Evidence for a short spin diffusion length in permalloy from the giant magnetoresistance of multilayered nanowires

We present magnetization and giant magnetoresistance (GMR) measurements performed on two series of electrodeposited $Py/\mathrm{Cu}$ multilayered nanowires $(Py={\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ of diameter φ=90 nm. The multilayers of the first series are composed of a conventional stacking of Py and Cu layers and the GMR is studied as a function of the Cu thickness for a constant Py thickness. The multilayers of the second series are composed of $Py/\mathrm{Cu}/Py$ trilayers uniformly distributed along the filament and separated from each other by thick Cu layers. For this second series, magnetic and GMR properties were investigated as a function of the Py layer thickness and our magnetization measurements demonstrate that, for Py layers thinner than about 90 nm, the magnetic moments of the two Py layers of a trilayer are approximately antiparallel at zero field. Analysis of the GMR data using the Valet-Fert model allows us to estimate that the spin diffusion length in Py, ${\mathcal{l}}_{\mathrm{sf}}^{\mathrm{}\left(\mathrm{Py}\right)\mathrm{}},$ is between 3.3 and 5.3 nm.