Influence of the thickness of the FeCoGd layer on the magnetoresistance in FeCoGd-based spin valves and magnetic tunnel junctions

Both normal and inverse magnetoresistance effects have been observed in spin valves (SVs) of FeCoGd(t)/Cu/FeCoGd and magnetic tunnel junctions (MTJs) of FeCoGd(t)/AlO/FeCo with different thicknesses ( t) of the FeCoGd layer. At room temperature, for the giant magnetoresistance (GMR) ratio in the SVs, it remains positive when t <= 17.2 nm and changes to negative when t >= 18.4 nm; for the tunnelling magnetoresistance ratio in the MTJs, it remains positive when t <= 15.4 nm and changes to negative when t >= 16.5 nm. At low temperatures, ranging from 5 to 300K in the SVs, we find that for t = 17.2 nm, the GMR ratio remains negative when T is lower than 50 K, while it becomes positive when T is higher than 75 K. However, for t = 18.4 nm, the GMR ratio is always negative in the whole temperature range. Studies on the magnetic property of the FeCoGd layer indicate that the above results are due to the competition of magnetic moments between Gd and FeCo subnetworks in the FeCoGd layer changes with t and T as well.