  Perpendicular shape anisotropy (PSA) and double magnetic tunnel junctions
(DMTJ) offer practical solutions to downscale spin-transfer-torque Magnetic
Random-Access Memory (STT-MRAM) beyond 20 nm technology nodes, whilst retaining
their thermal stability and reducing critical currents applied. However, as
these modern devices become smaller and three-dimensionally (3D) complex, our
understanding of their functional magnetic behavior is often indirect, relying
on magnetoresistance measurements and micromagnetic modelling. In this paper,
we review recent work that was performed on these structures using a range of
advanced electron microscopy techniques, focusing on aspects specific to the 3D
and nanoscale nature of such elements. We present the methodology for the
systematic transfer of individual SST-MRAM nano-pillars from large-scale arrays
to image their magnetic configurations directly using off-axis electron
holography. We show that improved phase sensitivity through stacking of
electron holograms can be used to image subtle variations in DMTJs and the
thermal stability of < 20 nm PSA-STT-MRAM nano-pillars during in-situ heating.
The experimental practicalities, benefits and limits of using electron
holography for analysis of MRAM devices are discussed, unlocking practical
pathways for direct imaging of the functional magnetic performance of these
systems with high spatial resolution and sensitivity.
