Anomalous behavior of Co insertion to Al2O3 in CoFe/Al2O3/NiFe tunnel junctions

We fabricated a tunnel junction 20 nm CoFe/0.75 nm Al2O3/t Co/0.75 nm Al2O3/20 nm NiFe (t=0-1 nm) to investigate the effect of Co layer insertion on Al2O3 layer, using direct sputtering Al2O3 target. At room temperature the magnetoresistance (MR) ratio decreased as the Co layer thickness increased and above 0.4 nm Co the MR ratio became zero. With decreasing temperature, the MR ratio of 0.3 nm Co sample rapidly increased and reached 14.6% at 50 K. On the other hand, the MR ratio of 0 nm Co sample gradually increased and reached 4.6% at 4.8 K. Temperature dependence of the MR ratio was interpreted by two-step tunneling. MR was enhanced by Co insertion, although the mechanism was not fully understood. Anomalous skirts were found in MR curves of 0.3 nm Co sample below 150 K and their behavior resembled granular tunneling magnetoresistance. This phenomenon may be interpreted if the 0.3 nm Co layer sandwiched between Al2O3 insulating layers has a paramagnetic state above 150 K and remains in a superparamagnetic state below 150 K, provided the Curie temperature of 0.3 nm Co layer is 150 K. (C) 2000 American Institute of Physics. [S0021-8979(00)05320-2].