Numerical analysis of a novel MTJ stack for high readability and writability

Abstract

Recent advances in non-volatile spin transfer torque (STT)-RAM technology, which stores data by the spin orientation of a soft ferromagnetic material and shows current induced switching, have created interest for its use as embedded memory [1–2]. When a spin-polarized current passes through a mono-domain ferromagnet, the ferromagnet absorbs some of the angular momentum of the electrons. It creates a torque that causes a flip in the direction of magnetization in the ferromagnet. This is used in magnetic tunneling junction (MTJ) based spin torque transfer (STT) RAM cells where a thin insulator (MgO) is sandwiched between a fixed ferromagnetic layer (polarizer) and the free layer (storage node). Depending on the direction of the current flow (perpendicular to these layers in our study), the magnetization of the free layer is switched to a parallel (P: low resistance state) or anti-parallel (AP: high resistance state) state. The study in this abstract is done for the 22nm technology node (temp: 85°C) for a storage area of 2F×F in a 6F2 cell. Typical MTJ stacks employed for use in STTRAMs comprise of ferromagnetic or synthetic Antiferromagnetic (for higher stability) free layers with a single or double MgO barrier (SBFF and DBSAF) (Fig. 1a, b). In the authors' previous work the use of double MgO for better writability and single MgO for better readability has been shown. In this paper, we propose a novel bidirectional MTJ stack (Fig. 1) with antiparallel fixed layers (DBSAF-AP) that inherits the high readability of a single barrier stack with better writability than a double barrier stack by employing an intermediate metastable ferromagnetic state (Fig. 2 — to be described later). By using a self-consistent NEGF (for transport) and LLG (for magnetic dynamics) simulation framework [2,4] we show readability, writability and scalability of the proposed stack.