Kiyou Shibata1
The University of Tokyo1
In-situ heating, cooling, and electronic current application in transmission electron microscope (TEM) enable us to investigate dynamics of various physical properties of nano-scale structures. Besides the ability of simple imaging and diffraction, TEM can capture various physical processes with combination of dedicated imaging and spectral measurement for specific physical properties. Along with observation methods, focused-ion beam fabrication helps us with preparing various type TEM samples specialized for the measurement. In this talk, I would like to introduce some applications of in-situ TEM measurement and FIB fabrication for investigating dynamics of physical properties.<br/>In the former part, I will talk on a magnetic dynamics in thin lamella of a chiral magnet. Magnetic skyrmions are nano-scale magnetic vortices stabilized in magnetic materials with anti-symmetric spin interaction originating from structural symmetry breaking. They had been considered to be mobile under an extremely low electric current application from several experimental evidences, but dynamics of coexistence states of magnetic skyrmions and helical state had not been revealed by a direct observation. We prepared a thin lamella sample of B20 type FeGe using the FIB and connected to electrodes. We observed the dynamics of magnetic structures in chiral magnet of FeGe due to the current pulse application using in-situ a cooling and current application holder. By a Lorentz transmission electron microscopy, we observed the magnetic domain distribution in a lamella sample before and after the pulse current application. We analyzed the images and deduced the critical current density and current density and the pulse duration dependence of velocity of the magnetic domains [1].<br/>In the latter part, I would like to introduce an experimental result about thermal expansion of grain boundaries (GBs) investigated via electron energy-loss spectroscopy (EELS). The GBs are nano-scaled localized structures distinct from that of bulk and can possess peculiar physical properties due to their atomic arrangements. Concerning a coefficient of thermal expansion (CTE), although experimental and theoretical studies generally concluded that increasing amounts of GBs enhance the CTE of polycrystals, it is still not clear how the presence of GBs modifies the CTE of polycrystals. In order to reveal the thermal expansion at GBs, we used bicrystals of SrTiO3 (STO) to investigate how specific GBs contributes to the CTE enhancement. For the measurement of CTE, we utilized a plasmon loss spectroscopy, which has been recently applied for the direct measurement of thermal expansion in 2-D materials[2]. Lamella samples including GBs were fabricated from several types of STO bicrystals and mounted on a MEMS heating tip by SEM/FIB. Using a heating holder, we obtained EELS spectra of the plasmon loss region at various temperatures. Analyzing plasmon peak map in varied temperature, we obtained CTE for bulk and GBs from the bicrystals. Our analysis indicates that CTE of GBs can be larger than bulk and is different possibly depending on the local atomic arrangements at GBs [3].<br/>References<br/>[1] K. Shibata <i>et al.</i>, <i>Nano Lett.</i> <b>18</b>(2), 929–933 (2018).<br/>[2] X. Hu <i>et al.</i>, <i>Phys. Rev. Lett.</i> <b>120</b>, 055902 (2018).<br/>[3] K. Liao, K. Shibata, and T. Mizoguchi, <i>submitted</i>.<br/>Acknowledgements<br/>These works were done in collaboration with Dr. T. Tanigaki, Dr. T. Akashi, Dr. H. Shinada, Dr. K. Harada, Dr. K. Niitsu, Prof. D Shindo, Dr. N. Kanazawa, Prof. Y. Tokura, Prof. T. Arima, Dr. K. Liao, and Prof. T. Mizoguchi. These studies were supported by the special postdoctoral researcher program of RIKEN, JSPS KAKENHI (Grant Nos. 24224009, 15H05456, 17H06094, 18K14117, 19H00818, and 19H05787), and JST CREST (Grant Nos. JPMJCR16F1, JPMJCR1993). The STEM observation and EELS were obtained with the support program “Nanotechnology Platform at NIMS” of MEXT and helpful supports from Dr. F. Uesugi at NIMS.