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Giant Magnetoresistance without Defect Scattering

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Schep, K.M., Kelly, P.J. & Bauer, G.E., 1995. Giant Magnetoresistance without Defect Scattering Phys. Rev. Lett. 74, 586.

Essay about this article

There are two primary causes for GMR, the electronic or band structure of the multilayer and the scattering by impurities, both in the bulk of the layers and due to roughness of the interfaces between layers. These two were well known by those who used phenomenological Hamiltonians with adjustable parameters, but prior to 1995 there were no calculations to properly assess the magnitude of the two origins to determine which cause produced the dominant effect.

The role of band structure was first assessed in 1995 by Gerrit Bauer’s group in Delft, Holland. They considered a metallic superlattice, which is a multilayer where the basic motif of a magnetic and nonmagnetic layer is periodically repeated, without impurities or roughness scattering at the interfaces, and calculated its band structure so as to evaluate the conductivity based on, essentially, the Landauer formalism [1]. This method relates the conductivity to the transmission across the superlattice, and is appropriate to evaluate structures without scattering by defects, i.e., in the ballistic regime. They calculated the conductivity for two configurations for the magnetic layers, parallel and antiparallel, and then determined the Current Perpendicular to the Plane of the layers Magnetoresistance (CPP-MR) ratio which they defined as the difference in the conductivity for the two configurations divided by the conductivity in the antiparallel configuration.

They found a ratio of 120%, which is comparable to measured values on several cobalt-copper multilayers, therefore they concluded that in the ballistic regime, at least for the CPP geometry, the change of the band structure, especially near the Fermi level, as the magnetic configuration goes from parallel to antiparallel is a dominant contribution to GMR. It should be pointed out that while the GMR ratio was realistic, the individual conductivities were much higher than those measured; they realized defect scattering played an important role in the current parallel to the plane, or the Current In Plane (CIP) geometry and could also contribute to CPP, and that this scattering would produce more realistic values for the conductivities.

1 Electronic Transport in Mesoscopic Systems, S. Datta (Cambridge University Press, 1995).

Discussion Question

Why is the ballistic regime more applicable to CPP-MR than CIP-MR?

The above article is reprinted with permission from Schep, K.M., Kelly, P.J. & Bauer, G.E., 1995. Phys. Rev. Lett. 74, 586. Copyright (1995) 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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