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Abstract:
Spintronic is a multidisciplinary field and a new research area. New
materials must be found for satisfying the different types of demands.
The search for stable half-metallic ferromagnets and ferromagnetic
semiconductors with Curie temperatures higher than room temperature is
still a challenge for solid state scientists. A general understanding of
how structures are related to properties is a necessary prerequisite for
material design. Computational simulations are an important tool for a
rational design of new materials. The new developments in this new field
are reported from the point of view of material scientists. The
development of magnetic Heusler compounds specifically designed as
material for spintronic applications has made tremendous progress in the
very recent past. Heusler compounds can be made as half-metals, showing
a high spin polarization of the conduction electrons of up to 100% in
magnetic tunnel junctions. High Curie temperatures were found in
Co-2-based Heusler compounds with values up to 1120K in Co2FeSi. The
latest results at the time of writing are a tunnelling magnet resistance
(TMR) device made from the Co2FeAl0.5Si0.5 Heusler compound and working
at room temperature with a (TMR) effect higher than 200%. Good
interfaces and a well-ordered compound are the precondition to realize
the predicted half-metallic properties. The series Co2FeAl1-xSix is
found to exhibit half-metallic ferromagnetism over a broad range, and it
is shown that electron doping stabilizes the gap in the minority states
for x = 0.5. This might be a reason for the exceptional temperature
behaviour of Co2FeAl0.5Si0.5 TMR devices.
Using x-ray diffraction (XRD), it was shown conclusively that Co2FeAl
crystallizes in the B2 structure whereas Co2FeSi crystallizes in the
L2(1) structure. For the compounds Co2FeGa or Co2FeGe, with Curie
temperatures expected higher than 1000 K, the standard XRD technique
using laboratory sources cannot be used to easily distinguish between
the two structures. For this reason, the EXAFS technique was used to
elucidate the structure of these two compounds. Analysis of the data
indicated that both compounds crystallize in the L2(1) structure which
makes these two compounds suitable new candidates as materials in
magnetic tunnel junctions.
