Control of offset field and pinning stability in perpendicular magnetic tunnelling junctions with synthetic antiferromagnetic coupling multilayer

Abstract

In a magnetic tunnelling junction (MTJ) with perpendicular magnetic anisotropy (PMA), offset field (Ho) of the free layer is usually controlled by using a synthetic antiferromagnetic (SAF) coupling structure, which is composed of an antiferromagnetic coupling (AFC) layer sandwiched by two ferromagnetic (FM) layers. However, Ho increases significantly as the size of MTJ devices shrinks to accommodate high density. In addition, magnetostatic field in PMA SAF structure tends to destabilize the antiferromagnetic (AFM) alignment of the SAF layers, in contrast to the in-plane anisotropy SAF, where the closed flux forms stable AFM magnetic configuration. Here, we present a double SAF structure to control Ho, while maintaining high magnetic stability of the reference layer (RL). The double SAF consists of FM1/AFC/FM2/AFC/FM3 multilayer. An AFM layer like PtMn is added to further stabilize the magnetic configuration of the double SAF. As the magnetization of other FM layers (FM1 and FM2) is aligned oppositely, the magnetostatic field acting on the RL (FM3) layer is significantly reduced due to cancellation effect from its adjacent layers. Both simulation and experimental results demonstrate that the double SAF layers provide high stability for the RL in addition to the reduction of Ho. Our results on MTJ devices show that the AFM pinned double SAF has the highest RL stability. The RL switch rate decreases as the thickness of the CoFe inserted layer between AFM and the pinned layer (Co/Pt multilayer) increases due to improved exchange coupling.

Strong interlayer exchange coupling and high post-annealing stability in perpendicularly magnetized [Pt/Co]/Ru/[Co/Pt] structures

“…Stray fields in the UL that are generated in adjacent magnetic layers in both the pinned and free structures as well as those in the FL that are generated in the pinned structure, can increase dramatically as the cell size enters the nanoscale range. [3][4][5][6][7] This can make the UL unstable, thus causing back-hopping write failures during current-induced magnetization switching, as well as occurrence of cross-talk in MTJ cell arrays. [5][6][7] Several structures were proposed for overcoming this drawback, such as stepped-edge MTJs, 3 counter-bias-magnetic-field layers, 4 and double SyFs.…”

“…[3][4][5][6][7] This can make the UL unstable, thus causing back-hopping write failures during current-induced magnetization switching, as well as occurrence of cross-talk in MTJ cell arrays. [5][6][7] Several structures were proposed for overcoming this drawback, such as stepped-edge MTJs, 3 counter-bias-magnetic-field layers, 4 and double SyFs. 7 These structures, however, increase the fabrication complexity by requiring additional stages such as etching or the insertion of magnetic layers.…”

“…[5][6][7] Several structures were proposed for overcoming this drawback, such as stepped-edge MTJs, 3 counter-bias-magnetic-field layers, 4 and double SyFs. 7 These structures, however, increase the fabrication complexity by requiring additional stages such as etching or the insertion of magnetic layers. One simple solution can be to stabilize the SyF pinned structure by increasing the intrinsic AP-IEC strength.…”

Impact of Magnetic Coupling and Density on STT-MRAM Performance

“…The deviation of energy barriers along the two switching directions has a significant impact on the retention and the STT-switching characteristics of MTJ devices, as reported in [9][10][11]. In the extreme case where H z s intra exceeds the FL coercivity (H c ), defined as the reverse field needed to drive the magnetization of a ferromagnet to zero, the bistable states will disappear [5].…”

Magnetization Reversal Process Study in Exchange Coupled Synthetic Antiferromagnetic Multilayers and Patterned Structures

“…Synthetic antiferromagnetic (SAF) multilayers [1] show promise as soft underlayers for perpendicular recording media [2, 3], high moment write head material [4], high frequency application [5] and magnetic tunnel junctions [6,7]. SAF multilayers are comprised of two ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the ferromagnetic layers are coupled through antiferromagnetic interlayer exchange.…”