Interfacial origin of inverse magnetoresistance in uncoupled Fe/Si/Fe/Ru sandwiches

Experimental evidence of inverse magnetoresistance for ferromagnetic layers separated by a Si layer is reported. A series of Fe/Si sandwiches have bee n prepared by ion-beam sputtering at room temperature onto a glass substrat e with the following nomenclature: glass/Si-20 nm/Fe-5 (nm)/Si-x (nm)/Fe-5 nm/Ru-2 nm. Magnetization measurements have been performed at 300 K and sho w no evidence of antiferromagnetic exchange coupling. However, the magnetor esistance curves recorded at 300 K are very interesting and show a reversed magnetoresistance for sandwiches with Si spacer layer thicknesses between 1.2 and 1.5 nm. Indeed, the resistivity is smaller at zero field than at sa turation. This reversed magnetoresistance is due to the superparamagnetic i nterfaces and finds its origin in the difference of the electronic nature o f the Fe/Si interfaces and Fe/Ru interfaces. Indeed, iron silicide Fe1-ySiy at Fe/Si interfaces have scattering spin asymmetry ratios (alpha = rho dow n arrow rho up arrow) larger than 1, whereas, Fe with Ru impurities at the Fe/Ru interfaces present scattering spin asymmetry ratios lower than 1. (C) 1998 American Institute of Physics. [S0003-6951(98)00150-8].