Direct observation of the perpendicular shape anisotropy and thermal stability of p-STT-MRAM nano-pillars

TL;DR

This study uses off-axis electron holography to directly observe the magnetic configuration and thermal stability of perpendicular shape anisotropy in nano-pillars for STT-MRAM, confirming its robustness up to 250°C.

Contribution

It provides the first direct, spatially resolved magnetic measurements of PSA-STT-MRAM nano-pillars as a function of temperature, validating their thermal stability.

Findings

PSA is confirmed by micromagnetic configuration maps.

PSA remains stable up to at least 250°C.

Magnetic induction decreases moderately with temperature.

Abstract

Perpendicular shape anisotropy (PSA) offers a practical solution to downscale spin-transfer torque Magnetic Random-Access Memory (STT-MRAM) beyond the sub-20 nm technology node whilst retaining thermal stability of the storage layer magnetization. However, our understanding of the thermomagnetic behavior of PSA-STT-MRAM is often indirect, relying on magnetoresistance measurements and micromagnetic modelling. Here, the magnetism of a FeCoB / NiFe PSA-STT-MRAM nano-pillar is investigated using off-axis electron holography, providing spatially resolved magnetic information as a function of temperature, which has been previously inaccessible. Magnetic induction maps reveal the micromagnetic configuration of the NiFe storage layer (60 nm high, 20 nm diameter), confirming the PSA induced by its 3:1 aspect ratio. In-situ heating demonstrates that the PSA of the FeCoB / NiFe composite storage layer is maintained up to at least 250 degrees centigrade, and direct quantitative measurements reveal the very moderate decrease of magnetic induction with temperature. Hence, this study shows explicitly that PSA provides significant stability in STT-MRAM applications that require reliable performance over a range of operating temperatures.