Abstract
Novel developments in X-ray based spectro-microscopic characterization techniques have increased the rate of acquisition of spatially resolved spectroscopic data by several orders of magnitude over what was possible a few years ago. This accelerated data acquisition, with high spatial resolution at nanoscale and sensitivity to subtle differences in chemistry and atomic structure, provides a unique opportunity to investigate hierarchically complex and structurally heterogeneous systems found in functional devices and materials systems. However, handling and analyzing the large volume data generated poses significant challenges. Here we apply an unsupervised data-mining algorithm known as DBSCAN to study a rare-earth element based permanent magnet material, Nd2Fe14B. We are able to reduce a large spectro-microscopic dataset of over 300,000 spectra to 3, preserving much of the underlying information. Scientists can easily and quickly analyze in detail three characteristic spectra. Our approach can rapidly provide a concise representation of a large and complex dataset to materials scientists and chemists. For example, it shows that the surface of common Nd2Fe14B magnet is chemically and structurally very different from the bulk, suggesting a possible surface alteration effect possibly due to the corrosion, which could affect the material’s overall properties.
Highlights
The projection images of the sample are acquired as the energy of the incoming X-rays is tuned from 6000 eV to 7023 eV with step size at 1 eV, covering the Nd L2 at ~6722 eV and L3 edges at ~6208 eV
We studied a small piece of Nd2Fe14B sample crushed from a magnet rod (Goodfellow, item #531-114-16) using the energy resolved transmission X-ray microscope (TXM) at beamline 6-2C of the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC National Accelerator Laboratory
The X-rays from a 56 pole 0.9 Tesla wiggler pass through several mirrors and are focused to about 200 microns for serving as the secondary source for the microscope
Summary
The projection images of the sample are acquired as the energy of the incoming X-rays is tuned from 6000 eV to 7023 eV with step size at 1 eV, covering the Nd L2 at ~6722 eV and L3 edges at ~6208 eV. One image of the sample as well as one reference image (with the sample moved out of the field of view) are recorded for flat field correction by applying Beer-Lambert law[45]. The data went through several image processing steps including the magnification correction, the spatial registration of images taken at different energies, and the normalization of the pixel spectrums. All of these were performed using an in-house developed software package known as the TXM-Wizard[2]
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