Size dependence of the thermal stability factor in a perpendicular CoFeB/MgO magnetic tunnel junction studied by micromagnetic simulations

Abstract

Size dependence of the thermal stability factor (Δ) for perpendicular CoFeB/MgO-based magnetic tunnel junctions (p-MTJs) was investigated using the string method in the framework of finite element micromagnetics. The simulation revealed that Δ showed significant size dependence, and the reversal mode changed from coherent rotation to domain wall (DW) motion as MTJ size increased. It also elucidated that Δ depended on the exchange stiffness constant in DW mediated reversal region. These Δ simulations can reproduce the experimental data and can be helpful for MTJ device design.

Thermal stability for domain wall mediated magnetization reversal in perpendicular STT MRAM cells with W insertion layers

“…We find w dw ∼ = 11−15 nm for all D and t W values. These are smaller than reported previously for PMA thin films where w dw was measured by magneto-optical Kerr microscopy imaging 20,21 , but are consistent with device-level micromagnetic simulation results 6 . However, unlike ε dw and ∆, w dw does not depend monotonically on t W : it is largest for t W = 1.1 Å, smallest for t W = 1.5 Å, while the values for t W = 2.0 Å and 2.6 Å are in between.…”

“…with E b (H) calculated assuming domain wall-mediated M reversal (DWMR). Micro-magnetic 6 and atomistic 7 simulations suggest that MTJ FL with perpendicular magnetic anisotropy (PMA) can prefer DWMR down to D ≈ 25 nm. In that case, E b,dw = Dε dw t FL where t FL is the FL thickness and ε dw = √ 8M s H k A ex is the DW energy density (M s is saturation magnetization, H k is the net PMA field and A ex is the exchange stiffness constant of the FL).…”

Thermodynamic properties and switching dynamics of perpendicular shape anisotropy MRAM

“…We see that the reduction in the thermal stability factor is in agreement with our previous data, suggesting a significant and non-linear reduction of shape anisotropy as the tower free layer thickness is reduced. The loss of shape anisotropy begins as the free layer is reduced to thickness below 30 nm, with a greater reduction as the thickness approaches the width [5,22]. This is expected, since a free layer of 8 nm is only marginally above the 5 nm width.…”

“…These features are often predicted to make MRAM the universal memory of the future, replacing both DRAM and SRAM on the market. However, for this to be a reality, MRAM needs to be scaled down to a sub-20 nm dimension to compete with the density of DRAM while maintaining a compromise of sufficiently high thermal stability for data retention and a low writing current [2][3][4][5].…”

Spin-Torque-Triggered Magnetization Reversal in Magnetic Tunnel Junctions with Perpendicular Shape Anisotropy

“…This can be understood by considering that, at these small dimensions, reversal of the magnetic volume is almost coherent, and thus, is roughly proportional to the device area. This decrease significantly reduces the retention time of the memory [13][14][15]. A proposal to counter this decrease is to double the IPMA by using two Fe-Co(B)/MgO interfaces [16].…”