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X-Ray Standing Waves on Surfaces
Published in Arthur T. Hubbard, The Handbook of Surface Imaging and Visualization, 2022
The X-ray standing wave (XSW) technique is a sensitive tool for determining the position of atoms within a crystal, adsorbed onto a surface, or distributed within the crystal or at the interface. The technique is based on the X-ray standing wave field that arises as a result of the interference of coherently related incident and reflected plane waves (Figure 64.1) and is described by the theory of dynamical diffraction of X-rays.1–3 When two coherently related traveling plane waves having the same wavelength pass through each other, their superposition results in a standing wave of period D = ʎ/2 sinϘ, where X is the wavelength of the traveling waves, and 20 is the relative angle between them. The generation of a standing wave requires both an incident and a reflected wave, and the latter can be generated by either Bragg diffraction or total external reflection.
Electronic structures of MgO/Fe interfaces with perpendicular magnetization revealed by hard X-ray photoemission with an applied magnetic field
Published in Science and Technology of Advanced Materials, 2019
Shigenori Ueda, Masaki Mizuguchi, Masahito Tsujikawa, Masafumi Shirai
In this paper, we discussed the electronic and magnetic states of the Fe film near the MgO/Fe interface by comparing the valence-band and Fe 2p core-level spectra for the MgO (2 nm)/Fe (1.5 and 20 nm)/MgO(001) structures. In contrast, Yang et al. [24] reported soft X-ray standing-wave (SW) PES for the AlOx (~1.2 nm)/MgO (~1 nm)/wedged-Fe (0–20 nm) structure on the [MoSi2/Si]80 multilayer with 4.98 nm periodicity. Since the SW period is same as the period of the multilayer and typical resolution in depth is ~1/10 of the period, they were able to derive the depth-resolved sample structure and magnetization profile for the in-plane magnetization component near the MgO/Fe interface from SW-PES experiments. Although the sample fabrication method, sample structure, and photon energy in Ref [24] are different from this work, it is expected that the SW technique with HAXPES yields more precise depth distribution of electronic and magnetic states with a few Å resolution in depth near the MgO/Fe interface, as a future direction.
Direct observation of spin-resolved valence band electronic states from a buried magnetic layer with hard X-ray photoemission
Published in Science and Technology of Advanced Materials, 2021
Various methods used in vacuum ultraviolet (VUV) and soft X-ray PES are also realised in HAXPES experiments. For example, magnetic circular and linear dichroism (MCD and MLD) in HAXPES [8–11], X-ray standing-wave (XSW) HAXPES [12], hard X-ray angle-resolved PES (HARPES) for band dispersion [13,14], XSW-HARPES [15], HAXPES combined with X-ray total reflection [7], and spin-resolved HAXPES [10,11,16,17] have been reported in the last decade. Among them, spin-resolved HAXPES can be a potential candidate for the direct probe of the bulk-sensitive spin-dependent valence band electronic structures of magnetic materials. However, due to extremely low cross-section of the valence electrons in the hard X-ray region relative to the VUV and soft X-ray regions (see Figure 1), the reported spin-resolved HAXPES (spin-HAXPES) experiments have limited in the Fe 2p core–level of Fe based ferromagnetic materials [10,11,16,17]. In addition, the extremely low efficiency of a typical spin detector, such as a Mott detector [18–20] whose figure of merit (FOM) is in the order of 10−4, prevents us from accessing the spin-HAXPES experiments in the valence band region. While Kozina et al. [17] reported spin-HAXPES experiments in the vicinity of the Fermi-level (EF) for body-centred cubic FeCo(001), bulk-sensitive spin-HAXPES for the entire valence band region has not been reported until now. To realise the spin-HAXPES experiments for the valence band region, either the development of the X-ray light source or the improvement of FOM in the spin detector is required, since the extremely low cross-section and FOM represent the photon-hungry and electron-hungry experimental aspects, respectively.