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

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 X2YZ 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.

[1]  S. Wolf,et al.  Magnetic damping and spin polarization of highly ordered B2 Co2FeAl thin films , 2014 .

[2]  B. Geisler,et al.  First-principles study of spin-dependent thermoelectric properties of half-metallic Heusler thin films between platinum leads , 2014 .

[3]  Jianhua Zhao,et al.  Magnetic and Gilbert damping properties of L21-Co2FeAl film grown by molecular beam epitaxy , 2013 .

[4]  J. Okabayashi.  Large anisotropic Fe orbital moments in perpendicularly magnetized Co 2 FeAl Heusler alloy thin films revealed by angular-dependent x-ray magnetic circular dichroism , 2013 .

[5]  M. Gabor,et al.  Perpendicular magnetic anisotropy in Ta/Co2FeAl/MgO multilayers , 2013 .

[6]  B. Johansson,et al.  Magnetic properties and temperature-dependent half-metallicity of Co2Mn(Ga1−xZx) (Z=Si, Ge, Sn) from first-principles calculation , 2013, Journal of physics. Condensed matter : an Institute of Physics journal.

[7]  M. Hehn,et al.  Magnetic and structural anisotropies of Co 2 FeAl Heusler alloy epitaxial thin films , 2011, 1108.4043.

[8]  Claudia Felser,et al.  Simple rules for the understanding of Heusler compounds , 2011 .

[9]  M. Hehn,et al.  Static and dynamic magnetic properties of epitaxial Co2FeAl Heusler alloy thin films , 2011 .

[10]  B. Hillebrands,et al.  Effect of annealing on Co 2 FeAl 0.5 Si 0.5 thin films: A magneto-optical and x-ray absorption study , 2011 .

[11]  Koichiro Inomata,et al.  Temperature dependence of tunneling magnetoresistance in epitaxial magnetic tunnel junctions using a Co2FeAl Heusler alloy electrode , 2010 .

[12]  G. Fecher,et al.  Charge transfer and tunable minority band gap at the Fermi energy of a quaternary Co2(MnxTi1−x)Ge Heusler alloy , 2010 .

[13]  Yongbing Xu,et al.  Interfacial structure and magnetic properties of Co2FeAl0.5Si0.5/MgO heterostructures , 2010 .

[14]  E. Jedryka,et al.  Coherent tunneling and giant tunneling magnetoresistance in Co 2 FeAl / MgO / CoFe magnetic tunneling junctions , 2010 .

[15]  J. Woicik,et al.  Effects of annealing on the local structure of Fe and Co in CoFeB/MgO/CoFeB tunnel junctions: An extended x-ray-absorption fine structure study , 2010 .

[16]  T. Miyazaki,et al.  Low damping constant for Co2FeAl Heusler alloy films and its correlation with density of states , 2009 .

[17]  H. Sukegawa,et al.  Structure and spin polarization of outermost surface of the Co 2 FeAl 0.5 Si 0.5 full-Heusler alloy studied by spin-polarized ion-scattering spectroscopy , 2009 .

[18]  B. Koopmans,et al.  Correlation between magnetism and spin-dependent transport in CoFeB alloys. , 2008, Physical review letters.

[19]  T. Koide,et al.  X-ray absorption spectroscopy and x-ray magnetic circular dichroism of epitaxially grown Heusler alloy Co2MnSi ultrathin films facing a MgO barrier , 2007 .

[20]  Kazuhiro Hono,et al.  Ab initio predictions for the effect of disorder and quarternary alloying on the half-metallic properties of selected Co2Fe-based Heusler alloys , 2007 .

[21]  T. Miyazaki,et al.  Interfacial structure and half-metallic ferromagnetism in Co2MnSi-based magnetic tunnel junctions , 2006 .

[22]  B. Aktaş,et al.  Erratum: Doping and disorder in the CO2MnAl and Co2MnGa half-metallic Heusler alloys , 2006, cond-mat/0607652.

[23]  N. Tezuka,et al.  Structural dependence of the tunnel magnetoresistance for magnetic tunnel junctions with a full-Heusler Co2Fe(Al,Si) electrode , 2006 .

[24]  B. Aktaş,et al.  Effect of doping and disorder on the half metallicity of full Heusler alloys , 2006, cond-mat/0603811.

[25]  G. Schönhense,et al.  Electronic structure and spectroscopy of the quaternary Heusler alloy Co2Cr1−xFexAl , 2006, cond-mat/0601670.

[26]  Marek Przybylski,et al.  Magnetic properties and spin polarization of Co 2 MnSi Heusler alloy thin films epitaxially grown on GaAs(001) , 2005 .

[27]  Yoshio Miura,et al.  Atomic disorder effects on half-metallicity of the full-Heusler alloys Co 2 ( Cr 1 − x Fe x ) Al : A first-principles study , 2004 .

[28]  N. Papanikolaou,et al.  Slater-Pauling behavior and origin of the half-metallicity of the full-Heusler alloys , 2002 .

[29]  Shoji Ishida,et al.  Search for Half-Metallic Compounds in Co2MnZ (Z=IIIb, IVb, Vb Element) , 1995 .

[30]  V. L. Moruzzi,et al.  Generalized Slater-Pauling curve for transition-metal magnets , 1983 .