SPIN-VALVE EFFECT IN SOFT FERROMAGNETIC SANDWICHES

We demonstrate in a variety of systems that the in-plane resistivity of sandwiches of soft ferromagnetic layers separated by nonmagnetic metallic layers depends on the relative angle between their magnetizations. We observe this phenomenon, which we term the spin-valve effect, in sandwiches where we are able to control the relative angle between the magnetizations of two ferromagnetic layers either by constraining one layer through exchange anisotropy or by fabricating layers with different coercivities. In the first case, for example Si/50 angstrom Ta/60 angstrom NiFe/25 angstrom Cu/40 angstrom NiFe/50 angstrom FeMn/50 angstrom Ta we have seen relative changes in resistance of more than 4% at room temperature in a range of in-plane field of 0 to 15 Oe. In a system where the layers have different coercivities, Si/8 X (30 angstrom Fe/60 angstrom Ag/30 angstrom Co/60 angstrom Ag) we observed a relative change of 1.6% at room temperature for fields between 0 and 50 Oe. Since the ferromagnetic layers are essentially decoupled and have high squareness, one can rule out any mechanism requiring scattering by domain walls. The usual anisotropic magnetoresistance in these structures is much smaller than the spin-valve effect. In contrast to noble metals, when using Ta, Al, Cr or Pd spacers of similar thickness (20 to 150 angstrom) between layers of permalloy, only the anisotropic magnetoresistance is observed. We believe the spin-valve effect to be related to spin-dependent scattering at the interface and within the ferromagnetic layers, in balance with spin-dependent relaxation within the layers. We also report the observation of a weak exchange-like coupling between the ferromagnetic layers.