Reduced Exchange Interactions in Magnetic Tunnel Junction Free Layers with Insertion Layers

TL;DR

This study investigates how inserting tungsten layers affects the exchange interactions and magnetic anisotropy in CoFeB free layers of magnetic tunnel junctions, revealing a trade-off crucial for device optimization.

Contribution

It provides the first detailed measurement of exchange constants in CoFeB layers with W insertions across a broad temperature range, showing significant dependence on W thickness.

Findings

Exchange constant decreases sharply with W layer thickness.

Perpendicular magnetic anisotropy peaks at about 0.3 nm W.

Results inform optimization of magnetic tunnel junction performance.

Abstract

Perpendicularly magnetized CoFeB layers with ultra-thin non-magnetic insertion layers are very widely used as the electrodes in magnetic tunnel junctions for spin transfer magnetic random access memory devices. Exchange interactions play a critical role in determining the thermal stability of magnetic states in such devices and their spin torque switching efficiency. Here the exchange constant of free layers incorporated in full magnetic tunnel junction layer stacks, specifically CoFeB free layers with W insertion layers is determined by magnetization measurements in a broad temperature range. A significant finding is that the exchange constant decreases significantly and abruptly with W insertion layer thickness. The perpendicular magnetic anisotropy shows the opposite trend; it initially increases with W insertion layer thickness and shows a broad maximum for approximately one monolayer (0.3 nm) of W. These results highlight the interdependencies of magnetic characteristics required to optimize the performance of magnetic tunnel junction devices.