A highly thermally stable sub-20 nm magnetic random-access memory based on perpendicular shape anisotropy

Abstract

A new approach to increase the downsize scalability of perpendicular STT-MRAM is presented. It consists of significantly increasing the thickness of the storage layer in out-of-plane magnetized tunnel junctions (pMTJ) as compared to conventional pMTJ in order to induce a perpendicular shape anisotropy (PSA) in this layer. This PSA is obtained by depositing a thick ferromagnetic (FM) layer on top of an MgO/FeCoB based magnetic tunnel junction (MTJ) so that the thickness of the storage layer is of the order of or larger than the diameter of the MTJ pillar. In contrast to conventional spin transfer torque magnetic random access memory (STT-MRAM) wherein the demagnetizing energy opposes the interfacial perpendicular magnetic anisotropy (iPMA), in these novel memory cells, both PSA and iPMA contributions favor the out-of-plane orientation of the storage layer magnetization. Using thicker storage layers in these PSA-STT-MRAMs has several advantages. Due to the PSA, very high and easily tunable thermal stability factors can be achieved, even down to sub-10 nm diameters. Moreover, a low damping material can be used for the thick FM material thus leading to a reduction of the write current. The paper describes this new PSA-STT-MRAM concept, practical realization of such memory arrays, magnetic characterization demonstrating thermal stability factor above 200 for MTJs as small as 8 nm in diameter and possibility to maintain the thermal stability factor above 60 down to 4 nm diameter.

Impact of Dzyaloshinskii-Moriya interactions on the thermal stability factor of heavy metal/magnetic metal/oxide based nano-pillars

“…The evolution of the intermediate states and their interpolation have been repeated until the maximum relative energy error on the last interpolated MEP path is less than a user-selected numerical tolerance (in this work, 10 -5 ). The above described procedure has been successfully applied to analyse the thermal stability factor of perpendicular shape anisotropy STT-MRAM cells37 .Our samples are square dots of Pt/Co/AlOx with a Co layer of 0.6nm and various lateral sizes. The following parameters have been used for Co38 : saturation magnetization Ms=1.09 kA/m, uniaxial anisotropy constant Ku=1.25e6 J/m 3 , exchange stiffness Aex=10pJ/m and Gilbert damping parameter α=0.5.…”

From MTJ Device to Hybrid CMOS/MTJ Circuits: A Review

“…Reducing D after 30 nm degrades Δ and limit its use, since, for nonvolatilty, Δ for the FL needs to be more than 40 [121]. Shape anisotropy MTJ is the recent development [349,350] to overcome this scaling issue while maintaining Δ. Here the magnetization switching is solely achieved by current without changing the material for the device.…”

Magnetic Nanowires and Nanotubes

“…This was made possible by the move of the flash industry to stack up to tens of actives layers for storage nowadays, which is the reason why we all have flash sticks and disks in our devices. It is interesting to see that the MRAM industry is sneaking into 3D to push forward the retention of cells at dimensions below 20 nm (Watanabe et al 2018, Perrissin et al 2018). Thus, we should aim at demonstrating functions of wires and tubes for reading, writing, sensing, computing, to remain competitive on the long run.…”