Consequently, one of the most plausible spin arrangements of this system is a c (2 × 2) ordering of Mn atoms on the ferromagnetic Fe substrate. The other possible spin arrangement is a p (2 × 2) ordering of Mn atoms ferromagnetically coupled with the ferromagnetic Fe substrate. In both cases, the c (2 × 2) surface reconstruction is necessary to reproduce the experimental results properly.

Spin Polarized Photoelectron Diffraction from W4f Core Level.

So far, most of the experimental results for the core-level spin-resolved photoemission from non-magnetic materials have been interpreted by a simple free-atom model. However, the multiple scattering exerts some influence on the spin polarization. We thus have theoretically investigated the effect of multiple scattering on the spin polarization in W4ƒ photoemission spectra from W(110) surface. The present multiple scattering calculation improves the agreement with the experimental results in comparison with the calculation using the free-atom model. To understand the effect of multiple scattering on spin polarization, we calculated the spin polarization from emitter atoms in the individual surface layers. We found that the forward scattering plays an important role. We also studied the effect of thermal factor on spin polarization, and found that it was quite small.

Observation of a Ferromagnetic-to-Paramagnetic Phase Transition on a Ferromagnetic Surface Using Spin-Polarized Photoelectron Diffraction: Gd(0001)

The ferromagnetic system Gd(0001) has been studied by spin-polarized photoelectron diffraction, using the highly spin-polarized $^{7}S$ and $^{9}S$ multiplet peaks in Gd $4s$ and $5s$ spectra. Temperature-dependent spin asymmetries derived from the $^{7}S$-to- $^{9}S$ peak intensity ratios exhibit pronounced changes over a range of temperatures spanning both the bulk Curie temperature ${T}_{\mathrm{cb}}$ and a surface transition temperature ${T}_{\mathrm{cs}}$ that is found to be $\ensuremath{\approx}85\mathrm{K}$ above ${T}_{\mathrm{cb}}$. The surface transition also appears to span several atomic layers.

Spin-polarized photoelectron diffraction of nanoscale manganese structures

Magnetic ordering in bulk Mn and nanoscale Mn islands deposited on highly oriented pyrolytic graphite has been investigated with spin-polarised photoelectron diffraction (SPPED). Results for the bulk film show the expected step in the ratio of intensities of the Mn 3 s core level 5 S and 7 S multiplets. This occurs at a temperature of 505 ± 20 K or 5.3 times T N , the bulk Néel temperature and compares well with similar data from Mn compounds. In contrast the equivalent step observed with mesoscopic Mn islands of mean size 6 nm is found to occur at 85 ± 15 K, that is, at about T N . The high proportion of surface atoms (∼25%) in the nanoscale system produced an enhanced photoemission signal from the surface allowing a deconvolution of surface and interior contributions to the 3 s spectra. We thus show that the step in the ratio of multiplet intensities arises entirely from the surface component of the 3 s spectrum. This supports a recent interpretation that the step is due to a surface specific transition in longrange magnetic order that can be well above T N,bulk for antiferromagnets and T C,bulk for ferromagnets.

CHARACTERIZATION OF THE MAGNETIC STRUCTURE OF Cr on Pd(001) BY SPIN-POLARIZED AND ANGLE-RESOLVED PHOTOELECTRON DIFFRACTION

Recent investigations of the magnetic structure of Cr layers on Pd (001) have shown that two structures appear dependent on the number of Cr layers which have an in-plane antiferromagnetic structure and a layer antiferromagnetic structure, respectively. It is shown that these magnetic structures should be distinguishable experimentally by spin-polarized photoelectron diffraction experiments. The photoelectron intensity from the Cr 2 p 1/2 core level is calculated for normal incidence of left and right circularly polarized light. It is shown that there is a strong dichroism and spin polarization in the intensities. Dichroism and spin polarization are analyzed with respect to their sources, and thus the contribution of the magnetic scattering is separated. It is an essential contribution for a monolayer Cr on Pd (001).

COMPARISON OF THE SPIN POLARIZATION OF THE PHOTOELECTRON DIFFRACTION SPECTRA FOR DIFFERENT STRUCTURE MODELS OF MAGNETITE (111) LAYERS

Layers of magnetite structure were observed in deposing iron oxide on Pd by LEED measurements. We consider the magnetic structure of these layers. It is investigated whether spin-polarized and angle-resolved photoelectron diffraction can be used to dstinguish between the Zuo and the Mizoguchi model, respectively. In the two models we have a different distribution of the Fe 2+ and Fe 3+ ions. This difference is incorporated into the calculation by spin-dependent scattering phase shifts. The results of the calculation show that there is a cancellation of the magnetic scattering in the Mizoguchi structure whereas there remains a contribution in the Zuo model. Thus, we conclude that these structures should be distinguishable by spin-polarized angle-resolved photoelectron diffraction.

Spin-polarized photoelectrons excited by circularly polarized radiation from a nonmagnetic solid.

We report a spin analysis of core-level photoelectrons excited by circularly polarized x rays from a nonmagnetic solid. In a combined experimental and theoretical study, we show that the spin-orbit-split W 4 f 7/2 and 4 f 5/2 photoemission lines from W~110! exhibit high spin polarizations of opposite sign that vary with energy and emission direction. These results suggest the study of the magnetic structure of nonmagnetic/ferromagnetic interfaces formed on high-atomic-number substrates by spin-polarized photoelectron diffraction. Photoelectrons from unpolarized atoms can be highly spin polarized when ejected by circularly polarized ~CP! light, due to the Fano effect: 1~a! although the dipole operator does not act upon the electron spin explicitly, the photon angular momentum is partially transferred to the photoelectron spin by the spin-orbit ~SO! interaction. Shortly after its prediction in 1969, this effect was observed in the valence levels of alkali-metal 1~b! and rare-gas atoms 2 in gaseous and solid-state phases, and today it is well known as the basic mechanism behind the GaAs source for spin-polarized electrons. 3 In particular, photoelectrons excited by CP light from p, d, and f shells can acquire substantial spin polarization over a wide photon energy range. 4 Up to now, the spin analysis of photoelectrons from unpolarized targets has mostly served fundamental interests of ~i! understanding photoemission ~PE! dynamics through quantum mechanically ‘‘complete’’ experiments 5,6 and ~ii! characterizing the symmetry of valence bands in nonmagnetic solids. 7

A structural determination using magnetic X-ray circular dichroism in spin-polarized photoelectron diffraction

The first structural determination with spin-polarized, energy-dependent photoelectron diffraction using circularly-polarized X-rays is reported for Fe films on Cu(001). Circularly-polarized X-rays produce spinpolarized photoelectrons from the Fe 2p doublet, and intensity asymmetries in the 2p 3 2 level are observed. Fully spin-specific multiple scattering calculations reproduce the experimentally-determined energy and angular dependences. A new analytical procedure which focuses upon intensity variations due to spindependent diffraction is introduced.

Spin-polarized core photoelectron diffraction from magnetic and non-magnetic surface layers

A method is considered to calculate core photoelectron spectra including multiple scattering contributions and all curved wave corrections. Furthermore this method allows to calculate the spectra in dependence on the polarization (linearly or circularly) of the X-ray beam and on the spin-polarization of the photoelectrons. The results show that the spin-polarized spectra give more detailed information on the structure of the surface layer. Results are shown for the Sb 4 d 3 2 and 4 d 5 2 spectra of the Sb InP(110) surface where we found a strong circular dichroism of the spin-polarization. Furthermore the system Cr Ag(100) is investigated where experimental spectra are available for the Cr 2 p 1 2 level. In this case it is shown to what extent the spectra depend on the orientation of the magnetization of the Cr layers besides the dichroism. The results should encourage experimentalists to do spin-polarized measurements with circularly polarized light.

Monte Carlo study of magnetic order at ferromagnetic and antiferromagnetic surfaces: Implications for spin-polarized photoelectron diffraction.

We have used Monte Carlo simulations on simple-cubic Ising lattices with modified surface interaction parameters to model phenomenologically the temperature dependence of magnetic order near ferromagnetic and antiferromagnetic surfaces. These results are also discussed in connection with previous experiments suggesting surface-specific magnetic transition temperatures for semi-infinite systems, with special emphasis on spin-polarized photoelectron diffraction as a probe of short-range magnetic order. The calculated spin-spin correlation functions show no evidence of a high-temperature transition in short-range magnetic order. However, over a plausible range of choices for the surface interaction parameters, these correlation functions do show distinct surface transitions in long-range magnetic order that can be well above ${\mathit{T}}_{\mathit{N},\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}$ for antiferromagnets (both frustrated and nonfrustrated) and well above ${\mathit{T}}_{\mathit{C},\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}$ for ferromagnets. Thus, prior spin-polarized photoelectron data from antiferromagnetic ${\mathrm{KMnF}}_{3}$ and MnO may be explainable via such surface magnetic transitions, although further theoretical and experimental work are necessary to make this connection quantitative and definitive.

Imaging short-range magnetic order by spin-polarized photoelectron holography.

We extend spin-polarized photoelectron diffraction to the direct imaging of magnetic spin configurations by using holographic-inversion methods. Theoretical calculations for ${\mathrm{Mn}}^{2+}$ and ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ions in linear chains show that it is possible via two different spin-selective image functions to determine both the positions and orientations of magnetic scatterers relative to a given type of emitter. Such holographic images thus should provide direct information on short-range magnetic spin configurations near surfaces.

Spin-polarized photoelectron diffraction using circularly polarized x rays

The observation of spin-dependent photoelectron diffraction using circularly polarized x rays is reported for Fe films on Cu(001). Circularly polarized x rays produce spin-polarized photoelectrons from the Fe 2p doublet, and intensity asymmetries in the 2${\mathit{p}}_{3/2}$ level are observed. Fully spin-specific multiple-scattering calculations reproduce the experimentally determined energy and angular dependences. An analytical procedure is introduced which removes exchange shift contributions to the Fe 2p asymmetry, and focuses instead upon intensity variations due to spin-dependent diffraction.

Photoelectron Diffraction and Synchrotron Radiation

The basic concepts of photoelectron diffraction are briefly introduced. The increasing number of synchrotron beam lines devoted to this type of experiments is explained by the variety of surface phenomena that can be studied. New developments, such as the measurement of full hemispherical angular distributions, photoelectron holography, energy-scanned photoelectron diffraction, chemical-state resolved photoelectron diffraction, and spin-polarized photoelectron diffraction all benefit from the special attributes of synchrotron radiation. PACS numbers: 61.14.-x, 79.60.—i

Theory of spin polarized photoelectron diffraction

We discuss several aspects of the theory of spin-polarized photoelectron diffraction (SPPD). This method makes use of multiplet splittings of core-level binding energies to produce photoelectron peaks with high spin polarization (for example, the two principal peaks associated with Mn 3s emission from Mn 2+). We consider three possible mechanisms for spin-dependent photoelectron scattering and diffraction: exchange scattering by valence electrons (3d 5 for Mn 2+), spin-orbit scattering (which is not expected to yield large effects if the sample does not have a net magnetization), and spin-dependent inelastic scattering (which cannot yet be dealt with in a fully quantitative way, but is estimated to be less important than the other two). The fact that SPPD involves internal sources of polarized electrons references to their respective emitters implies that it can be employed to study magnetic order in both anti-ferromagnets and ferromagnets and at temperatures above their respective Néel or Curie points. The effects of exchange scattering on Mn 3s emission from Mn 2+ in KMnF 3 have been incorporated into a single-scattering cluster model of the diffraction process via either the Dirac-Hara or Kohn-Sham local density approximations. This model is applied to several cases: a single Mn 2+ scatterer, small clusters of Mn 2+ scatterers, and full clusters appropriate to the (110) surface of KMnF 3, with all atoms included. These calculations demonstrate that SPPD should be a short-range probe of magnetic order, a result consistent with conclusions reached in several prior studies of photoelectron diffraction without spin resolution. They also illustrate the perturbative nature of these effects, which are only about ca. 5–15% of the total intensity; this in turn leads to several possible simplifications in the theory. We have in addition phenomenologically modelled the decreases of short-range order with increasing temperature by using a Gaussian modulation of spins; this model produces curves of SPPD spin asymmetry versus Gaussian width which qualitatively agree with those observed experimentally (Phys. Rev. Lett. 55 (1986) 1227). Direct comparisons of the observed angular dependence of these spin asymmetries with theory for (110) KMnF 3 are also encouraging in that trends from one direction to another are in general predicted correctly. However, the maximum degree of asymmetry predicted by theory (13%) is somewhat below that of experiment (17%). Possible reasons for the discrepancies seen between theory and experiment are discussed, together with likely future directions of study.

Spin-polarized photoelectron diffraction and valence-band photoemission from MnO(001).

Spin-polarized photoelectron diffraction (SPPD) has previously been proposed as a technique for studying short-range magnetic order in magnetic materials, and the first experimental study of this kind was performed on the ionic antiferromagnetic ${\mathrm{KMnF}}_{3}$ [B. Sinkovic, B. Hermsmeier, and C. S. Fadley, Phys. Rev. Lett. 55, 1227 (1985)]. We present here a much more detailed study of SPPD for the antiferromagnetic transition-metal oxide MnO with a (001) surface orientation. The Mn 3s and Mn 3p multiplets have been studied using both low-energy (192.6 eV) and high-energy (1486.7 eV) x-ray excitation and the intensity ratios I${(}^{5}$S(\ensuremath{\uparrow}))/I${(}^{7}$S(\ensuremath{\downarrow})) and I${(}^{5}$P(\ensuremath{\uparrow}))/I${(}^{7}$P(\ensuremath{\downarrow})) have been measured as a function of both direction and temperature. Data obtained with the lower excitation energy and resulting in kinetic energies of 50--100 eV show an abrupt change or step in both the I${(}^{5}$S(\ensuremath{\uparrow}))/I${(}^{7}$S(\ensuremath{\downarrow})) and I${(}^{5}$P(\ensuremath{\uparrow}))/I${(}^{7}$P(\ensuremath{\downarrow})) intensity ratios at \ensuremath{\approxeq}530\ifmmode\pm\else\textpm\fi{}20 K or \ensuremath{\approxeq}4.5 times the N\'eel temperature ${\mathit{T}}_{\mathit{N}}$.This change is interpreted to be a new type of short-range-order transition occurring at what is denoted ${\mathit{T}}_{\mathrm{SR}}$. Also, these same quintet or septet intensity ratios show a weak peak at ${\mathit{T}}_{\mathit{N}}$, suggesting for the first time that SPPD has sensitivity to long-range magnetic order. Data obtained for the I${(}^{5}$S(\ensuremath{\uparrow}))/I${(}^{7}$S(\ensuremath{\downarrow})) intensity ratio with the higher excitation energy show no such effects, a result consistent with the much weaker exchange scattering expected at such energies. Additional x-ray photoelectron spectroscopy spectra and azimuthal scans of Mn and O core-level intensities are considered and these establish that (i) the sample surface had good stoichiometry and was very clean and well ordered, (ii) the SPPD effects observed at ${\mathit{T}}_{\mathrm{SR}}$ are not due to any surface structural change, and (iii) a single-scattering cluster (SSC) theoretical model is at least a qualitatively reasonable starting point for describing such effects. We also compare experimental results for the magnitudes of these steps with calculations based upon exchange scattering in the spin-polarized SSC model [B. Sinkovic and C. S. Fadley, Phys. Rev. B 31, 4665 (1985)], and conclude that there is at least qualitative agreement. A final aspect of our data concerns the temperature dependence of the Mn 3d--dominated valence-band spectra: These spectra are found to show no measurable change in crossing ${\mathit{T}}_{\mathrm{SR}}$, but by contrast exhibit a large 0.4-eV increase in width in going below ${\mathit{T}}_{\mathit{N}}$, which is in contradiction to recent theoretical predictions.

ChemInform Abstract: Spin‐Polarized Photoelectron Diffraction: A Probe of Short‐Range Magnetic Order

Abstract Spin-polarized photoelectron diffraction (SPPD) is a unique probe for obtaining surface sensitive near-neighbor magnetic order information in magnetic materials, including antiferromagnets. A brief review on the principles of x-ray photoelectron diffraction and SPPD is given, followed by a summary of results on Mn containing compounds obtained by the author and other co-workers. Variations in the diffraction pattern of the dominant Mn 3s multiplet, as a function of temperature, produced abrupt changes in the spin asymmetries, of up to 20% for KMnF 3 and MnO, at temperatures well above their Neel temperatures. There also appears to be a strong dependence in the spin asymmetry on emission direction. Complete corroboration of the Mn 3s spin asymmetry curves from MnO is found in the Mn 3p curves and good agreement is found with spin-polarized single-scattering cluster calculations.

Temperature effects in the theory of spin-polarized photoelectron diffraction

We outline an extension of the previously-developed zero-temperature spherical-wave single-scattering-cluster theory of spin-polarized photo-electron diffraction (SPPD) to finite temperature. We show explicitly how the spin-spin correlation function ⟨s(r0) · s(r)⟩ enters into the thermal averages over near neighbors, thus allowing SPPD spectra to be computed as a function of temperature if the correlation function is known. The spin-dependent part of the diffraction may be isolated by defining a spin asymmetry for a 3s emission process S3s, which is the ratio of spin-up to spin down intensities as normalized to the high-temperature or paramagnetic limit. A computation of S3s for a standard correlation function derived from a Monte Carlo analysis shows a complex dependence on temperature, but does not exhibit the step-like behavior at temperatures of several times TN where a step has been observed experimentally for KMnF3 and MnO. These results thus confirm that the steps observed in SPPD represent a new effect that goes beyond present theoretical models of the disappearance of shortrange magnetic order.

Spin-polarized photoelectron diffraction: a probe of short-range magnetic order

Spin-polarized photoelectron diffraction (SPPD) is a unique probe for obtaining surface sensitive near-neighbor magnetic order information in magnetic materials, including antiferromagnets. A brief review on the principles of x-ray photoelectron diffraction and SPPD is given, followed by a summary of results on Mn containing compounds obtained by the author and other co-workers. Variations in the diffraction pattern of the dominant Mn 3s multiplet, as a function of temperature, produced abrupt changes in the spin asymmetries, of up to 20% for KMnF 3 and MnO, at temperatures well above their Néel temperatures. There also appears to be a strong dependence in the spin asymmetry on emission direction. Complete corroboration of the Mn 3s spin asymmetry curves from MnO is found in the Mn 3p curves and good agreement is found with spin-polarized single-scattering cluster calculations.

Evidence for a high-temperature short-range-magnetic-order transition in MnO(001).

We report the observation of spin-polarized photoelectron diffraction for antiferromagnetic MnO(001). Data at kinetic energies near 100 eV for the spin-split Mn $3s$ multiplet show abrupt changes in the $^{5}S:^{7}S$ intensity ratio at a temperature of 530\ifmmode\pm\else\textpm\fi{}20 K that is 4.5 times the N\'eel temperature. An additional temperature-dependent study done at higher electron kinetic energies, along with azimuthal diffraction measurements, support the conclusion that this abrupt change is due to a new type of high-temperature short-range-magnetic-order transition.