Inverse giant magnetoresistance inFe∕Cu∕Gd1−xCoxspin-valves

Abstract

For bottom spin-valves of $\mathrm{NiO}∕\mathrm{Fe}∕\mathrm{Cu}∕{\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$, giant magnetoresistance has been measured as a function of thickness and composition of $\mathrm{GdCo}$ layers as well as temperature. For all spin-valves involved here, the giant magnetoresistance has been attributed to contributions of spin-dependent scattering at interfaces and in bulk. The interfacial contribution produces positive giant magnetoresistance ratio for various compositions of $\mathrm{GdCo}$ alloys. However, the bulk contribution produces negative one for the Co contents from $50\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ to a critical value ${x}_{\mathrm{C}}(R)$ (between $69\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ and $77\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$) and positive one for higher Co contents. It is suggested that the interfacial asymmetric factor of spin-dependent scattering is larger than 1.0 at the $\mathrm{Cu}∕\mathrm{GdCo}$ for various alloy compositions of $\mathrm{GdCo}$ and that the bulk asymmetric factor in $\mathrm{GdCo}$ layer is smaller and larger than 1.0 for Co contents below and above ${x}_{\mathrm{C}}(R)$, respectively. For spin valves with Co contents below and/or above ${x}_{\mathrm{C}}(R)$, the giant magnetoresistance ratio varies nonmonotonically and/or monotonically as a function of the $\mathrm{GdCo}$ layer thickness (temperature) for a specific temperature (a specific $\mathrm{GdCo}$ layer thickness), respectively. The change of the bulk asymmetric factor in the $\mathrm{GdCo}$ layer with the alloy composition can be attributed to the variation of either the spin alignment of Co and Gd atoms or the spin polarization of the $\mathrm{GdCo}$ layer.