Masako Sakamaki

Depth-resolved characterization of buried magnetic interfaces is vital for advancing spintronic devices, yet consistent analysis remains challenging. Fluorescence-yield depth-resolved X-ray magnetic circular dichroism (XMCD) enables quantitative profiling of spin and orbital moments, but conventional background (BG) subtraction is empirical and operator-dependent. We present a systematic framework using unsupervised t-distributed stochastic neighbor embedding similarity mapping to automate and optimize BG subtraction. Validation tests confirm robustness against spectral noise while revealing strong sensitivity to BG subtraction, highlighting the critical role of optimized BG treatment. Applied to a Graphene/L1 ₀ -FePd thin film, the method reproduces interfacial spin suppression and orbital enhancement with expert-level consistency. This framework thus provides a standardized and reproducible approach to depth-resolved XMCD, broadly applicable to interfacial magnetism in spintronic materials.

In this study, a method for reflection-mode soft x-ray absorption spectroscopy was developed to realize three-dimensional chemical-state imaging. Soft x rays from a pinhole were reflected by the sample, and the magnified image was observed with a two-dimensional detector. This technique was applied to a Co film with an Au-island-covered surface to obtain the surface chemical state images with a spatial resolution of several tens of micrometers. Furthermore, the soft x-ray reflection spectra within and outside the Au layer were extracted from the images by changing the photon energy. Distinct differences were observed at the Co absorption edge. By considering anomalous x-ray scattering around the Co L-edges in the simulation, the reflection spectrum near the absorption edge in the nm depth resolution was reproduced. In the region without the Au layer, the results were well reproduced, assuming that 4 nm CoO was formed at the surface. These results demonstrate the feasibility of three-dimensional imaging of the chemical states in multilayer films.

We develop a bond-length mapping technique without two-dimensional scanning by combining photoelectron emission microscopy with extended X-ray absorption fine structure (EXAFS) analysis. Several tens of thousands of EXAFS data are simultaneously acquired, which corresponds to a data acquisition time less than 1 second per spectrum. We apply this technique to an Fe/BaTiO ₃ thin film, and two-dimensional distribution of the Fe–Fe bond length at the Fe surface is observed. From the Fe K-edge EXAFS analysis, we find that the Fe–Fe bond length is ∼2.52 Å over the whole area with a Gaussian distribution of ∼0.01 Å, which implies that this technique realizes a bond-length mapping with precise bond-length determination.

A fluorescence-yield wavelength-dispersive x-ray absorption spectroscopy technique in the soft x-ray region, by which the x-ray absorption spectra are recorded without scanning the monochromator, has been developed. The wavelength-dispersed soft x rays, in which the wavelength (photon energy) continuously changes as a function of the position, illuminate the sample, and the emitted fluorescence soft x rays at each position are separately focused by an imaging optics onto each position at a soft x-ray detector. Ni L-edge x-ray absorption spectra for Ni and NiO thin films taken in the wavelength-dispersive mode are shown in order to demonstrate the validity of the technique. The development of the technique paves the way for a real-time observation of time-dependent processes, such as surface chemical reactions, with much higher gas pressure compared to the electron-yield mode, as well as under magnetic and electric fields.

High signal-to-background (S/B) ratio depth-resolved soft X-ray absorption spectroscopy (XAS) is achieved by an energy-selective detection of the fluorescence with a diffraction grating at different emission angles. We apply this technique to a Au/FeCo/BaTiO ₃ thin film, and selectively observe the Fe and Co Lα fluorescences. The S/B ratio for the Co L-edge depth-resolved XAS is then enhanced by nearly two orders of magnitude. We observe the spectra at different probing depths, and find from the layer-resolved analysis that ∼60% of bottom Co is oxidized. A significant improvement in the observation of the buried interface state is thus successfully demonstrated.

We study an electric field-induced redox reaction at the interface of GdO _x /Co thin film, by means of soft X-ray absorption spectroscopy.

Effects of electric fields on magnetization of ferromagnetic Ni films grown on a Cu(001) single crystal covered with antiferromagnetic NiO overlayer are investigated by means of X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), and magneto-optical Kerr effect (MOKE). The growth of the NiO overlayer on the Ni film is confirmed by XAS, and it is revealed by XMCD and MOKE that the NiO/Ni films show a spin reorientation transition from in-plane to perpendicular magnetization with increasing Ni thickness. It is also observed that the coercive field of the Ni films increases as the NiO thickness increases, possibly due to the interaction with antiferromagnetic NiO. Remanent magnetization of the Ni film is found to be modified by the application of electric fields. The possible origin of the electric-field effects is discussed, and some change in magnetic anisotropy of the Ni film is suggested. [DOI: 10.1380/ejssnt.2018.186]

Effect of energetic C60 cluster ion irradiation on magnetic properties in FeRh thin films was examined by SQUID and XMCD measurements. The XMCD signal for the C60 irradiated samples is larger than that for the C1 samples in the initial stage of the irradiation. In contrast, as for the irradiation with larger ion fluence, the XMCD spectrum for the sample irradiated with C1 ion is larger than that for the sample irradiated with C60 cluster ion. Although the magnetization for the C60 irradiation samples continuously decreases with increasing in the ion fluence, the saturation magnetization for the C1 irradiation samples increases with increasing in the ion fluence. Considering these different behaviors in the irradiation induced ferromagnetism, the cluster ion can effectively deposit the apparent elastic collision energy at the surface region in the film, which effectively causes the ferromagnetic state at the surface of the films.