How Spin-transfer Torque MRAMs May Simplify Embedded Systems

May 23, 2022 by Darshil Patel

New magneto-resistive RAMs are said to simplify embedded designs by eliminating the need for batteries or capacitors.

In computational systems, the rise of big data has heightened the demand for non-volatile, energy-efficient data storage.

The most widely used memory elements are flash random access memories (RAMs), dynamic RAMs (DRAMs), and static RAMs (SRAMs). However, another memory technology called magneto-resistive RAM (MRAM) has garnered acclaim in recent years. MRAM, which uses electron spin to store information, is said to combine high capacity and high speed with high power efficiency.

Current computation systems require large high-speed caches for workloads. These computers also need non-volatile memory to prevent memory loss during power failure. To address these two requirements, DRAM, SRAM, and flash specialist Memphis Electronic recently introduced a new spin-transfer torque (STT) MRAM to avoid memory loss in embedded systems in the event of power failure.


MRAMs can simplify embedded designs

MRAMs can simplify embedded designs. Image (modified) courtesy of Memphis Electronic


STT is one subset of MRAM memory technology that is gaining prominence even among industry giants like IBM and Samsung. How exactly does this memory architecture function compared to more established non-volatile memory technologies


MRAMs: An Overview

MRAMs have several other advantages, including the ability to resist radiation, operate at extreme temperatures, and resist tampering. These features make them suitable for a variety of industrial and automotive applications.

The general structure of MRAMs consists of two ferromagnetic plates separated by an insulating layer. One of the plates is a permanent magnet fixed to a particular polarity, while the other plate's polarity changes via an external field to store bits.


Structure of an MRAM cell

Structure of an MRAM cell. Image courtesy of Cyferz [CC BY-SA 3.0]

The bits are stored in the form of electrical resistance of the cell. When the magnetic polarity of both the ferromagnetic layers is the same, the cell resistance is lower than when the polarities are opposite. High or low resistance refers to binary 1 or 0, and the polarity of the free ferromagnetic layer changes by applying a relatively high current. 

The first MRAMs, called toggle-MRAMs (T-MRAMs), used magnetic fields to change the electron spin. T-MRAMs are easier to develop but difficult to scale up. The latest MRAMs use a spin-polarized current to switch the spin of electrons. Spin-transfer torque MRAMs (STT-MRAMs) and spin-orbit torque MRAMs (SOT-MRAMs) are such devices. They are faster, more efficient, and easier to scale up than T-MRAMs.


Spin-polarized, Current-based MRAMs

STT-MRAMs use spin-polarized current for the fixed layer, so the electrons in the layer spin in a particular direction. Some of these electrons tunnel through the dielectric layer to change the magnetic polarity of the free layer, thereby changing the device's resistance.


Diagram of an STT-MRAM

Diagram of an STT-MRAM. Image courtesy of Blocks and Files

STT MRAMs can deliver high speed by using a very high write current. However, this affects the longevity of the cell. SOT-MRAMs overcome this challenge by separating the write and read current paths to raise both speed and endurance. Here, an extra layer is required to set the polarity of the free layer. Even so, these MRAMs present cost and complexity challenges.  


Diagram of a SOT-MRAM

Diagram of an SOT-MRAM. Image courtesy of Blocks and Files



Players in the STT-MRAM Arena

Several companies are now developing STT-MRAMs, including IBM, Samsung, Everspin, Avalanche Technologies, Crocus, and Spin Transfer Technologies. As mentioned, Memphis Electronic also announced its plans to showcase its new STT-MRAMs from Netsol at embedded world 2022. These MRAMs are said to save significant board space and reduce the number of passive components.

The new Netsol MRAM technology integrates an additional magnetic tunnel junction (MTJ) layer on top of the CMOS fabrication process to combine low latency with high speed read and write operations and high endurance. They are introduced in the 28nm technology process with the help of Samsung Foundry.

The new MRAM's target applications include smart meters, tire pressure monitoring systems, drone flight data capturing, ultrasound, and MRI scanners.