Perpendicular Spin Transfer Torque Magnetic Random Access Memory (p-STT-MRAM) needs to combine high speed and low critical switching current at technological nodes below 30 nm. Higher STT write efficiency is therefore required. Double magnetic tunnel junctions (DMTJ) stacks have been proposed to achieve this goal [1-5]. The DMTJ critical switching current is decreased by the combined effect of the STT exerted from two antiparallel polarizers on the storage layer (SL). The retention is increased by doubling the SL/MgO interfaces contributing to the layer’s effective perpendicular anisotropy (PMA). Despite the efficiency advantage, the development of p-DMTJ presents several complexities. First, ensuring enough PMA for the top polarizer [6] and second, controlling the stray field compensation of both polarizers acting on the SL. A p-DMTJ structure having a switchable reference layer was recently proposed [7], showing through magnetic simulation similar performance as conventional p-DMTJ with static polarizers. In this work, we report experimental evidence of the working principle for this type of novel p-DMTJ. We fabricated and optimized an ASL-DMTJ structure having a switchable polarizer acting as a switching assistance layer (ASL). Fig.1.(a) and (b) show the two switching sequences, from AP to P and from P to AP states respectively. Both reversals show a transition through an intermediate resistance state (IRS). Differing from the working principles proposed in [7], the ASL is designed to switch its magnetic orientation during the write operation. This maximizes the STT efficiency independently of the switching direction, while also increasing the SL retention, since the assistance layer always aligns parallel to the SL in standby Fig.2(a)-(b). Validation of the switching sequences was possible by performing time-resolved resistance measurements Fig.2.(c)-(b).

![](https://assets.underline.io/uploads/markdown_image/1/image/0c7395b35a105c6f992d8d15bd884588.jpg)

Figure 1. (a) AP→P and (b) P→AP reversal sequence through an intermediated resistance state.

![](https://assets.underline.io/uploads/markdown_image/1/image/938d95a5c9fad68f070f40573955c5dd.jpg)

Figure 2. (a) RH of ASL-DMTJ (b) RV loops of ASL-DMTJ. (b) Real time AP→P switching sequence and (d) real time P to AP switching sequence both transiting through the IRS with (I) applied field, (II) at zero field.

Real time investigation of Double magnetic tunnel junction with switchable assistance layer for high efficiency STT-MRAM

The use of double magnetic tunnel junctions (DMTJ) is a well-known strategy to enhance STT-MRAM efficiency by increasing thermal stability (Δ) while reducing the critical switching current (Ic). In these structures, the Storage layer (SL) is sandwiched between two tunnel barriers, each with its corresponding fixed polarizer. For the case of perpendicular DMTJ, the SL Δ is increased due to the two interfacial anisotropy contributions arising from both magnetic metal-oxide interfaces. If the top (TP) and bottom (BP) polarizers are set antiparallel, the Ic is found both theoretically [1] and experimentally [2] to be reduced due to additive spin-transfer torques. Another feature of DMTJ is the possibility of switching one of the polarizers, allowing to tune the STT depending on the polarizers parallel (Read Mode) or antiparallel (Write Mode) configuration [3]. Fig.1 (a) and (b) show the structure of such p-DMTJ with switchable TP and the two operation modes, where the polarizers are coupled to a hard Co/Pt synthetic antiferromagnetic multilayer (SAF). In this study, we investigate the possibility of replacing the TP SAF by a single magnetic layer realizing a p-DMTJ structure as depicted in Fig.1.(c). The structure proposed comprises a SAF based BP acting also as a fixed reference layer, a composite SL and the top free polarizer called assistance layer (ASL). The ASL is designed to have an out-of-plane anisotropy and a coercivity lower than that of the SL. In standby, the magnetic coupling between the SL and the ASL sets the magnetization of these two layers in parallel configuration, reinforcing the Δ of the SL. During write, the ASL acts as a polarizer inducing an additional STT, thus reducing the Ic. To investigate this concept, we fabricated, characterized and modeled these devices to understand the ASL working conditions. We demonstrate an increase by a factor of 5 in the figure of merit Δ/Ic of these structures compared to conventional MTJ stack for STT-MRAM with double barriers and a non-magnetic ASL Fig.2.References: [1] Daniel C. Worledge,IEEE MAGNETICS LETTERS, Volume 8 (2017) [2] G. Hu, J. H. Lee, J. J. Nowak, IEEE International Electron Devices Meeting (IEDM), (2015) [3] P.-Y. Clément, C. Baraduc, M. Chshiev, B. Diény,IEEE 6th International Memory Workshop (IMW), (2014) [4] K. Nishioka, H. Honjo, S. Ikeda,Symposium on VLSI Technology Digest of Technical Papers, (2019)

Simplified Double Magnetic Tunnel Junction with switchable Assistance layer for improved STT-MRAM performance

**Session chair:** Denys Makarov, Helmholtz-Zentrum Dresden-Rossendorf


GOL - MRAM, Magnetic Logic and Related Devices II

technical paper

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O2: MRAM and Magnetic Tunnel Junctions

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Real time investigation of Double magnetic tunnel junction with switchable assistance layer for high efficiency STT-MRAM

Simplified Double Magnetic Tunnel Junction with switchable Assistance layer for improved STT-MRAM performance

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