While conventional charge-based memory is approaching fundamental scaling limits, several so-called “emerging memories” have migrated from laboratory samples to integrated products. Among various emerging memory technologies, MRAM (magnetoresistive random access memory) has been making impressive progress, ahead of other emerging memories, and has demonstrated the capability to be a successor to DRAM or SRAM. MRAM data is stored via magnetic moments. Parallel or anti-parallel magnetic moments in MRAM stacks present the “0” or “1” state. In earlier generations of MRAM, these states were switched by current-induced magnetic field but that is an obstacle for scaling.

The invention of ST (spin-torque) MRAM, which is switched by injecting spin-polarized tunneling current, removes the scaling limitation. In an invited paper at the International Electron Devices Meeting, researchers from Everspin Technologies will describe how they built the largest functional ST-MRAM circuit ever built, a 64-Mb device with good electrical characteristics. The work shows that MRAM technology is fast approaching commercialization.

Everspin MRAM products employ a one transistor, one magnetic tunnel junction (MTJ) memory cell for the storage element. The MTJ is composed of a fixed magnetic layer, a thin dielectric tunnel barrier and a free magnetic layer. When a bias is applied to the MTJ, electrons that are spin polarized by the magnetic layers traverse the dielectric barrier through a process known as tunneling. The MTJ device has a low resistance when the magnetic moment of the free layer is parallel to the fixed layer and a high resistance when the free layer moment is oriented anti-parallel to the fixed layer moment.

ST-MRAM uses an alternate method for programming an MTJ element that has the potential to further simplify the MRAM cell and reduce write power. Programming is accomplished by driving current directly through the MTJ to change the direction of polarization. The read operation is accomplished by sensing the MTJ resistance, just like Toggle MRAM.

Everpsin says that ST-MRAM products will offer a new storage class memory solution for non-volatile buffers and caching applications as well as deliver a new nanosecond-class, gigabyte-per-second non-volatile storage tier. Using a spin-polarized current for switching, ST-MRAM can overcome scaling limitations to address persistent DRAM applications in densities from megabits to gigabits.

The device has a wide separation between applied and breakdown voltages— the more separation, the wider the device operation margin.

This “shmoo plot,” is a graphical display of the ST-MRAM’s performance over a range of voltages. The green area signifies there were no failures of the memory as voltages increased, indicating that its design is robust.