A Systematic Assessment of W-Doped CoFeB Single Free Layers for Low Power STT-MRAM Applications

Abstract

Spin-transfer torque magnetoresistive random access memory (STT-MRAM) technology is considered to be the most promising nonvolatile memory (NVM) solution for high-speed and low power applications. Dual MgO-based composite free layers (FL) have driven the development of STT-MRAMs over the past decade, achieving data retention of 10 years at the cost of higher write power consumption. In addition, the need for tunnel magnetoresistance (TMR)-based read schemes limits the flexibility in materials beyond the typical CoFeB/MgO interfaces. In this study, we propose a novel spacerless FL stack comprised of CoFeB alloyed with heavy metals such as tungsten (W) which allows effective modulation of the magnet properties (M-s, H-k) while retaining compatibility with MgO layers. The addition of W results favours a delayed crystallization process, in turn enabling higher thermal budgets up to 180 min at 400 degrees C. The presence of tungsten reduces the total FL magnetization (M-s) but simultaneously increasing its temperature dependence, thus, enabling a dynamic write current reduction of similar to 15% at 2 ns pulse widths. Reliable operation is demonstrated with a WER of 1 ppm and endurance > 10(10) cycles. These results pave the way for alternative designs of STT-MRAMs for low power electronics.