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

Thursday, 9 October 2014: 16:10
Expo Center, 1st Floor, Universal 5 (Moon Palace Resort)
S. E. Lee, T. H. Shim, J. P. Hong, and J. G. Park (Hanyang University)
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 (Jex) 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 Jex 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