Effect of Tantalum spacer thickness and deposition conditions on the properties of MgO/CoFeB/Ta/CoFeB/MgO free layers

TL;DR

This study investigates how Ta spacer thickness and growth conditions affect the magnetic properties and stability of MgO/CoFeB/Ta/CoFeB/MgO free layers in tunnel junctions, optimizing for anisotropy and damping balance.

Contribution

It provides new insights into the effects of spacer thickness and deposition conditions on magnetic anisotropy and damping in MgO/CoFeB-based tunnel junctions.

Findings

Thicker Ta spacers increase anisotropy but also damping.

Optimal junction stability achieved at 22 nm diameter with balanced properties.

Inserting a sacrificial Mg layer reduces interface roughness and linewidth broadening.

Abstract

To get stable perpendicularly magnetized tunnel junctions at small device dimensions, composite free layers that comprise two MgO/FeCoB interfaces as sources of interface anisotropy are generally used. Proper cristallisation and annealing robustness is typically ensured by the insertion of a spacer layer of the early transition metal series within the FeCoB layer. We study the influence of the spacer thickness and growth condition on the switching metrics of tunnel junctions thermally annealed at 400$^\circ$C for the case of 1-4 \r{A} Ta spacers. Thick Ta spacer results in a large anisotropies indicative of a better defined top FeCoB/MgO interface, but this is achieved at the systematic expense of a stronger damping. For the best anisotropy-damping compromise, junctions of diameter 22 nm can still be stable and spin-torque switched. Coercivity and inhomogeneous linewidth broadening, likely arising from roughness at the FeCoB/Ta interface, can be reduced if a sacrificial Mg layer is inserted before the Ta spacer deposition.