Theory of magnetoresistance in concentrated ferromagnetic alloys (invited)

Existing values of the magnetoresistance (ρ∥−ρ⊥)/ρ0 for crystalline Ni1−xFex, Ni1−xCox, Ni1−xMnx, Fe1−xCrx, and Fe1−x-Vx alloys depend on x in a way which disagrees with the traditional Smit theory, based on the Born approximation. The reasons are related to the variation of the 3d wave-function amplitude between chemically different atoms and, in turn, to a variation of s-d scattering intensity. Experimental atomic moments indicate that this 3d amplitude variation is large. The variation is related to the fact that d-d scattering is strong and resonant. We predict large deviations from Nordheim’s rule ρ↓∝ x(1−x) where ρ↓ is the spin-down resistivity, similar to the deviations found experimentally. Correspondingly, (ρ∥−ρ⊥)/ρ0 is much larger for x≂0 than for x≂1.