In Situ TEM and Electron Holography Investigation of the Perpendicular Shape Anisotropy and Thermal Stability of STT-MRAM Nano-Pillars

Magnetic random-access memory (MRAM) is a non-volatile memory based on the storage of one bit of information by a ferromagnetic memory carrier. Spin-transfer torque (STT) MRAM involves the use of a magnetic tunnel junction (MTJ) comprising an MgO tunnel barrier sandwiched between a magnetically-pinned reference layer and a switchable storage layer. The electrical resistance of the MTJ changes significantly when the layers are magnetized in parallel and antiparallel states, providing a system of readable / writable ‘0’ or ‘1’ binary information. Increasing the areal bit density of STT-MRAM requires a decrease in the in-plane size or diameter of the nano-patterned MTJ, but reduces the thermal stability of the ultrathin storage layer magnetization. One solution is to increase the storage layer thickness to larger than its diameter, so that its out-of-plane aspect ratio provides additional thermal stability through perpendicular shape anisotropy (PSA). However, our knowledge of the thermal stability of these STT-MRAM nano-pillars is often indirect, relying on magnetoresistance measurements and micromagnetic modelling. In order to understand fully the effect of PSA and its associated thermomagnetic behavior, it is necessary to examine the effect of temperature directly.<br/><br/>The advanced transmission electron microscopy (TEM) technique of off-axis electron holography allows imaging of magnetization within nano-scale materials. Here, we use electron holography to image the micromagnetic configuration of the nano-pillars in presence of PSA and to visualize its thermal stability through <i>in-situ </i>heating. Magnetic induction maps display the micromagnetic configuration of FeCoB / NiFe PSA-STT-MRAM nano-pillars, confirming the PSA induced by the 3:1 aspect ratio of the NiFe storage layer (~ 60 nm high, ≤ 20 nm diameter). <i>In-situ</i> heating demonstrates that the PSA of the FeCoB/NiFe composite storage layer is maintained up to at least 250°C and is coupled with consistent direct measurements of magnetic induction. The magnetic states in a closely-packed array of inverted Co nano-pillars are also imaged, demonstrating they exhibit PSA after application of magnetic fields in directions perpendicular to their elongated axis. However, the interactions between the nano-pillars act to restrict their magnetic states from relaxing in the same direction. This study shows explicitly that PSA provides significant stability in STT-MRAM applications that require reliable performance over a range of operating temperatures.