Scaling and power of thermally written MRAM

Current nonvolatile magnetoresistive random access memory (MRAM) designs that are being pursued by industry for commercial markets are not well suited to military and space missions. They consume a fair amount of power, making them unsuited for long battery-powered missions. They are not designed for radiation environments, requiring weight-restrictive shielding, and are susceptible to upsets caused by stray magnetic fields, which require additional magnetic shielding. This work will focus on designs that are attempting to eliminate these restrictions for military and space applications. Most nonvolatile RAM technologies currently being developed use a transistor as a part of the memory cell. As photo lithography becomes smaller and smaller, this transistor becomes a significant part of the cell size. To keep this device small, on the order of the lithography, it is important to reduce the current through the cell. In addition, as elements are made smaller, the thermal excitation of the elements becomes progressively more important as a design consideration. To make the elements adequately stable, generally the anisotropy must be increased. Increased anisotropy means larger drive fields are required to switch the elements. Larger drive fields in turn require larger current densities in the conductors and this causes an increase in the accompanying heating. Thermal writing or thermally assisted writing exploit the heating that occurs and can be exploited to substantially reduce the drive currents which are required to write a memory element. A comparison is made between MRAM single free layer, toggle bits, Ru coupled free layer and several thermal write bits for scaling, writing, writing temperature and stability.