Abstract
Nano-oxide-layers (NOLs) inserted between the pinned layer and the antiferromagnetic (AF) layer were found to increase the magnetoresistance (MR) ratio. The ferromagnetic coupling between top and bottom pinned layers through NOLs is necessary to maintain high exchange fields and MR ratios. We have investigated the coupling between top and bottom pinned layers through various NOLs in lrMn-base spin valves. The bottom lrMn-base spin valves (Si/Ta/Cu/IrMn/bottom- CoFe/NOL/top-CoFe/Cu/CoFe/Cu/Ta) were deposited by magnetron sputtering, and NOL layers were formed by using oxygen-plasma oxidation or natural oxidation on 1 nm metallic layers. Naturally oxidized Co90Fe10 layers show a ferromagnetic coupling between top and bottom CoFe layers. The plasma-oxidized Co90Fe10 layers show non-coupling between pinned CoFe layers, plausibly due to the formation of non-ferromagnetic oxides, leading to a low MR. By using XPS analyses, we confirmed that only Fe-oxides were formed. Naturally oxidized Ni80Fe20-oxides show the same results as those of naturally oxidized Co90Fe10-oxides. In contrast, plasma-oxidized Ni80Fe20-oxides exhibit a biquadratic coupling between top and bottom pinned CoFe layers. Consequently, anomalous transfer curves were observed while measured along exchange bias direction. The highest MR was obtained when the applied field was along the direction perpendicular to the exchange bias direction. Both naturally oxidized and plasma-oxidized Fe-oxide layers exhibit similar biquadratic coupling to those of plasma-oxidized Ni80Fe20-oxides. To investigate the role of Fe in these spin valves, nano-Fe-nitrides were inserted in the pinned CoFe layers, prepared by using nitrogen plasma. MR ratio was still enhanced and ferromagnetic coupling was observed between top and bottom CoFe layers. Based on these results, we suggested that the biquadratic coupling between pinned CoFe layers results from the existence of specific Fe-oxides. The possible mechanisms for the biquadratic coupling are (1) induced biquadratic anisotropy by antiferromagnetic Fe-oxides, or (2) loose spins in ferromagnetic Fe-oxides.