Abstract
In order to utilize perpendicular spin-torque-transfer magnetic-random-access-memory (p-STT MRAM) as a storage class memory, the achievement of performing multi-level-cell (MLC) operation is important in increasing the integration density of p-STT MRAM. For a double pinned perpendicular magnetic tunneling junction spin-valve performing MLC (i.e., four-resistance level) operation, the uniformity in the resistance difference between four-level resistances was investigated theoretically and experimentally. The uniformity in the resistance difference between four-level resistances was strongly dependent on the top MgO tunneling-barrier thickness. Particularly, the most uniform resistance difference between four resistance states could be achieved at a critical top-MgO tunneling thickness (i.e., ∼1.15 nm).
Highlights
Perpendicular spin-transfer-torque magnetic-random access memory (p-STT MRAM) has been intensively researched as an embedded memory and a stand-alone memory to replace the current dynamic-random-access-memory (DRAM) because of many advantages such as non-volatile characteristics, low power consumption (10 year), and write and erase endurance (>1016).1–6 In addition, storage class memory (SCM) has been proposed to cover a memory hierarchy between the current DRAM and NAND flash memory, called a three dimensional cross-point memory.7 SCM has 10–100 times faster latency than NAND, while it provides real-time access to data like DRAM at a lower cost
We investigated the dependency of the top MgO tunnelingbarrier thickness on the MLC (4-level resistance states) operation mechanism change and resistance difference uniformity between 4level resistance states, which is closely related to the resistance area [RA(Ω/μm2)] and tunneling magnetic ratio (TMR) of the top MTJ (ΔR = RP ⋅ TMR)
These results indicate that AP state 3 (RAP3) would have the highest resistance, while the P state (RP) would have the lowest resistance
Summary
We have demonstrated a p-STT MRAM with a double pinned pMTJ spin-valve that exhibits 4-level resistance, but the resistance difference was not uniform. We investigated the dependency of the top MgO tunnelingbarrier thickness on the MLC (4-level resistance states) operation mechanism change and resistance difference uniformity between 4level resistance states, which is closely related to the resistance area [RA(Ω/μm2)] and tunneling magnetic ratio (TMR) of the top MTJ (ΔR = RP ⋅ TMR)
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