Electrical Resistivity of Ferromagnetic Nickel and Iron Based Alloys
Fert, A. & Campbell, I.A. (1976) Electrical resistivity of ferromagnetic nickel and iron based alloys J.Phys. F:Metal Phys. 6,849
Essay about this article
As part of his doctoral thesis at the Université de Paris-Sud in Orsay France, Albert Fert, together with his advisor Ian Campbell, set about to verify the details of Mott’s picture of electrical conduction in the ferromagnetic transition metals. For example, they looked for deviations of the resistivity from Matthiessen’s rule--the sum of the resistivity from different sources--that follows from the assumption that there is one type of current carrier and that the scattering processes that cause resistance are independent. For a metal containing impurities this rule holds that the total residual resistivity is the sum of the residual resistivities if the impurities are present separately at the same concentrations as in the alloy.
From an extensive series of resistivity measurements on the ferromagnetic transition-metals alloyed with varying concentrations of one or two species of impurities [binary and ternary dilute alloys] Fert concluded that there were substantial deviations from Matthiessen’s rule; the one carrier model is inadequate. However, Mott’s two-current model was able to explain the extant data. While Mott’s original model envisaged two currents carried in parallel, Fert extended the model to include mixing of the spin-up and spin-down currents and found that with his modification the two-current model has much more general validity than was originally believed.
A critical conclusion of Fert from his studies, ca. 1967-73, of the two-current model was that certain impurities increased the resistivity of metallic alloys far more than others. In Table 1 of his 1976 article we discern the origins of the idea of Giant Magnetoresistance (GMR); while the spin-dependent resistivity of all the elements is of the order of 10 micro-ohm-cm/at%-impurity in one of the two spin channels, the ferromagnetic atoms distinguish themselves in having an unusually small resistivity in the other channel. When one makes the assumption that the two spin channels conduct independently, which is a good as long as the spin-flip scattering between channels is small compared to the non-spin-flip (yet spin-dependent) scattering, the overall resistivity is arrived at by positing that the two channels conduct as if they were resistors in parallel, and the effect of the channel with the depressedis to lower the overall resistivity.
From his studies Fert concluded when the magnetic moment of an impurity is antiparallel to the host magnetization, or if the moments of ternary impurities are antiparallel, the resistivity is higher than when they are parallel. The question that remained was how can one switch the moments from parallel to antiparallel other than by varying the amount of ternary impurity in the alloys?
Campbell, I.A. & Fert, A. (1968) Phys.Rev.Lett. 21,1190 and Pomeroy, A. (1967) Phil. Mag. 15, 977.
What differences can you discern between Fert’s picture of conduction in ferromagnetic metals and Mott’s model?
The full text of Fert/Campbell, 1976 was provided with kind permission of the Institute of Physics and IOP Publishing Limited.
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