標題: Atomic origin of the spin-polarization of the Co2FeAl Heusler compound
作者: Liang, Jaw-Yeu
Lam, Tu-Ngoc
Lin, Yan-Cheng
Chang, Shu-Jui
Lin, Hong-Ji
Tseng, Yuan-Chieh
材料科學與工程學系
Department of Materials Science and Engineering
關鍵字: XMCD;Heusler;synchrotron radiation
公開日期: 25-Feb-2016
摘要: Using synchrotron x-ray techniques, we studied the Co2FeAl spin-polarization state that generates the half-metallicity of the compound during an A(2) (low-spin) -> B-2 (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 B-2 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 A(2) whereas the charge transfer turned the dominance to Co upon B-2 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 B-2. 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.
URI: http://dx.doi.org/10.1088/0022-3727/49/7/075005
http://hdl.handle.net/11536/129630
ISSN: 0022-3727
DOI: 10.1088/0022-3727/49/7/075005
期刊: JOURNAL OF PHYSICS D-APPLIED PHYSICS
Volume: 49
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