Heusler compounds: Multifunctional materials for spintronics

Tremendous progress has been made recently in the development of
magnetic Heusler compounds specifically designed as materials for
spintronic applications [1]. While problems in the field of
spintronics remain, the use of half-metallic Heusler compounds
provides a prospect for novel solutions.
Heusler compounds can be made with high spin polarization and high
Curie temperature as well as high spin injection efficiency, either
very low or high damping, tunable magnetic moment (low and high
magnetic moments can be realized), and tunable anisotropy. There is,
therefore, great potential that many materials-related problems
present in current-day 3d metal systems can be overcome.
The handling of interfaces with respect to their chemical properties
(atomic diffusion and roughness), electronic properties (e.g.,
Schottky barrier design), and spin properties (injection and pumping)
remains a big challenge. The potential exists for new phenomena and
applications with the use of novel materials in the Heusler compound
family - for example, the use of semi-conducting Heusler compounds as
non-ferromagnetic spin conductors.
High spin polarization and high Curie temperatures were found in
Co2-Heusler compounds, with Curie temperatures up to 1120 K in
Co2FeSi. Mn2YZ compounds (Y = Mn, Cr; Z = Al, Ga, Si, Ge, Sb) such as
Mn3Ga are ferrimagnets with low magnetic moments despite their high
Curie temperatures. Due to the Jahn Teller instability of manganese in
these materials, some of them show a tetragonal distortion, which
renders out-of-plane magnetization in thin films possible.
Semiconducting half-Heusler compounds such as TiNiSn have attracted
attention as potential candidates for thermoelectric applications.
These complex C1b compounds can be designed as n- and p-type
thermoelectrical materials with exceptionally large figure of merit,
ZT≈1.5 at high temperatures.
The potential for applications of these ternary compounds as
rationally designed, multifunctional materials will be discussed.
[1] C. Felser, G. H. Fecher, and B. Balke, Angew. Chem. Int. Ed., vol.
46, pp. 668-699, January 2007.