Ultrathin perpendicular free layers for lowering the switching current in STT-MRAM

TL;DR

This paper demonstrates that ultrathin perpendicular free layers in STT-MRAM significantly reduce switching current density while maintaining high thermal stability, enabling more efficient memory devices.

Contribution

The study introduces a novel multilayer structure with ultrathin free layers that lowers switching current density without compromising thermal stability.

Findings

Lowest $J_{c0}$ of 4 MA/cm$^2$ achieved with 1.2 nm free layers.

Thermal stability factor $ riangle_{dw}$ up to 150 for smallest devices.

Ultrathin free layers enhance spin-transfer torque efficiency.

Abstract

The critical current density $J_{c0}$ required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (STT-MRAM) cell is proportional to the product of the damping parameter, saturation magnetization and thickness of the free layer, $\alpha M_S t_F$. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of sub-monolayer thickness, we have reduced $t_F$ while minimizing $\alpha$ and maximizing $M_S$, ultimately leading to lower $J_{c0}$ while maintaining high thermal stability. Bottom-pinned MRAM cells with device diameter in the range of 55-130 nm were fabricated, and $J_{c0}$ is lowest for the thinnest (1.2 nm) FLs, down to 4 MA/cm$^2$ for 65 nm devices, $\sim$30% lower than 1.7 nm FLs. The thermal stability factor $\Delta_{\mathrm{dw}}$, as high as 150 for the smallest device size, was determined using a domain wall reversal model from field switching probability measurements. With high $\Delta_{\mathrm{dw}}$ and lowest $J_{c0}$, the thinnest FLs have the highest spin-transfer torque efficiency.