Giant magnetoresistance of magnetically soft sandwiches: Dependence on temperature and on layer thicknesses

We present comprehensive results on the magnetoresistive properties of spin-valve sandwiches comprising glass/M(1)/Cu/${\mathrm{Ni}}_{80}$ ${\mathrm{Fe}}_{20}$/${\mathrm{Fe}}_{50}$ ${\mathrm{Mn}}_{50}$/Cu, where M(1) is a ferromagnetic transition metal or alloy (Co,Ni,${\mathrm{Ni}}_{80}$ ${\mathrm{Fe}}_{20}$). We discuss the thermal variation of the magnetoresistance (ΔR/R) and its dependence on the thicknesses of the layers constituting the active part of the spin-value sandwich [i.e., M(1)/Cu/NiFe]. An almost linear decrease of ΔR/R is observed between 77 and 320 K. For a given ferromagnetic material, ΔR/R extrapolates to zero at a temperature ${\mathit{T}}_{0\mathrm{S}\mathrm{V}}$ significantly lower than the Curie temperature, and independent of the ferromagnetic layer thickness. We have identified spin-↑ and spin-↓ intermixing by spin-wave scattering as responsible for the thermal decrease of the magnetoresistance. We show that the magnetoresistance arises within the ‘‘active’’ parts of the ferromagnetic layers of thickness of about 90 Å located next to the M/Cu interfaces. We give a phenomenological expression relating ΔR/R to the longer of the two spin-dependent mean free paths, and to current shunting in the inactive part of the sandwich. The thickness of the active region is independent of temperature.