[Zurück]

G. Parkinson, U. Diebold, L. Malkinski, J. Tang:
"Tailoring the Interface Properties of Magnetite for Spintronics";
in: "Chapter 3 in: Advanced Magnetic Materials, edited by Leszek Malkinski, Intechweb, May 24", Intech, 2012, 978-953-51-0673-1, S. 61 - 88.

The field of spintronics originates from the discovery of giant magnetoresistance (GMR) by
Fert and Grünberg [1, 2] in 1988, for which they were awarded Nobel Prize in 2007. This
effect, first observed in nanostructures comprised of two thin magnetic layers of Fe
separated by a 1-2 nm thick Cr spacer, was both qualitatively and quantitatively different
from the prior-known phenomenon of anisotropic magnetoresistance. GMR leads to
magnetoresistance much larger than anisotropic magnetoresistance. Given the exciting
nature of the effect, the underlying mechanism was promptly investigated and quickly
understood [3]. Essentially, GMR can be described in terms of spin filtering; conduction
electrons are polarized in one ferromagnetic layer, maintain spin memory while traveling
through a thin spacer, and then enter the second magnetic layer. The scattering of electrons
in this second magnetic layer depends on the direction of the magnetization relative to the
first (polarizing) layer. The electrons pass through the two layers almost unperturbed if their
respective magnetization is parallel. In contrast they experience enhanced scattering for an
antiparallel magnetization configuration. This magnetization-dependent scattering potential
can be explained through the availability (or non availability) of electron states in the second
material around the Fermi level in the spin-up and spin down bands. Consequently,
spintronics relies on materials in which a spin asymmetry exists in the density of states at
the Fermi level. Such differences down in magnetically ordered materials arise from
exchange interactions between magnetic atoms. The extreme case of energy band splitting
occurs in half metals, where only one spin orientation can be occupied by electrons at the
Fermi level. Therefore, half-metals should behave as a conductor for the electron current
when electron spins match the direction of magnetization and as an insulator when the spin
direction opposes the magnetization.