Spin-polarized electron energy-loss spectroscopy (SPEELS)

Bärbel Fromme

Electron energy-loss spectroscopy (EELS) is a widely used experimental technique for investigations of the excitation spectra of atoms, molecules and solids. In this method, advantage is taken of the possibility of excitation by electron impact: An incident electron beam of fixed primary energy is inelastically scattered at the target and the energy distribution of the scattered electrons is measured with respect to the incident electron energy. This energy distribution - the electron energy-loss spectrum - directly reflects the target excitations, because an excitation leads to the appearance of electrons in the scattered beam, which suffered a characteristic energy-loss corresponding to the excitation energy. If other properties of the scattered electron beam, as angular distribution or polarization, are measured additionally, the kind of interaction between incident electrons and target, responsible for target excitation and inelastic scattering process, respectively, can be inferred.

Beside electronic transitions, collective vibronic excitations as phonons and plasmons are observable with EELS. In addition, Auger transitions and vibronic excitations of adsorbed molecules can be investigated. Concerning optical absorption spectroscopy, EELS has several advantages, especially when low-energy electrons are used: Then EELS is a very surface sensitive method. Due to the very small mean free path length of electrons with 30 - 100 eV energy, the incident electrons are able to penetrate the first few atomic layer of a solid only. Therefore, investigations of the electronic structure of the clean and adsorbate-covered surface of a solid, important for the understanding of catalytic processes for example, are possible. Also thin films and layers and there magnetic coupling to the substrate, essential for the understanding and development of data storage devices, can be examined. Further, low-energy EELS is especially predestined for the excitation and examination of dipole-forbidden transitions. The reason is the "breakdown" of dipole-selection rules for low-energetic electrons: The probability of electric monopole as well as higher multipole transitions and excitations by electron-exchange increases with decreasing energy of the incident electrons.

Both, investigations of magnetic coupling as well as exchange-excitation processes require the use of polarized electrons, the spin-polarized electron energy-loss spectroscopy (SPEELS). In the Institute of Applied Physics we use a very sophisticated SPEELS-version with both, a polarized primary electron beam and the polarization analysis of the scattered electrons (2.). With such an experimental setup only, it is possible to proof exchange processes unambiguously and to examine their behavior, because inelastically scattered incoming and emitted target electrons become distinguishable, if their spin direction is different: If the polarization P_s(DE) of the electrons, scattered with energy-loss DE, deviates from the polarization of the primary electron beam P₀, the target-excitation, requiring an excitation-energy equal to DE, has been accompanied by an exchange of incoming and target electrons of opposite spin-direction. Such exchange processes are usually called spin-flip exchange processes, although not the spin of an electron is flipped during the scattering process, but two electrons of different spin are exchanged. Non-flip exchange processes, where the exchange occurs between electrons of identical spin direction are of course not provable by SPEELS, because these electrons remain indistinguishable. Non-flip exchange processes therefore cannot be distinguished from direct scattering without exchange.

In the last few years, we concentrated on the electronic structure of bulk and surface of the antiferromagnetic transition-metal oxides NiO, CoO and MnO. Especially the dipole-forbidden d-d excitations of bulk and surfaces transition-metal ions and their behavior has been thoroughly investigated by SPEELS. Applications of the oxides, their physical properties and the central significance of the 3d-electrons of the transition-metal ions for these properties is briefly summarized in 3., followed by a brief survey about our SPEELS-results obtained at the oxides (4.).

2. Experimental setup

The experimental setup of our SPEELS-experiment is scheduled in Fig. 1. Significant experimental parameters are summarized in Tab. 1.

                           Fig. 1: Experimental setup of our SPEELS-experiment [1]                                        

                                Tab. 1: Significant experimental parametrs.

The polarized electrons are generated by a conventional GaAs-source [2]. Electrons, scattered under the fixed scattering angle of 90°, given by the axes of the electron optics are energy analyzed by a spherical 180°-spectrometer. Measurements in others than specular scattering geometry are possible by rotating the sample (Fig. 1). Spin-analysis of scattered as well as incident electrons is done by a conventional high-energy (100 keV) Mott-detector [3]. For the measurements of the polarization of the incident electrons, a repulsive potential is applied to the target, so that the primary electrons reach the Mott detector without interaction with the target atoms.

The targets were single crystals. Measurements were performed at freshly in situ cleaved as well as sputtered surfaces.

3. Transition-metal oxides and their 3d-states

Transition-metal oxides form a fascinating class of compounds with a wide range of technical applications: They are used in catalysis, lasers and magnetic recording as well as in sensors for gases, high pressure and magnetic fields for example. For a lot of physical properties, responsible for the variety of applications of the oxides, especially the transition-metal (TM) 3d-electrons and their behavior are of central significance. The 3d-electrons remain localized at the TM-ions in several oxides and do not show band-like, but quasi atomic-like character. In contrast to free atoms or ions, where the d-states are degenerate, the d-states of the transition-metal ions in the oxides are energetically split due to the crystal-field, provided by the surrounding oxygen-ions. Transitions between 3d-states are of great influence on the optical properties and often determine the color of the compounds. For example, the d-d transitions of the Cr³⁺-ions in Cr₂O₃ are responsible for the green color of this compound, used in ceramic glazing since centuries. The equivalent transitions at Cr³⁺, embedded in the different crystal-field of Al₂O₃, provide the beautiful red color of ruby and are used for the generation of laser light in the ruby [4]. In addition, the energetic shift of these transitions due to the pressure dependence of the crystal-field splitting is used in sensors for very high pressures of up to more than 10⁹ Pa.

The knowledge of the behavior of adsorbates is essential for the understanding of the catalytic processes at the oxides’ surfaces. Here, the localized 3d-states are also of considerable importance, because the crystal-field splitting of surface transition-metal ions, which deviates from that of bulk-ions due to missing oxygen, can be used to determine the adsorption-sites of molecules: If the adsorbed molecule occupies regular surface-sites, it replaces the missing oxygen-ions at the surface and the surface crystal-field splitting is found to be altered after adsorption. It is similar to the crystal-field splitting of bulk transition-metal ions again. No changes are expected, if the adsorbed molecule resides at other sites, as surface defects for example. By use of this effect, the adsorption of NO on regular surface-sites at the Ni²⁺-ions of NiO was shown; OH-molecules on the contrary were found to be adsorbed at defect sites [5, 6].

In the transition-metal oxides with incompletely-filled 3d-shell like Cr₂O₃, MnO, FeO, CoO and NiO the localization of the TM3d-electrons is responsible for their insulating nature. These oxides were among the first solids found, where band-pictures fail in describing a wide range of physical properties: From simple band-structure calculations, the 3d-electrons are expected to form an incompletely-filled 3d-conduction-band, leading to metallic conductivity of the oxides similar to that of the corresponding transition-metals [7], which is actually not observed. In fact, these oxides belong to the class of Mott-Hubbard or charge-transfer insulators, because a strong Coulomb correlation prevents the electrons from forming a 3d-band and localizes them at the transition-metal. The electrons cannot move freely and an energy of several electron volts is needed for the electron-transfer between neighboring transition-metal ions. It was the discovery of this insulating behavior of the transition-metal monoxides in 1937 [8] and the contradiction to the young field of band-structure calculations, which started the intense investigations in their electronic structure, lasting up to know. Especially in the last few years, the growing interest in sensors and catalysis and in particular the discovery of high-temperature superconductivity in the Cu-O-perovskites forced the intensification of studies concerning all oxides’ bulk as well as surface electronic structure.

Excitations within the crystal-field multiplet are transitions between 3d-states and therefore strongly forbidden by dipole-selection rules: All d-d transitions violate the parity selection rule Dl = ± 1 (Laporte rule). The multiplicity changing transitions violate the spin-selection rule Ds = 0 additionally. Whereas in NiO and CoO multiplicity conserving triplet-triplet and quartet-quartet transitions as well as multiplicity changing triplet-singlet and quartet-doublet transitions occur, in MnO with its half-filled 3d-shell all transitions from the ⁶A_1g (⁶S) crystal-field ground-state are multiplicity changing and forbidden by the parity- as well as the spin-selection rule.

Generally, electric dipole-forbidden transitions remain forbidden in the crystal field, but the parity selection rule is slightly released by an admixture of odd-parity parts to the even-parity wave function due to symmetry distortions arising from lattice vibrations [9, 10, 4]. But nevertheless, the dipole matrix-element remains small, leading to optical absorption coefficients two to three orders of magnitude lower than that of dipole-allowed transitions across the optical gaps of the oxides. Those transitions, which are forbidden by the spin-selection rule additionally, can occur in optical absorption spectra due to spin-orbit interaction. Their optical transition probabilities have been estimated to be seven orders of magnitude lower than those of dipole-allowed transitions [9]. Nevertheless, both kinds of transitions are visible in optical absorption spectra, but the latter with very weak intensity.

We now studied the dipole-forbidden d-d transitions of bulk and surface transition-metal ions in MnO, CoO and NiO by SPEELS with low-energetic electrons of 20 - 130 eV. It could be shown that most transitions are excellently excited by electron-exchange processes. The excitation energies of a variety of bulk and surface d-d transitions was measured here for the first time. By use of spin-resolved, scattering-geometry dependent measurements it was shown that different scattering mechanisms can contribute to the only parity-forbidden multiplicity conserving transitions of NiO and CoO, whereas the additionally spin-forbidden sextet-quartet transitions are exclusively excited by electron-exchange.

4. Results

Most of our results have been already published or will be published soon [11-17] and are summarized here briefly.

The efficiency of electron energy-loss spectroscopy especially for excitation and examination of dipole-forbidden d-d transitions may be demonstrated here by three excellent energy-loss spectra of the gap region of NiO, CoO and MnO (Fig. 2). The d-d excitations appear in the energy-loss range up to » 6 eV and give rise to sharp energy-loss peaks, clearly demonstrating the localized, atomic-like nature of the participating initial and final states (3.). In contrast to optical absorption spectroscopy, where the intensities of d-d excitations and dipole-allowed interband transitions across the optical gap deviate by several orders of magnitude (3.), in the energy-loss spectra here, the d-d excitation intensities are of the same order of magnitude as that of the dipole allowed ones, which give rise to the broad, more continuously distributed intensity occurring at energy-losses above » 4 - 5 eV. Especially for MnO, with its only multiplicity-changing, parity- and spin-forbidden d-d transitions, the intensities of the dominant d-d excitation with 2.82 eV excitation energy and the gap-transitions are of nearly identical size.

At several primary electron energies, corresponding to inner excitation thresholds, the d-d excitations are resonantly enhanced due to the formation and decay of a temporarily formed compound state [18, 17]; the spectra of Fig. 2 were recorded at such resonance primary energies. For the experimental determination of the excitation energies, their assignment to certain d-d transitions and the comparison with calculated values, knowledge and use of the resonance primary energies is essential: All d-d excitation are excellently visible here. The intensities are strongly reduced at off-resonant primary energies and the dominant d-d excitations only remain clearly visible, the weaker ones are not or hardly observable and can be measured in resonance only.

Especially the combination of the special advantages of spin-polarized electron energy-loss spectroscopy with resonant scattering is of great importance for measurement and assignment of the d-d excitations: Whereas the main d-d excitation peaks are of lower excitation energy and appear in the optical gap, the weaker d-d transitions of excitation energies above »  4 eV are superimposed with the onset of transitions across the optical gap, which increase rapidly with increasing energy-loss (Fig. 2). These d-d excitations and those of low intensity, which are superimposed by very intense ones are hardly visible in the spin-integrated spectra. But they are clearly observable in the spin-flip spectra (Fig. 3a, c) or in the polarization P_S(DE) of the scattered electrons (Fig. 3b, d). By use of spin-resolved measurements in resonance, it was possible to measure and assign nearly all sextet-quartet d-d transitions of MnO. For CoO, some of the d-d excitations of higher excitation energies have been measured here for the first time.

Spin-resolved scattering geometry dependent electron energy-loss measurements have been performed for several primary energies in and off-resonance. With these measurements, the central significance of electron-exchange up to more than 100 eV primary energy was shown for the d-d excitations in NiO, CoO and MnO. In resonance especially exchange excitations are found to be enhanced and the multiplicity changing as well as multiplicity conserving d-d excitations are completely exchange-determined independent of the scattering geometry; the intensity of the scattered electrons is angular wide spread, but with a distinct maximum in specular scattering geometry. Off-resonance, the angular wide spread exchange scattering is superposed by non-flip dipole-scattering processes, which are strongly confined to the small dipolar-lobe around specular scattering geometry, if slightly allowed multiplicity conserving d-d transitions are excited. Excitation by inelastic dipole-scattering is nearly completely missing in the spin-forbidden, multiplicity-changing d-d excitations of MnO, as expected. (Dipole scattering has been described in detail by several authors ([19] for example). Excitation by dipole-scattering is possible for dipole-allowed transitions only and not accompanied by electron exchange. The scattering intensity is often confined to a small angular range around specular scattering geometry, the so-called dipolar lobe). The dominant 2 eV d-d excitation of CoO, which is discussed controversially in the literature and often assigned to a quartet-doublet excitation, could now definitely identified as quartet-doublet excitation (⁴T_1g ® ⁴A_2g (⁴F)) by our scattering geometry dependent spin-polarized electron energy-loss measurements.

At the freshly cleaved NiO-surface, d-d excitations of the surface Ni-ions have been found and examined by scattering-geometry dependent SPEELS-measurements. A different scattering-geometry dependence of spin-flip- and non-flip intensity was observed for bulk and surface d-d excitations, providing the possibility to distinguish between both. Taking advantage of this possibility and the high intensity of all d-d excitations at resonance primary energies, two surface excitations have been measured here for the first time. The excitation energies are in  good agreement with calculated ones [5, 15].

References

 [11] B. Fromme, M. Schmitt, E. Kisker, A. Gorschlüter, and H. Merz, Phys. Rev. B 50, 1874 (1994)

Resonant Electron Exchange Excitations in Transition-Metal Oxides

[17] B. Fromme, U. Brunokowski, and E. Kisker, Phys. Rev. B in print, will be published 15.10. 1998

d-d excitations and interband transitions in MnO - a spin-polarized electron energy-loss study -

Localized d-d excitations in NiO(100) and CoO(100)

[19] H. Ibach and D. L. Mills, Electron Energy Loss Spektroscopy and Surface Vibrations, Academic Press, New York 1982