A conceptual material design for magnetic tunneling junction cap layer realizing a steep NiFeAlOx interface is proposed. Tunnel magneto resistance stack of capNiFeAlOxCoFeRuCoFePtMnTasub was prepared. Maximum magnetoresistance (MR) ratios of nonmagnetic-NiFeZr, Zr, Ta, Ru, and Rh caps at 0 V were 55%, 28%, 50%, 43%, and 42%, respectively. The decrease of MR ratio and the increase of resistance area product RA with Ru cap compared to Ta cap correlate with the partial oxidation of the NiFeAlOx interface occurring in additional postannealing, which was confirmed by focused-ion-beam-transmission-electron- microscope-energy-dispersive-x-ray-fluorescence observation. Since standard electrode potential is Ta<Fe<Ni<Ru, it is supposed that NiFe with Ru cap is positively charged, the NiFeAlOx interface is easily oxidized during annealing by negatively charged oxidizing species, and increase of RA and decrease of MR ratio occur. RA with Rh cap was even higher than that with Ru cap, consistent with the higher electrode potential of Rh than that of Ru. On the other hand, as the electrode potential of Ta is lower than NiFe, we think that NiFe with Ta cap is negatively charged, the oxidation of the NiFeAlOx interface is suppressed, and the lower RA and higher MR ratio than those with Ru cap are obtained. Since the electrode potential of Zr is still lower than Ta, we expected an even higher MR ratio with Zr cap. However, magnetism and magnetostriction of ZrNiFeAlOxsub sample changed significantly at first and saturated rapidly in sequential postannealing processes, indicating that the diffusion of Zr to NiFe proceeds at the beginning and saturates promptly. Therefore, we introduced nonmagnetic-NiFeZr alloy cap and the highest MR ratio of 55% was realized, without suffering from the diffusion of Zr. The highest MR ratio with nonmagnetic-NiFeZr cap is consistent with the lowest electrode potential of Zr, and we believe that nonmagnetic-NiFeZr cap realizes a steep NiFeAlOx interface without the oxidation of NiFe.
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