PALE:ClassicArticles/GMR/Article7
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Boundary Resistance of the Ferromagnetic-Nonferromagnetic Metal Interface
van Son, P.C., van Kampen H. & Wyder, P. 1987. Boundary Resistance of the Ferromagnetic-Nonferromagnetic Metal Interface. Phys. Rev. Lett. 58, 2271
Essay about this article
When dissimilar metals are joined there is a redistribution of electrons at the interface such that the Fermi levels in the two metals are at the same energy. Due to the mobility of conduction electrons they readily achieve this equilibration, and leave one side of the interface with an excess charge and the other with a deficit. The depletion regions are very narrow because of the strong screening of the Coulomb interaction between these mobile electrons and the charged ion background. They extend over a few angstroms in metals, however in semiconductors they can be of the order of a thousand angstroms because of the paucity of mobile conduction electrons to screen out the Coulomb forces.
When at least one of the metals is ferromagnetic, the densities of electrons parallel and antiparallel to the magnetization [spin-up and spin-down electrons] are unequal. Therefore in addition to there being a transfer of charge across an interface between a normal and ferromagnetic metal, or between two ferromagnetic metals, there is an additional transfer of spin density or magnetization. All of this occurs when the system is in equilibrium, i.e., before applying an electric field.
Upon applying an external field the system is taken out of equilibrium and a current flows across the interface between dissimilar metals. Once again there is a redistribution of charge and spin with one crucial difference: the length scale of the spin or magnetization redistribution or accumulation. While charge imbalances are readily screened out by strong Coulomb forces, even those produced by driving the system out of equilibrium, the spin accumulation has a much longer length scale and can be of the order of submicrons [10-7 meters], see Figure 1.
Figure 1. Position dependence of the potential differences 
near an F-N interface. This figure originally appeared as Figure 2 in P.C. van Son, H. van Kampen and P. Wyder, Phys. Rev. Lett. 58, 2271 (1987).
As disussed in the 1987 work of van Son, van Kampen, and Wyder, as well as Johnson and Silsbee’s 1987 paper, below, the length scale for the spin or
magnetization accumulation is determined by the mean free path of the conduction electrons and the spin-flip scattering length. Most collisions between conduction electrons and other electrons, impurities, or the stationary ions may be spin-dependent, i.e., these events depend on whether the two entities colliding have their spins parallel or antiparallel, but they do not produce a reversal or flip the spin of the conduction electrons. Spin-flip scattering in a ferromagnetic metal is an inelastic process inasmuch as energy is required to take the spin from the one part of the Fermi sea polarized parallel to the background magnetization to the other part that is polarized antiparallel to the background magnetization. At low temperatures this energy is not readily available so that the spin-flip scattering length is rather long, in the range of submicrons, while the elastic spin-dependent scattering length, i.e., the mean free path, is short, on the order of nanometers. In a diffusive theory of electron transport [discussed in subsequent sections] the length scale for the spin accumulation is the root mean square of the mean free path and the spin-flip length, and is called the spin diffusion length.
See Also:
Mark Johnson and R.H. Silsbee, Phys. Rev.B 35, 4959 (1987) ibid. Phys. Rev. Lett. 60, 377 (1988).
Discussion Question
Is spin accumulation attendant to all GMR geometries, i.e., CIP as well as CPP?
The above article is reprinted with permission from the author(s) of P.C. van Son. H. van Kampen & P. Wyder, Phys. Rev. Lett. 58, 2271 (1987). Copyright (1987) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.
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