The influence of spin-independent disorder on giant magnetoresistance
| Autor: | E. Yu. Tsymbal, David G. Pettifor |
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| Rok vydání: | 1996 |
| Předmět: | |
| Zdroj: | Journal of Physics: Condensed Matter. 8:L569-L575 |
| ISSN: | 1361-648X 0953-8984 |
| DOI: | 10.1088/0953-8984/8/40/001 |
| Popis: | We demonstrate that the giant magnetoresistance (GMR) effect in magnetic multilayers can be explained quantitatively in terms of the scattering of electrons from a spin- independent random potential that arises from the grown-in defects within the multilayer. We have calculated the GMR ratio for Co/Cu and Fe/Cr multilayered systems within the Kubo- Greenwood formalism assuming that the on-site atomic energies are disordered randomly within a realistic spd tight-binding model. Our predictions are in good agreement with experiment and demonstrate how the GMR ratio depends on the features of the electronic band structure. In particular, we obtain the enhancement of GMR in Co/Cu multilayers at electron energies up to about 1 eV above the Fermi level that has recently been observed by Monsma et al (1995 Phys. Rev. Lett. 74 5260). We predict no such enhancement for Fe/Cr multilayers. The mechanism of giant magnetoresistance (GMR) in magnetic multilayers (1) is usually related to the spin dependence of the scattering processes. In the semiclassical models of GMR this spin dependence is introduced into the theory through a number of spin-dependent phenomenological parameters such as relaxation times and transmission coefficients (2, 3). In the quantum mechanical models the spin dependence of the scattering is assumed to arise from spin-dependent random potentials produced by magnetic impurities at the interfaces or in the bulk of the ferromagnetic layers (4-6). These models usually take the electronic structure of the host material to be spin independent, assuming that transport is carried out by the s electrons which are described by the free-electron model (2-5) or by the single- band tight-binding model (6). Recently, however, it has been shown that in the ballistic regime of conductance the spin dependence of the electronic structure already gives a sizeable contribution to GMR (7). Several attempts have been made at combining this realistic description of the electronic structure with spin-dependent scattering potentials for a predictive model of the diffusive regime (8-10). Unfortunately, these calculations imply unrealistically large values of the GMR compared to experiment. In this letter we demonstrate that a realistic prediction of GMR can be obtained within a model that introduces spin-independent disorder in the on-site atomic energy levels in conjunction with accurate spin-dependent electronic structure. In particular, we will show that this model predicts the recently observed enhancement of GMR for hot electrons in Co/Cu spin valves (11). Our model is motivated by experimental observations that magnetic multilayers contain a lot of defects such as vacancies, impurities and grain boundaries that are produced during the process of deposition (e.g. (12)). Each of these defects makes its own contribution to the scattering potential and, consequently, to the resistivity. However, on average there is no reason for this random scattering potential to be spin dependent. One would expect the appearance of a strong spin dependence only for the case when chemically |
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