Various types of memories are necessary in order to meet the requirements of an application. Flash memory is non-volatile and stores application code.
EEPROM is non-volatile and byte access that is well suited to store data and parameters. Unfortunately, these non-volatile memories are slow to write and have limited endurance (unlike volatile memories).
Volatile memories are fast to write, like DRAM, but consume a of lot energy as they need to be refreshed periodically. To decrease power, SRAM was invented but is more expensive and less dense than DRAM.
To resolve these trade-offs, researchers and silicon manufacturers have been looking for a technology that could replace all these memories with only one: universal memory. This memory must be non-volatile, fast access to read and write, byte accessible, unlimited endurance and low cost.
The Magnetoresistive Random Access Memory (MRAM) is in the race to become this memory. The market for MRAM can be split in two subgroups. The Toggle MRAM technology is used to manufacture products called Persistent SRAM. Products are in production from 128Kb to 16Mb. On the roadmap, the Spin Torque MRAM will replace DRAM with density planned from 16Mb to 4Gb.
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Both technologies rely on TMR (Tunnel MagnetoResistance) principle that occurs in a Magnetic Tunnel Junction (MTJ) which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough (typically a few nanometers), electrons can tunnel from one ferromagnet into the other one. Since this process is forbidden in classical physics, the tunnel magnetoresistance is strictly coming from quantum mechanics.
The bottom ferromagnet has a fixed magnet. The top ferromagnet can have its magnetism modified, changing the resistance of the structure. When both ferromagnets have the same magnetization direction, the resistance is low, and when magnetization directions are opposite, the resistance is high.
With the toggle technology, a write is accomplished by magnetic fields from current passing through bit and word lines. With the Spin-Torque technology, a Write is accomplished by spin polarized current passing through the MTJ.
In MRAM, each memory cell has an MTJ and the information is stored as a magnetic polarization and read as a resistance state. Therefore read and write operations are different. A Write applies a magnetic field and a Read applies a bias voltage of around 180mV. Toggle MRAM has excellent temperature range and long term reliability. Hence, it is well suited for applications such as automotive, aerospace, transportation and industrial.
Everspin Technologies offers a large portfolio of MRAMs with density from 128kb to 16Mb, with parallel asynchronous access or SPI interface. A large chunk of devices are qualified AEC-Q100 grade 1 and 3 to meet automotive applications requirements like Engine Control Unit, Airbag, steering control, Odometer, Lighting control or infotainment. Beside, MRAM is a perfect fit in data storage system, like RAID or industrial computing systems for its high endurance, byte access and non-volatility. Up to now, a battery back SRAM has been used that creates maintenance issues due to the limited lifetime of a battery. Medical applications, like dialysis machines, CT scanner ultra sound systems, point of sales applications, utility billing application are also target applications for MRAMs.