Heusler Compounds

 The Heusler compounds [7] were discovered in 1901. Heusler compounds are ternary X2YZ intermetallics
where X and Y are transition metals and Z is a main group element. The electrical and magnetic properties of
Heusler compounds range widely from metals to semiconductors (Fe2VAl [8]) and ferrimagnets to halfmetallic
ferromagnets (Co2FeSi [9,10]). The Cobalt based Heusler compounds, crystallizing in the L21
structure (space group Fm3m ), show some of the highest Curie temperatures (1100 K), high magnetic
moments (6μB/f.u.) and complete spin polarization at the Fermi level, leading to their description as halfmetallic
ferromagnets (HMFs). Those unique properties render HMFs and in particular Co based Heusler
HMFs good candidates for the integration in spintronic and spin logic devices [3]. Up to now, about 500
Heusler compounds with the overall stoichiometry X2YZ have been already reported. Apart from the ternary
X2YZ compounds, the quaternary compounds such as X2Y1-xY’xZ (example from current research Co2Mn1-
x
FexSi [11]) or X2YZ1-xZ’x (example Co2FeAl1-xSix [12]) are of particular interest, as recent ab initio
calculations indicated that these quaternary compounds with about 50% substitution will lead to a situation
where the Fermi edge is located close to the middle of the half-metallic gap and thus should lead to an
improved temperature stability of the spin polarization, in particular, if quasiparticle excitations are
appearing close to the band edges [13]. The substitued atom and the substitutional atom will share one
crystallographic position of the Heusler lattice. This requires a random distribution on the shared
crystallographic position from a crystallographic point of view. Such a local distribution can only be detected
by local methods such as spin echo nuclear magnetic resonance (NMR) [14].

The physical properties such as magnetisation and transport behaviour are strongly affected by the (local)
structure. Thus, also the performance of such a spin polarized material in a spintronic device will depend on
the (structural) quality of the material itself. In particular, it is well known that the relation between structure
and the preferred high spin polarisation is essential [25-29]. The Heusler compounds are known to exhibit
different structure types and band structure calculations revealed a decrease in spin polarisation and magnetic
moment if some types of disorder occur on certain atomic positions [25-29]. The structural quality of the
Heusler materials plays in important rule also in current perpendicular to the plane (CPP) devices [30]. The
optimisation of spin polarised materials for spintronic devices demands a precise control of the relations
between structure and properties, in particular of the magnetic and the structural properties. Thus, a
sophisticated state-of-the-art structural characterisation is a key tool in understanding, tuning and controlling
the macroscopic properties of spin polarised materials towards optimised performance in spintronic devices.

 

[7] Fr. Heusler. Verh. Dtsch. Phys. Ges., 12, 219 (1903).
[8] Y. Nishino, M. Kato, S. Asano, K. Soda, M. Hayasaki, and U. Mizutani, Phys. Rev. Lett. 79, 1909 (1997).
[9] S. Wurmehl, G. H. Fecher, H. C. Kandpal, V. Ksenofontov, H.-J. Lin, C. Felser, Appl. Phys. Lett. 88
032503 (2006).
[10] S. Wurmehl, G. H. Fecher, H. C. Kandpal, V. Ksenofontov, H. J. Lin, J. Morais, C. Felser, Phys. Rev. B
72 184434 (2005).
[11] B. Balke, G. H. Fecher, H. C. Kandpal, C. Felser, K. Kobayashi, E. Ikenaga, J.-J. Kim, and S. Ueda,
Phys. Rev. B 74, 104405 (2006).
[12] G. H Fecher and C. Felser J. Phys. D: Appl. Phys., 40, 1582 (2007).
[13] L. Chioncel, Y. Sakuraba, E. Arrigoni, M. I. Katsnelson, M. Oogane, Y. Ando, T. Miyazaki, E. Burzo,
and A. I. Lichtenstein, Phys. Rev. Lett. 100, 086402 (2008).
[14] S. Wurmehl, J. T. Kohlhepp, Invited topical review J. Phys. D: Appl. Phys. 41, 173002 (2008).