Spin transistor using tunneling barrier

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Spin tunneling devices were fabricated by the magnetron sputtering system with 5 targets, where the metal masks for forming the electrodes were changed in vacuum system. Magnetoresistance(MR) ratio increased from few % to over 20% and the reproductivity of tunneling junction was improved by changing the magnetic layers from polycrystalline NiFe to amorphous CoFeB layer. This is due to improvement of the flatness of interface because of the structure without polycrystalline grains. Oxidization process of Al layer were investigated by dc plasma and rf plasma discharge in pure oxygen. In case of dc plasma, the tunneling devices exhibit large MR ratio with spin valve type MR curves in the range of the Al thickness between 1.3 nm and 1.6 nm and MR ratio shows maximum value of 30% at 1.4 nm. The dispersion of MR ratio between seven samples prepared at same process becomes very small. In case of rf plasma, although the MR ratio is 15% at Al thickness of 1.6nm, it decreases with decreasing Al thickness and becomes less than 1% at 1.3 nm. The dispersion of MR ratio between samples is larger than that for dc plasma case. These results show that the dc plasma is more suitable for oxidization of Al layer compared with rf plasma. However, the MR ratio for dc plasma case decreases more steeply with increasing voltage bias compared with rf plasma case, which implies that the oxidization process by dc plasma also should be improved. It is considered that these results are due to the deference in the degree of oxidization of Al layer and interface roughness of tunneling barrier. The tunneling devises show good thermal stability, where MR ratio does not change below 300 C annealing in the vacuum.