Giant Magnetoresistance and Electronic Structure
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ABSTRACT The electronic structure of magnetic multilayers is expected to play an important role in determining their transport properties. We explain how the conductance through a ballistic point contact is related to simple geometrical projections of the Fermi surface. The essential physics is first discussed for simple model systems and then realistic results for magnetic metallic multilayers based on first principles band structure calculations are presented. The electronic structure is shown to make an important contribution to the perpendicular giant magnetoresistance. INTRODUCTION Electrical transport in metallic multilayers has been subject to extensive experimental and theoretical investigation. Most attention has been paid to the giant magnetoresistance (GMR) effect that arises in antiferromagnetically coupled magnetic multilayers when the anti-parallel (AP) magnetizations of adjacent magnetic layers are forced to become parallel (P) by an external magnetic field [1]. All of the experiments which have been performed so far have been in the diffusive transport regime, in which the sample dimensions are much larger than the mean free path. In this regime the conductivity is determined both by the electronic structure of the material and by the scattering at defects. This can be illustrated within the free electron model with two commonly used expressions for the Drude conductivity,
,~ nI_ I
OUDrude -ne2
=
20~ {IcF
}
ý.
(1)
depends both on electronic structure parameters (in curly brackets) such as the Fermi wave vector kF or the ratio between the density n and the mass m of the electrons, and on scattering parameters such as the mean free path f or the relaxation time r. The GMR is usually ascribed to a spin-dependence of the scattering properties which, within the free electron model, corresponds to assuming a spin-dependent f or r. The electronic structure parameters are on the other hand often taken to be spin-independent and constant throughout the multilayer. Recently, it was pointed out that the difference in the band structures for the AP and the P configurations can also make a large contribution to the GMR [2,3]. We identify the determination of the relative importance of electronic structure and scattering effects as a central issue in any study of the microscopic origin of GMR. Because both of these effects contribute to the diffusive conductivity it is difficult to distinguish between them on the basis of present transport measurements. In the ballistic transport regime it is possible to evaluate the effect of electronic structure on the GMR unambiguously, both experimentally and theoretically. ODrude
305 Mat. Res. Soc. Symp. Proc. Vol. 384 @1995 Materials Research Society
BALLISTIC TRANSPORT Consider two semi-infinite electrodes separated by an insulating barrier and only connected via a small opening in the barrier. When the diameter of the opening is much smaller than the mean free path and much larger than the electron wavelength, such a structure is referred to as a classical ball
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