Thermal Stability Enhancement of Cofeb Pinned Layer for Perpendicular-Magnetic Tunnel Junction with Multi-[Co/Pt]n-Synthetic-Anti-Ferro-Magnetic Layer

In recently years, research activities on magnetic tunnel junctions (MTJs) using crystalline MgO tunneling barrier layer has been performed for realizing perpendicular-spin-transfer-torque-magnetic-random-access-memory (p-STT-MRAM). In particular, to achieve perpendicular-magnetic-anisotropy (PMA) for p-MTJs grown on 12-inch TiN/W electrode, a design of synthetic-anti-ferro-magnetic (SyAF) layer being operated at the ex-tu annealing above 350 ^oC has been intensively studied. In general, the spin-valve has been fabricated with the structure of electrode / seed layer / CoFeB free layer / MgO tunneling barrier layer / CoFeB pinned layer / capping layer / SyAF layer / top electrode showing a high tunnel magneto-resistance ratio (TMR) of over 120 % [1]. However, the PMA characteristics of p-MTJs and the exchange coupling field (J_ex) of SyAF layer degraded when the ex-tu annealing temperature increased [2]. In addition, it has been reported that non-ferro-magnetic material such as Pd in multi-[Co/Pd]_nSyAF layer for spin-valves is diffused into the MgO tunneling barrier layer, deteriorating the crystallinity (L10) of the MgO tunneling barrier layer and CoFeB pinned layer after ex-tu annealing. In our paper, we investigated the thermal stability enhancement of CoFeB based p-MTJs fabricated with multi-[Co/Pt]_n SyAF layer and review the mechanism by which multi-[Co/Pt]_nSyAF layer enhances the thermal stability of p-MTJs.

Figures 1(a), (b) and (c) show the magnetization-magnetic field (M-H) curves of multi-[Co/Pd]_n SyAF layer, spin-valve structure with multi-[Co/Pd]_n SyAF layer, and Pd diffusion profile analyzed using secondary-ion-mass-spectroscopy (SIMS), respectively. Pd was diffused to the MgO tunneling barrier layer after ex-tu annealing at 275 ^oC, as shown in Fig. 1(c). According to our previous report, Pt diffused back from the MgO tunneling barrier layer after ex-tu annealing for CoFeB based spin-valves [3]. Thus, we investigated the thermal stability enhancement by using multi-[Co/Pt]_n SyAF layer rather than multi-[Co/Pd]_n SyAF layer. Figures 2(a) and (b) show the dependency of the M-H curves for PMA structure and SyAF structure on Co/Pt layer thickness and ex-tu annealing temperature, respectively. The PMA characteristics of multi-[Co/Pt]_n layer was degraded when the Co/Pt layer thickness and the ex-tu annealing temperature increased, as shown in Fig. 2(a). In addition, exchange coupling field of multi-[Co/Pt]_n SyAF layer degraded after ex-tu annealing at 400 ^oC.

In our presentation, we will present the thermal stability enhancement of CoFeB pinned layer for p-MTJs with multi-[Co/Pt]_n SyAF layer and review the mechanism by which the thermal stability enhancement. We solved the problem of degradation of PMA and J_ex of multi-[Co/Pt]_n SyAF layer through more experiments. Also, we report the effect of multi-[Co/Pt]_nSyAF layer spin-valves for p-MTJs as a function of annealing temperature by using vibrating-sampling-magnetometer (VSM), transmission-electron-microscopy (TEM), SIMS and current-in-plane-tunneling (CIPT) measurement. In addition, we review the mechanism why the magnetic properties of p-MTJs are varied by SyAF layer materials.

  * This work was financially supported by the IT R&D program of MOTIE/KEIT. [No. 10043398,  Fundamental technology development of materials of magnet resistance change (MR ratio >120%) and tunneling barrier for advanced low power memory device and No. 10041608, Embedded System Software for New-memory based Smart Device] and the Brain Korea 21 plus Project in 2014, Korea.

Reference [1-3]

[1] S. Ikeda et al., Nature materials 9, 721-724 (2010)

[2] C. Cheng et al., J. Appl. Phys., 110, 033916 (2011)

[3] D. Lee et al., Appl. Phys. Lett. 102, 212409 (2013)

Fig. 1. (a) M-H curves of multi-[Co/Pd]_n SyAF layer : as-deposition (left) and annealed(right), (b) spin-valve structure with multi-[Co/Pd]_n SyAF layer, (c) atomic component profile using SIMS profile : as-deposition (upper) and annealed (lower)

Fig. 2. M-H curves depending on (a) Co/Pt layer thickness and annealing temperature for multi-[Co/Pt]_n PMA structure and (b) for multi-[Co/Pt]_n SyAF structure