Magnetic Dichroism In Photoemission Research Articles

Photoelectron diffraction for probing valency and magnetism of 4f -based materials: A view on valence-fluctuating EuIr2Si2

We present and discuss the methodology for modeling $4f$ photoemission spectra, $4f$ photoelectron diffraction (PED) patterns, and magnetic dichroism effects for rare-earth-based materials. Using PED and magnetic dichroism in photoemission, we explore the electronic and magnetic properties of the near-surface region of the valence-fluctuating material ${\mathrm{EuIr}}_{2}{\mathrm{Si}}_{2}$. For the Eu-terminated surface, we found that the topmost Eu layer is divalent and exhibits a ferromagnetic order below 10 K. The valency of the next Eu layer, that is the fifth atomic layer, is about 2.8 at low temperature that is close to the valency in the bulk. The properties of the Si-terminated surface are drastically different. The first subsurface Eu layer (fourth atomic layer below the surface) behaves divalently and orders ferromagnetically below 48 K. Experimental data indicate, however, that there is an admixture of trivalent Eu in this layer, resulting in its valency of about 2.1. The next deeper lying Eu layer (eighth atomic layer below the surface) behaves mixed valently, but the estimated valency of 2.4 is notably lower than the value in the bulk. The presented approach and obtained results create a background for further studies of exotic surface properties of $4f$-based materials, and allow us to derive information related to valency and magnetism of individual rare-earth layers in a rather extended area near the surface.

Zen and the art of dichroic photoemission

The discovery of magnetic dichroism in photoemission is celebrating its 25th anniversary this year. Here a review of the underlying general theory for the angular and spin dependence of dichroic core-level photoemission is presented using both a single-particle model and a many-body approach. The established methods of angular momentum coupling offer an elegant and powerful way to analyse the magnetic dichroism and spin polarization in photoemission from core and localized valence levels. In the presence of core-valence interactions one can distinguish different fundamental spectra, which via sum rules are related to physical properties described by coupled tensor operators for spin and orbital moments. By separating the angular dependence from the physical information, different geometries can be distinguished to measure the magnetic circular dichroism (MCD), linear dichroism (LD), circular dichroism in the angular dependence (CDAD), and magnetic linear dichroism in the angular dependence (MLDAD). Various ways to probe the core-hole polarization are discussed, such as using the angular dependence, moment analysis of the spectral distribution, and resonant photoemission decay.

Photoemission investigation of GdTiGe

Spectroscopy of photoelectrons excited with monochromatized Al Kα X-ray and circularly polarized synchrotron radiation has been used to study electronic structure of ferromagnetic GdTiGe. A variation of the valence band in the vicinity of the Fermi level has been found between the spectra obtained at temperatures below and above the Curie point. It has been attributed to the reduction of exchange splitting in the paramagnetic phase. Magnetic dichroism in photoemission has been found for Gd 4f and 4d levels at the resonant Gd 3d–4f and Ti 2p–3d excitation energies.

A direct two-dimensional comparison of magnetic circular dichroism and magnetic linear dichroism in ultraviolet photoemission spectroscopy

The angular dependence of magnetic dichroism in photoemission (MDAD) from valence bands is more complicated than that from core levels. It gives access to spin-resolved information about the bands involved in the photoexcitation process. By using a unique ellipsoidal-mirror analyzer, we are able to observe MDAD from valence bands as a two-dimensional image, which gives a direct view of their angular dependence over a large slice through the Brillouin zone. In this work we have investigated Co films grown on a Cu(001) substrate. By using either circularly polarized light or linearly polarized light at a photon energy of 21 eV, magnetic circular dichroism and magnetic linear dichroism (MLD) from the Fermi edge shows different angular dependencies. This is due to the different excitation processes involved with the different light polarizations and it reflects the symmetry of the 3d bands of the Co. Furthermore, we also find that the sign of the MLD can change if differing initial states are involved. First principles calculations of Co bands and their photoelectron angular distributions with the different light polarizations will be presented. This shows that dichroism in valence band photoemission can provide valuable additional information on the electronic and magnetic properties of ultrathin films.

Surface vs. bulk magnetic properties of Co/Fe(001) and Fe/Co/Fe(001) as probed by linear magnetic dichroism in photoemission

Abstract The atomic-like behavior of the photoionization cross-section of core levels in solids is remarkably displayed in the observation of linear magnetic dichroism in the angular distribution (LMDAD) of photoelectrons. Nevertheless, structure-related effects are clearly visible as modulations of the dichroism signal, induced by photoelectron diffraction (PED). In this paper a comparison between the PED and LMDAD behaviours is presented for Fe 3p in Fe(0 0 1). In order to separate experimentally the surface and bulk contributions, ultrathin Co/Fe pseudomorphic overlayers were grown on the Fe(0 0 1) surface. Results confirms the PED nature of LMDAD modulations, which can be used as an estimate of the bulk sensitivity of the experiment.

Correlation effects and satellite intensities in photoemission from ferromagnetic interfaces: Co, Fe, Cr on Cu(1 1 13)

We present magnetic dichroism in photoemission (MCD) results obtained at the 2p core level of Co, Fe and Cr grown on Cu(1 1 13). By performing measurements under both magnetization and polarization reversal, we obtain the true MCD signal, as well as the small CDAD and MLDAD signals, which can always be present in these experiments. The 2p MCD spectra display strong magnetic signals at the main edges accompanied by satellite features.

Observation of component fine structure in the linear dichroism of Fe 3p photoelectron emission

We have investigated fcc Fe x Co 1− x /Cu(1 0 0) alloy films with linear magnetic dichroism in photoemission. Specifically we studied the emission from the Fe 3p core level. For the first time, we have observed fine structure for linear dichroism in the photoelectron emission of the Fe 3p core level and confirmed our assignments using spin-polarized photoelectron spectroscopy. This conclusively demonstrates the validity of the single electron, density of states model for the 3p manifold of 3d magnetic materials.

Magnetic dichroism in Co films on Cu(001) using unpolarized light

The thickness and temperature dependence of the angular distribution of the magnetic dichroism in photoemission from Co 2p core levels of Co/Cu(001) ultra-thin films has been investigated, using unpolarized Mg Kα radiation. Photoelectron spectra were collected for emission angles in a range of ±9° around the surface normal in the [001] azimuth. All asymmetry spectra were found to exhibit an identical spectral shape within the experimental error bar, but different scaling. The angular distribution of the Co 2p dichroic signal shows that at emission directions off the sample normal strong variations are present, in contrast to predictions from simple atomic theory. These crystallinity-related modulations were studied for Co films of different thicknesses [5, 3, 2, and 1.5 atomic monolayers (ML)] and different temperatures (100 and 300 K). They are higher for thicker films and lower temperatures. This experimental finding proves a diffraction picture, in which this modulation around the surface normal is explained by diffraction effects in forward scattering along the sample normal. As diffraction effects in magnetic dichroism combine both diffraction and magnetic dichroism, its behavior is influenced by both, structural and magnetic properties of the film.

Magnetic dichroism in core-level photoemission from Gd(0001)

This work reports on magnetic dichroism in photoemission from the Gd 4l(l=s,p,d,f ) core level of in-plane magnetized Gd films excited with linearly, circularly, and unpolarized light. Thin Gd films of about 100 ML thickness were evaporated onto a W(110) substrate at room temperature, and subsequently annealed to 700 K to form an ordered layer exhibiting a (0001) surface. The dichroism measurements were performed by physically rotating the sample azimuth so as to change the direction of the magnetization axis relative to the incident light. The magnetic dichroism asymmetries were obtained by calculating the normalized difference between spectra obtained with the two mutually orthogonal orientations of the magnetization. Strong asymmetries were observed, even if exciting the spectra by means of unpolarized light. The experimental results are compared to three-step photoemission cluster calculations based on a relativistic full potential algorithm respecting magnetic exchange.

Pseudomorphic Co/Cr(1 1 0) films investigated by magnetic dichroism in photoemission and MOKE

The electronic and magnetic properties of heteroepitaxial Co/Cr(1 1 0) films are investigated by magnetic dichroism in UV photoemission and the magneto-optical Kerr effect (MOKE). The Cr(1 1 0) host supports pseudomorphic growth of single crystalline and uniform Co films several nanometers in thickness. Pronounced magnetic dichroism occurs in the valence band region close to the Fermi edge. The observed spin-averaged valence band spectra and the asymmetry in photoemission intensity upon magnetization reversal are discussed in conjunction with the well-established spin-dependent band structure of body-centered cubic Co. The azimuthal dependence of the magnetic dichroic effect suggests two-fold easy axis behavior with the spontaneous in-plane magnetization aligned along [0 0 1], which is corroborated by complementary MOKE measurements.

Magnetic Dichroism and High Resolution Photoemission Study of Materials

Magnetic dichroism in photoemission and core absorption provides detailed information on electronic states in magnetic materials. High resolution resonance photoemission spectroscopy is a powerful tool to probe correlated electronic states. We report some results of Ni, pyrite compounds FeS2 and NiS2, Ce compounds CeNiSn and CePdSn, and perovskite Mn oxides Nd1-xSrxMnO3 with x=0.5 and 0.47.

Photoelectron diffraction in magnetic dichroism: Surface live magnetic layers in fcc Fe/Co(001)

We combine photoelectron diffraction and magnetic dichroism in photoemission to obtain magnetic and structural information regarding ultrathin films on an element-resolved basis. Crystallinity-induced effects in the magnetic dichroism of $2p$ core level photoelectrons present valuable structural information regarding magnetically inhomogeneous films. The location of ferromagnetic layers in structurally relaxed fcc Fe films (eight monolayers) grown at room temperature on Co/Cu(001) is found to be at the surface, with nonferromagnetic layers underneath. Tetragonally distorted fcc Fe films (one and three monolayers) are found to be entirely ferromagnetic.

Magnetic dichroism in photoemission from oxygen atoms adsorbed on cobalt surfaces

We report on the first observation of magnetic circular dichroism in the angular distribution MCDAD from O 2p photoelectrons from oxygen adsorbed on thin epitaxial cobalt films on W(110). Up to now, MCDAD effects in photoemission have only been observed from ferromagnetic thin films and single crystals. Here, we show evidence for a small splitting in the oxygen induced 2p x and 2p z orbitals when reversing the magnetization direction. The observed effect will be discussed with respect to related spin-resolved photoemission experiments.

Interfacial Magnetism of Eu/Gd(0001) Studied by Magnetic Circular Dichroism in Photoemission

We report on the fabrication of an atomically flat heteromagnetic rare-earth interface of 1-ML-Eu $(6\ifmmode\times\else\texttimes\fi{}6)$/Gd(0001), and its element-specific magnetic analysis by means of magnetic dichroism in photoemission, revealing a high net Eu magnetization at low temperatures. This magnetic Eu phase is due to a strong positive interlayer exchange coupling across the interface which overrules the weak negative intralayer coupling between Eu spins in the hexagonal two-dimensional lattice.

Magnetic dichroism in the angular distribution of Fe 2p and 3p photoelectrons: Empirical support to Zeeman-like analysis

We report on the measurements and analysis of Fe 2p magnetic dichroism in the angular distribution of the photoelectrons from remanently magnetized Fe(100) surfaces with unpolarized, monochromatized, x rays of 1486 eV energy, and with linearly polarized synchrotron radiation of 800 eV energy. The analysis of the dichroic photoemission intensity in the two experiments verifies the applicability of the atomic photoionization model which provides a consistent understanding of the differences between photoemission experiments with unpolarized and linearly polarized radiation. A comparison of the Fe 2p and Fe 3p dichroism spectra allows us to discuss, on an empirical basis, the validity of a Zeeman-like interpretation of Fe 3p hole sublevels connected to the observed magnetic dichroism in photoemission.

Magnetic dichroism in photoemission as interface magnetometer with atomic-layer resolution

Magnetic dichroism in photoemission as interface magnetometer with atomic-layer resolution

Magnetic interface formation at Fe/Cr/Fe(100)

Photoemission magnetic dichroism of Fe and Cr 3p core levels was employed to investigate the magnetic order at the Fe/Cr/Fe(100) interface. For submonolayer and monolayer Fe coverages on Cr(100) the interface system appears magnetically frustrated, with no net magnetization along the parallel (antiparallel) direction of the substrate magnetization. The analysis of the Fe 3p photoemission dichroism allows to conclude that, at these low coverages, Fe is magnetically ordered but along an axis lying at 90° from the substrate Fe(100) magnetization, favoring the interpretation of biquadratic coupling between the ultrathin iron overlayer and the bulk iron substrate, across the Cr interlayer.

Fe 2p photoemission magnetic dichroism with linearly polarized synchrotron radiation and with unpolarized A1 Kα radiation

We report on the measurements and analysis of Fe 2p magnetic dichroism in the angular distribution of the photoelectrons from remanently magnetized Fe(100)surfaces with unpolarized, monochromatized A1 Kα X-rays of 1486 eV energy (MDAD)and with linearly polarized synchrotron radiation of energy 800 eV (LMDAD). The two experiments verify the atomic photoionization model of (L)MDAD including the dichroism effect between field-averaged experiments of different chirality.

Magnetic interface formation at Cr/Fe(100) and Fe/Cr/Fe(100): Magnetic dichroism in photoemission study

The early stages of the growth of Cr/Fe(100) and Fe/Cr/Fe(100) interfaces have been investigated by magnetic dichroism in photoemission of Fe 3p and Cr 3p core levels as measured from chiral experiments employing linearly polarized synchrotron radiation. Evidence is obtained for a 30% larger magnetic moment of interface Cr atoms with respect to Cr atoms belonging to epitaxial ultrathin films and a 40% magnetic moment enhancement of top Fe interface atoms in the Fe/Cr/Fe(100) trilayer. The kinetic growth conditions (450 K) lead to a uniform overlayer growth, without intermixing, but dominated by islanding. As a consequence the formation of a single-surface ferromagnetic domain for Fe/Cr/Fe(100) is frustrated up to two Fe monolayer (ML) thickness. The line shape of Fe 3p photoemission in the frustrated regime is consistent with the presence of in-plane magnetic order at 90\ifmmode^\circ\else\textdegree\fi{} with respect to the substrate magnetization direction. The appearance of photoemission magnetic dichroism for Fe overlayer thicknesses exceeding 2 ML is interpreted as due to domain rotation towards the direction antiparallel to the Fe substrate magnetization.

Magnetic dichroism in photoemission as a spin-resolving probe for electronic correlations.

Magnetic dichroism with unpolarized x rays in angle-resolved photoemission has been systematically investigated for the 2 p core levels of Fe, Co, and Ni. In contrast to Fe 2 p, we find the dichroism in the Ni 2p spectra to be mainly governed by electronic correlations. The fine structure observed in the Co 2p spectrum indicates a distinct influence of many-body interactions also in this material. @S0163-1829~96!50446-7# Electronic interactions govern many phenomena in solidstate physics. Ferromagnetism, for instance, is a direct consequence of spin-dependent electronic correlations. Besides their influence in the ground state, correlations are very important for understanding electron spectroscopy results. These techniques always probe the many-electron system, even if the results may often be interpreted in the singleparticle limit. A famous example for the inadequacy of the single-particle picture is photoemission from Ni. Although a density functional approach can reproduce the average exchange splitting found in the experiment by a proper choice of the exchange correlation potential, 1 it fails to explain the photoemission satellite structures found, 2 yet another manifestation of electronic correlations. Their interpretation requires suitable many-electron formalisms, for example, a Hubbard-type approach as has been successfully applied to Ni. 3 A narrow d band in Ni—as compared to Fe and Co—is held responsible for the formation of correlation-induced spectral features. This bandwidth argument leads to an exciting aspect if surfaces and thin films are considered. It is well-known that the smaller number of nearest-neighbor atoms at the surface causes a dehybridization of the wave functions and thereby a band narrowing. A similar effect happens in monolayer films as a consequence of the reduced dimensionality, as long as hybridization with the substrate is negligible. As pointed out recently by Chen, 4,5 many-particle