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

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.

6 pages, 6 figures

References in corpus (2)

• CoFeB Thickness Dependence of Thermal Stability Factor in CoFeB/MgO Perpendicular Magnetic Tunnel Junctions
• Thermal stability for domain wall mediated magnetization reversal in perpendicular STT MRAM cells with W insertion layers

Cited by in corpus (3)

• Thermal stability for domain wall mediated magnetization reversal in perpendicular STT MRAM cells with W insertion layers
• Non-uniform Superlattice Magnetic Tunnel Junctions
• Multiscale approach for modeling magnetization properties of inhomogeneous ultrathin magnetic layers