Direct measurement of spin dependent mean free paths in metals (abstract)

Abstract

We describe results obtained using a new spin valve structure that is specifically designed to measure the spin up and spin down mean free paths in ferromagnetic metals. We report how these mean free paths can be measured more directly and with greater accuracy than previous methods, which were based mostly on indirect evidence from magnetoresistance in ternary alloys. The technique is based on the ‘‘backed’’ spin valve layered structure: substrate/seed/80 Å FeMn/50 Å NiFe/23 Å Cu/20 Å NiFe/t Å b/50 Å Ta where the back layer of material b with thickness t is probed by the rest of the structure, which forms a spin polarized conduction electron source. As t is varied the majority carrier mean free path λ+b in the layer b is obtained directly from the form taken by the change in film conductance between parallel and antiparallel magnetization states, ΔG, whose solution of the Boltzmann transport equation shows is well approximated by the form ΔG=ΔG0+ΔGb{1−exp([t−tx]/βλ+b)}, where ΔG0 arises from the 20 Å NiFe layer, tx is the high resistivity region of intermixing at the b layer interfaces, and β≂1 from observations with b=Cu. The minority carrier mean free path in a ferromagnetic layer b, λb, is obtained by comparing ΔG and λ+b with ΔG′, and λb′+, of a nonmagnetic b′ layer of similar resistivity to b; it is a less direct measurement than that of λ+ since it relies on the connection between conductivity and mean free path for the minority subband. We have obtained room temperature results for Ni80Fe20 (λ+=46±3 Å, 0<λ<6 Å), Fe (λ+=15±2 Å, λ=21±5 Å), and Co (λ+=55±4 Å, 0<λ<8 Å). The connection of spin dependent conductivity and mean free paths in ferromagnetic metals is crucial in exploring the mechanism of giant magnetoresistance and, more broadly, is central in all theories of transport in magnetic metals; this new technique should prove a powerful tool in measuring these fundamental quantities.