Magnetoresistive Random Access Memory: Present and Future

Magnetoresistive random access memory (MRAM) is regarded as a reliable persistent memory technology because of its long data retention and robust endurance. Initial MRAM products utilized toggle mode writing of a balanced synthetic antiferromagnet (SAF) free layer to overcome problems with half-selected bits that challenged traditional Stoner–Wohlfarth switching. With the development of spin transfer torque (STT) switching in perpendicular magnetic tunnel junctions, the capability for scaling MRAM products increased markedly, enabling a 1-Gb device in 2019. Ongoing research will allow scaling to even higher capacities. Compared to traditional memories, STT-MRAM can save power, improve performance, and enhance system data integrity, which supports the growing computing demands for everything from data centers to Internet of Things (IoT) devices. This article provides a review of the technology that enabled present toggle and STT-MRAM products, future STT-MRAM products, and new MRAM technologies beyond STT.

I. Introduction

Magnetoresistive random access memory (MRAM) is a superior form of nonvolatile working memory because of its high speed, robust endurance, and system power savings. A key element in each memory cell of MRAM is its magnetic tunnel junction (MTJ), which consists of a free layer (FL), a tunnel barrier, and a fixed layer. The information, either a 1 or a 0, is stored by the direction of the FL’s magnetization relative to the fixed layer’s magnetization. The FL can retain its magnetization direction without power, making MRAM a sought-after nonvolatile memory option. The data stored in each memory cell is typically read back by utilizing the tunnel magnetoresistance (MR) effect. When the FL’s magnetic moment is in an antiparallel state to the fixed layer moment, the MTJ is in a high resistance state and it is read as a “1.” If the FL is in a parallel state to the fixed layer direction, the MTJ has low resistance and it is read as a “0.” To write the bit, the direction of the FL magnetization must be reversed. There are several reversal mechanisms pertaining to MRAM technology, which are reviewed in this article.