Interplay of growth mode and thermally induced spin accumulation in epitaxialAl/Co2TiSi/AlandAl/Co2TiGe/Alcontacts

Abstract

The feasibility of thermally driven spin injectors built from half-metallic Heusler alloys inserted between aluminum leads was investigated by means of ab initio calculations of the thermodynamic stability and electronic transport. We have focused on two main issues and found that: (i) the interface between Al and the closely lattice-matched Heusler alloys of type Co2TiZ (Z = Si or Ge) is stable under various growth conditions; and (ii) the conventional and spin-dependent Seebeck coefficients in such heterojunctions exhibit a strong dependence on both the spacer and the atomic composition of the Al/Heusler interface. The latter quantity gives a measure of the spin accumulation and varies between +8 µV/K and −3 µV/K near 300 K, depending on whether a Ti-Ge or a Co-Co plane makes the contact between Al and Co2TiGe in the trilayer. Our results show that it is in principle possible to tailor the spincaloric effects by a targeted growth control of the samples.

Native defects in the Co2TiZ (Z=Si,Ge,Sn) full Heusler alloys: Formation an

“…Assuming an energy-independent scattering, our theoretical results predict a positive Seebeck coefficient for the bulk Co 2 TiZ materials, 16,20 a finding which is in contrast to the large, negative values reported experimentally, reaching as much as −30 µV/K in Co 2 TiSi/Ge and −50 µV/K in Co 2 TiSn.…”

“…19 In a previous study, we have proposed a thermal spin injector based on a thin, latticematched barrier layer of either Co 2 TiSi or Co 2 TiGe between metallic leads. 16 Moreover, we could show that, similar to the insulator case, the loss of half-metallicity is localized within few atomic layers in the proximity of the interface. Thus, its role in the filtering of transmission channels notwithstanding, we find the understanding of the bulk transport properties of particular importance for the design of Heusler-based heterostructures.…”

“…16,20 Indeed, for all these materials the DOS decreases above E F such that, in the approximation of an energy-independent scattering, the conductivity is expected to decrease with increasing energy E. One can then conclude that, at least in a range extending not far away from the Fermi energy, the presence of these four defects leads to a weak, nearly energy-independent scattering of the electrons. This generic behavior can be modified, however, if IBs are present.…”

Spin caloric transport from density-functional theory

“…18. If we made, in the absence of better knowledge, the assumption of energy-independent scattering, our theoretical results would predict a positive Seebeck coefficient for the bulk Co 2 TiZ materials [119,122], a finding which is in contrast to the large, negative values reported experimentally, reaching as much as −30µV/K in Co 2 TiSi/Ge and −50µV/K in Co 2 TiSn [119,120]. Calculating the conductivity via the Kubo-Greenwood formula, we are not limited to such an assumption; rather, the scattering properties of the point defects are taken into account within the framework of the CPA.…”

“…The calculations have shown that the hybridization of electronic states of Al with those of the Heusler material occurs in particular for the Cotermination, for which the interfacial bond length is shorter than for the TiSi or TiGe termination. Even in very thin films, however, one finds that the halfmetallic gap is recovered in the central layers of the Heusler alloy, already 3 to 4 atomic planes away from the interface [122]. The conductance of Al/Co 2 TiZ/Al heterostructures perpendicular to the planes is governed by the backscattering of carriers from the interfaces between Al and the Heusler alloy.…”