Enhanced Annealing Stability of Exchange-Biased Pinned Layer in Magnetic Tunnel Junction Using Ta/Ru/Ta/Ru Underlayer

Abstract

To realize highly sensitive analog sensor using magnetic tunnel junctions (MTJs), there is a demand for MgO-MTJs with a high signal-to-noise ratio. Thermal annealing is important for improving the crystalline quality of MgO barrier in order to enhance the tunneling magnetoresistance (TMR) ratio and decrease the noise level of MTJs. However, annealing at excessively high temperatures causes degradation of the exchange bias field ( $H_{\text {ex}}$ ) in the exchange-biased pinned layer (EB-PL). In this paper, we aim to develop a MgO-MTJ with high annealing stability by investigating the effect of underlayer structure on the annealing stability of EB-PL in MTJs. We fabricated two types of MTJ, one with a Ta/Ru underlayer (TR-UL) and one with a Ta/Ru/Ta/Ru underlayer (TRTR-UL). The results of evaluating the $H_{\text {ex}}$ of MTJs annealed in the temperature range of 320 °C–420 °C found that the MTJ containing a TRTR-UL (TRTR-MTJ) has higher annealing stability compared with that of the MTJ containing a TR-UL (TR-MTJ). Furthermore, the results of measuring the TMR ratio and 1/ $f$ noise level of the MTJs annealed at 380 °C showed that the TRTR-MTJ exhibits a higher TMR ratio and lower noise level than the TR-MTJ. To determine the mechanism by which the annealing stability improves, we carried out microstructural analysis of both MTJs and found that thermal Mn diffusion in the TRTR-MTJ was smaller than that in the TR-MTJ, and the crystallinity of IrMn on the TRTR-UL was found to be clearly improved compared with that on the TR-UL. As a result of $d$ -spacing analysis of each layer in the TRTR-UL, the second Ta layer deposited on the Ru layer was found to have $d$ -spacing corresponding to the (110) textured $\alpha $ -Ta structure and to be highly crystalline, whereas the first Ta layer was not found to have the components of (110) textured $\alpha $ -Ta structure. These results indicate that the Ta-on-Ru structure is responsible for the high crystallinity of the IrMn layer disposed on the TRTR-UL, resulting in the high annealing stability thanks to small Mn diffusion.