Atomistic simulations of perpendicular shape anisotropy MRAM

The continuous growth in demand for high performance computing necessitates that future random access memory (RAM) demonstrate low power consumption without sacrificing performance or reliability. A strong contender to fill this market is the non-volatile magnetic RAM (MRAM) based on a CoFeB-MgO-CoFeB magnetic tunnel junction. One of the CoFeB layers is pinned (reference layer), while the other CoFeB layer may reverse its magnetisation (free layer). These two allowed magnetic states represent the binary digits. Out of plane MRAM, whereby the diameter of the cell is larger than the CoFeB thickness, boasts a fast read-write time, relatively low power consumption and low bit-error rate. This technology is currently available on the market, but suffers from a loss of stability when the diameter is reduced to <20 nm, thus the memory density of this design is not competitive. An alternative design, is to increase the thickness of the free layer until it is larger than the diameter, thus the shape anisotropy is directed in-plane. This design is known as perpendicular shape anisotropy MRAM (PSA-MRAM) and has the potential for retaining the benefits of the MRAM on the current market while reduced to competitive node sizes.
The magnetic behaviours, stability, and properties of PSA-MRAM at reduced volumes is seldom explored in the current literature. This thesis utilises an atomistic spin model approach to explore the dynamic and equilibrium properties of PSA-MRAM. The use of an atomistic model is noteworthy, as it sits in an ideal position between micromagnetic and ab-initio density functional theory approaches. The continuum approach of the former does not capture finite size, edge or thermal effects, while the latter is not appropriate for the number of atoms that makeup the system.
The results show a change in reversal mechanism as the free layer is reduced in an external field from incoherent domain wall motion to coherent rotation. The shape anisotropy as the free layer is reduced is significant, as are the finite size effects due to the loss of exchange on the surfaces. Further, when subjected to a current density rather than an external field, the reversal mechanism is further complicated by these properties. Additionally, thermal fluctuations are capable of partially driving the reversal mechanism for small PSA-MRAM structures.