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 First
principles modelling of spin-polarised electron transport
Recently there has been a large effort to develop new electronic
devices that use
the electron spin to store or process information. The basic
physical phenomenon
that is utilized is that some materials display a difference
in the resistance for
transport of majority and minority spin electrons. Thus, there
is a great technological
interest to find new materials which have strong spin-dependent
transport properties.
The spin-dependent transport properties of materials can be
accurately modelled
using first principles quantum transport methods.
Based on such calculations, Butler et. al. [1] suggested that
an MgO crystal sandwiched between Fe electrodes should display
a giant magnetoresistance, i.e. the resistance of junctions
with parallel alignment of the electrode spins were found
to be orders of magnitude smaller than the resistance of junctions
with anti-parallel alignment. Subsequently, there has been
a number of experiments that try to realize such materials,
however, so far the measured magneto-resistances have been
much smaller than predicted by theoretical studies[2].
We have used the Atomistix ToolKit (ATK)[3,4] to model the
spin-dependent transport across MgO
layers coupled with Fe electrodes, and investigated how the
interface structure and defects in the MgO
layers affect the spin transport. The results of the calculations
will be discussed, and compared with
experimental studies.
References
1. W. H. Butler, X.-G. Zhang, T. C. Schulthess, and
J. M. . Maclaren, Spin-dependent tunnelling conductance
of Fe/MgO/Fe sandwiches, Phys. Rev. B 63, 054416 (2001).
2. S. S. P. Parkin, C. Kaiser, A. Panchulla, P. M.
Rice, B. Hughes, M. Samant, and S.H. Yang, Giant
tunnelling magnetoresistance at room temperature with MgO(100)
tunnel barriers, Nature Materials
3, 862 (2004).
3. M. Brandbyge, J.-L. Mozos, P. Ordejon, J. Taylor,
and K. Stokbro, Density functional method for
nonequilibrium electron transport, Phys. Rev. B. 65, 165401
(2002).
4. Atomistix Tool Kit (ATK). www.atomistix.com
Dr Kurt Stokbro, born 1965, is an educated physicist, cofounder and vice president at Atomistix A/S, and associate professor at Copenhagen University. He is coordinating a number of EU and Danish-funded R&D projects, with a total budget of more than 2 Mio Euro. Dr. Stokbro has published more than 50 papers and review articles in international journals, held a large number of lectures as invited speaker and has organized 3 international conferences in nanoscience. The TranSIESTA method, developed by Dr. Stokbro and co-workers, has become the de facto standard for electronic transport calculations in nanostructures, and has laid the foundation for the company Atomistix A/S.
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