We present comprehensive results on the magnetoresistive properties of spin-valve sandwiches comprising glass/M(1)/Cu/NisoFe2o/Fe5OMn»/Cu, where M(1) is a ferromagnetic transition metal or alloy (Co, Ni, Ni80Fe20). We discuss the thermal variation of the magnetoresistance (AR/R) and its dependence on the thicknesses of the layers constituting the active part of the spin-value sandwich [i.e., M( 1)/Cu/N(Fe]. An almost linear decrease of b,R/R is observed between 77 and 320 K. For a given ferromagnetic material, bR /R extrapolates to zero at a temperature T"sy 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 0 thickness of about 90 A located next to the M/Cu interfaces. We give a phenomenological expression relating AR /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.
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