Highlights

Magnetic Phase Transition Probed by High Momentum Resolution EELS

Scientific Achievement

We have used High Angular Resolution Electron Channeling Electron Spectroscopy (HARECES) to demonstrate the temperature dependence of the magnetic linear dichroism (MLD) through a magnetic phase transition for the first time. Definitive and reproducible electron energy-loss spectra (EELS) exhibiting the temperature dependence of the magnetic linear dichroism (MLD) were obtained by HARECES on an antiferromagnetic hematite (a-Fe₂O₃) single crystal using a nanoprobe in a transmission electron microscope.

Many past attempts for detecting MLD using conventional EELS techniques showed inconsistent results due to the lack of angular resolution and signal-to-noise ratio, specimen quality, and instrumental stability. The present study with HARECES using a high quality hematite sample, overcame the above difficulties. The HARECES is a high momentum resolution EELS technique developed at EMC, Argonne.

The Fe L₂₃ EEL spectra acquired above and below the Morin temperature (263 K) clearly showed changes due to the temperature dependent magnetic phase transition, and agreed with the calculation based on the atomic multiplet method for the octahedrally coordinated Fe³⁺ ion in hematite, showing distinct dependence of Fe L₂₃ spectra on the orientation of the magnetic moments. However, the diffraction pattern did not show apparent changes, indicating the change in spectra was primarily of magnetic origin and not a structural effect. Furthermore, this high quality EELS data showed an excellent agreement with published synchrotron-based x-ray MLD spectra, and can be used for quantitative analyses.

Significance

Since the spin transport occurs through the bulk of multi-layered nanostructures, techniques, which are directly sensitive to the magnetic anisotropy within a bulk and its interfaces, are of great interest.  The significance of the work is that the quality of the linear dichroic signal obtained by this work is significantly better than the previously obtained spectra by conventional methods. Therefore, it can be used for quantitative analyses of the magnetic dichroism with high reliability.  Hence, this will serve as a powerful tool for the direct investigation of magnetic nanostructures and their interfaces with increasing spatial resolutions.

A part of this work has been published in Micros. Microanal. 14 (Suppl.2), 1366-1367 (2008), and as a result, the authors have been invited for a book chapter contribution in “Linear and Chiral Dichroism in Electron Microscope” (editor P. Schattschneider), and international collaborations.

Performers

Y. Ito (Northern Illinois U.; Argonne-MSD); N. J. Zaluzec, A. N. Chiaramonti, D. J. Miller (Argonne-MSD); M. van Veenendaal (Northern Illinois U.; Argonne-APS)