I. Introduction
Recently, THE spin-transfer-torque random access memory (STT-RAM) has been regarded as a promising substitute for conventional memory such as Flash memory, SRAM, and DRAM owing to its fast writing speed, compact bit-cell structure, great scalability, and non-volatility [1]–[8]. As the CMOS technology scales down to the 22-nm node, conventional memories are confronted with scalability problem [9], [10]. Thus, the STT-RAM with great scalability is widely researched as a next-generation memory.