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.