Electron holography observation of electron spin polarization around charged insulating wire

Microscopy, 2024, 73(4), 367–375 DOI: https://doi.org/10.1093/jmicro/dfad056 Advance Access Publication Date: 11 November 2023 Letter Electron holography observation of electron spin polarization around charged insulating wire Takafumi Sato1 , Keiko Shimada2 , Zentaro Akase1 , Hideyuki Magara1 , Takeshi Tomita2 and Daisuke Shindo2,* 1 2 To whom correspondence should be addressed. E-mail: Abstract We report direct observation by electron holography of the spin polarization of electrons in a vacuum region around a charged SiO2 wire coated with Pt–Pd. Irradiating the SiO2 wire with 300 keV electrons caused the wire to become positively charged due to the emission of secondary electrons. The spin polarization of these electrons interacting with the charged wire was observed in situ using a phase reconstruction process under an external magnetic field. The magnetic field of the spin-polarized electrons was simulated taking into account the distribution of secondary electrons and the effect of the external magnetic field. Key words: electron holography, electron spin polarization, charging effect, secondary electron, insulating wire Introduction The microstructure of various materials has been widely studied using transmission and analytical electron microscopy [1–10], and electric and magnetic fields both inside and outside materials have been investigated using Lorentz transmission electron microscopy [11], differential phase contrast scanning electron microscopy [12,13] and electron holography [14–24]. The magnetic flux distribution and domain structure of various magnetic materials have been successfully analyzed with these techniques. The purpose of this study was to directly observe the electron spin polarization of secondary electrons around non-magnetic insulating materials by using electron holography. Previous studies used electron holography to systematically investigate charging phenomena in insulating materials. When thin insulating specimens were irradiated with highenergy electrons, secondary electrons were emitted from the specimens, and the specimens became positively charged. As the charging effect increased, the emitted secondary electrons tended to be attracted to the positively charged specimens. The distribution of secondary electrons emitted around charged microfibrils in sciatic nerve tissues was observed by detecting electric field variations due to the motion of secondary electrons. The orbits of secondary electrons around the microfibrils were identified by using reconstructed phase [25,26] and amplitude [27,28] images captured using electron holography under disturbance-free conditions [28], which is similar to quantum nondemolition [29–31] and negative-result [32] measurements. The accumulation and distribution of secondary electrons around the surfaces of various insulators such as epoxy resin [33] and ferroelectric BaTiO3 and cellulose nanofibers [15] have also been investigated. In addition, the energy distribution of secondary electrons has been estimated using a secondary electron analyzer developed for a 300-kV transmission electron microscope [34]. Most of the secondary electrons were found in the energy range below 20 eV. Furthermore, a study of a charged mica specimen revealed that it is possible to detect the magnetic field due to the electron spin polarization of secondary electrons caused by applying an external magnetic field [15]. In the study reported here, we investigated the magnetic flux distribution caused by the spin polarization of secondary electrons around a SiO2 wire under the application of an external magnetic field. We compared reconstructed phase images with simulated phase images. Methods In the previous study with a thin specimen of non-magnetic mica prepared by a focused ion beam method, the magnetic field due to the spin polarization of secondary electrons under the external magnetic field could be detected [15]. To obtain a clear and strong signal of the magnetic field due to electron spin polarization, we constructed a simple geometric structure containing a thin insulating wire. The SiO2 wire with a diameter 0.65 μm was prepared by burning quartz glass with an intense H2 –O2 gas flame. The wire and its base fixed to a Received 15 February 2023; Revised 26 October 2023; Editorial Decision 2 November 2023; Accepted 10 November 2023 © The Author(s) 2023. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/ licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site–for further information please contact . * Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 368 external magnetic field applied by the magnetizing stage [36]. When the beam tilt is corrected, the image position of a biprism wire-electrode is not changed after the application of external magnetic field. Figure 1c is an optical microscope image showing the configuration of the wire and the poles of the electromagnet. Holograms were observed at the center of the wire. To estimate the electric potential of the wire, we used the ELFIN calculation code [37] to perform electric field analysis using Maxwell’s equation in integral form. Before observing each hologram of the SiO2 wire, we observed the reference hologram without the specimen with and without external magnetic field. By using the reconstructed phase images obtained from these holograms, we removed the system artifact and noises of the microscope system and the external magnetic field. Results and discussion In conventional electron holography, electric and magnetic information can be obtained from a hologram by superimposing an object wave containing electric and/or magnetic field information and a reference wave without information about the electric and magnetic fields in a vacuum region. In an experiment investigating electron spin polarization using conventional electron holography, a reference wave without electric and magnetic information cannot be obtained. Since the secondary electrons are widely distributed around the wire, there is an electric field due to the secondary electrons and a magnetic field due to the constant external magnetic field and a magnetic field due to the electron spin polarization. Since the external magnetic field is uniform in the reference and object wave regions, the phase shifts in the two r (...truncated)