Understanding STT-MRAM

Nearly all of today's products use one or a combination of charge-based, volatile memories, DRAM and SRAM, and non-volatile memories NOR and NAND flash. These existing memories have significant advantages that led to market dominance over the last 30 years. They also come with drawbacks that cloud their future, since systems consistently need to be faster, smaller, more reliable, and less expensive to compete effectively over the next five to ten years.

There are new disruptive technology challengers coming into the market, specifically non-volatile memories (NVM) such as resistive RAM (RRAM) and phase-change RAM (PCRAM) that promise high performance, low power consumption, and unlimited endurance. Magnetic RAM (MRAM) is one of these emerging technologies.

Note that MRAM is not new—it was first presented at industry conferences more than 25 years ago. It created a lot of excitement back then since it was so different from the charge-based memories. There are now a few different types of MRAM being deployed today, including field-switching and heat-assist MRAM. All have distinguishing characteristics making the technology useful for different kinds of niche applications.

Spin-Transfer Torque (STT) MRAM, however, is well suited for many mainstream applications, particularly as a storage technology, since it delivers the high performance of DRAM and SRAM, has the low power and low cost of flash memory, scales well below 10nm, and leverages existing CMOS manufacturing techniques and processes. Since it is non-volatile, STT-MRAM will also retain its data indefinitely when the power is lost or completely turned off.

Unlike a much-anticipated newcomer RRAM, the basic physics behind MRAM as a storage device is well understood. With MRAM, a memory cell is comprised of a magnetic tunnel junction (MTJ), which has been widely used as read head for hard-disc drives for many years. Early MRAM devices utilised in-plane MTJ (iMTJ) where the magnetic moments (a vector having a magnitude and direction) stay parallel to the substrate silicon surface (figure 1).

Figure 1: In-line MTJ diagram.

There is now another, more-optimised version of MTJ, called perpendicular MTJ (pMTJ), where the magnetic moments are perpendicular to the silicon substrate surface (figure 2).

Figure 2: Perpendicular MTJ diagram.

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