# Advances in Emerging Memory Technologies: From Data Storage to Artificial Intelligence
## 3 Overview of Emerging Memory Technologies
### Emerging Memory Technologies
#### 3.1.3 Magnetic Memory  
MRAM (magnetic RAM) is a memory that uses the magnetism of electron spin to provide non-volatility (Figure 4). MRAM stores information in magnetic material integrated with silicon circuitry.  
Toggle MRAM uses a one transistor, one MTJ (magnetic tunnel junction) cell to provide a simple high-density memory. During a read, the pass transistor is activated and data are read by comparing the resistance of the cell to a reference device. During writes, the magnetic field from Write Line 1 and Write Line 2 writes the cell at the intersection of the two lines but does not disturb other cells on either line. Another MRAM technology uses a spin torque transfer property, which is the manipulation of the spin of electrons with a polarizing current, to establish the desired magnetic state of the free layer to program, or write, the bits in the memory array. Spin transfer torque MRAM (STT-MRAM) provides a significant reduction in switching energy compared to toggle MRAM and is highly scalable, enabling higher density memory products.  
The advantage of MRAM is fast switching speed compared to other nonvolatile memories, with \(\sim\)1-10 ns read and write erase times and very good endurance (up to 10\({}^{15}\) cycles).  
One major concern with MRAM is the scalability that is difficult due to the complexity of etching many layers with good conformity. Moreover, MRAM etching generally uses ion beam etching, which is not suitable for extensive scaling. The other generally reported issue is MRAM data retention. However, recent material and stack development allowedsignificantly improved retention performances [83], which can be attributed to the adoption of a dual MgO interface instead of the standard single layer.  
MRAM also suffers from small ON/OFF current ratio compared to the other emerging memories: STT-MRAM has a small window and TMR reduces with temperature [83; 84]. Multilevel is difficult to achieve and MRAM requires a good sense amplifier. One decade ago, magnetic memories were envisaged for SRAM replacement (last level cache) due to their high speed and endurance. However, the retention and reliability of MRAM has been highly improved, opening the range of applications to eDRAM (Samsung [85]), embedded (Samsung [85], GlobalFoundries [86], Intel [87]), Industrial (Everspin [88]), and even automotive (TSMC) applications.  
Intel recently presented significant improvements in MRAM technology [89; 90] for embedded applications. STT-MRAM is also proposed by GF for Embedded, MCU, and IoT applications [86]. Avalanche is shipping perpendicular MRAM for SRAM (manufactured at partner foundries) but also announced the production in 2020 of a 22 nm MRAM for Flash replacement for nonvolatile embedded applications [84]. The limitation of spin memories was retention, but recent achievements by TSMC have shown that this technology can be envisaged for automotive applications in the near future [82].  
Several products already exist in the market, such as the spin transfer torque MRAM for DDR3 and the DDR4 (DRAM) product of Everspin using STT-MRAM, serial peripheral interface (SPI) and parallel interface MRAM (using toggle MRAM technology) from Everspin, and the SPSRAM (a persistent SRAM using STT-MRAM technology with serial peripheral interface) and SPNOR (perpendicular STT-MRAM for embedded Flash and embedded SRAM used in system-on-chips) memories from Avalanche technology.  
In summary, MRAM offers excellent endurance suitable for DRAM and SRAM applications, but has also shown recent increased stability for embedded applications. Stack complexity is generally invoked as a device limitation. The next challenge will be to increase its scalability and capacity.