Atomic origin of the spin-polarization of the Co2FeAl Heusler compound

Jaw Yeu Liang, Tu Ngoc Lam, Yan Cheng Lin, Shu Jui Chang, Hong Ji Lin, Yuan Chieh Tseng

Research output: Contribution to journalArticle

2 Scopus citations

Abstract

Using synchrotron x-ray techniques, we studied the Co2FeAl spin-polarization state that generates the half-metallicity of the compound during an A2 (low-spin) → B2 (high-spin) phase transition. Given the advantage of element specificity of x-ray techniques, we could fingerprint the structural and magnetic cross-reactions between Co and Fe within a complex Co2FeAl structure deposited on a MgO (0 0 1) substrate. X-ray diffraction and extended x-ray absorption fine structure investigations determined that the Co atoms preferably populate the (1/4,1/4,1/4) and (3/4,3/4,3/4) sites during the development of the B2 phase. X-ray magnetic spectroscopy showed that although the two magnetic elements were ferromagnetically coupled, they interacted in a competing manner via a charge-transfer effect, which enhanced Co spin polarization at the expense of Fe spin polarization during the phase transition. This means that the spin-polarization of Co2FeAl was electronically dominated by Fe in A2 whereas the charge transfer turned the dominance to Co upon B2 formation. Helicity-dependent x-ray absorption spectra also revealed that only the minority state of Co/Fe was involved in the charge-transfer effect whereas the majority state was independent of it. Despite an overall increase of Co2FeAl magnetization, the charge-transfer effect created an undesired trade-off during the Co-Fe exchange interactions, because of the presence of twice as many X sites (Co) as Y sites (Fe) in the Heusler X 2 YZ formula. This suggests that the spin-polarization of Co2FeAl is unfortunately regulated by compromising the enhanced X (Co) sites and the suppressed Y (Fe) sites, irrespective of the development of the previously known high-spin-polarization phase of B2. This finding provides a possible cause for the limited half-metallicity of Co2FeAl discovered recently. Electronic tuning between the X and Y sites is necessary to further increase the spin-polarization, and likely the half-metallicity as well, of the compound.

Original languageEnglish
Article number075005
JournalJournal of Physics D: Applied Physics
Volume49
Issue number7
DOIs
StatePublished - 29 Jan 2016

Keywords

  • Heusler
  • synchrotron radiation
  • XMCD

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