Giant magnetoresistance and tunnel magnetoresistance effects in FeCoGd-based spin valves and magnetic tunnel junctions

Giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) were studied in spin valves of FeCo∕Cu∕(FeCo)1−xGdx and magnetic tunnel junctions of FeCo∕AlO∕(FeCo)1−xGdx, respectively. When the FeCoGd layer is thick enough, both GMR and TMR ratios change their signs from positive to negative at the compensation composition with 0.293⩽x0⩽0.337 as the Gd content is increased. This scenario is originated from a competition of rare-earth and transition-metal spins in FeCoGd layers with antiferromagnetic coupling. Accordingly, it is deduced that in the FeCoGd layer the spin polarizations PN of electrons at the Fermi level and tunneling spin polarization PTSP are negative and positive for x x0.