A variety of magnetic memory devices, including MRAM, spin-torque RAM, plated wire, and bubbles, either are or have been under investigation as candidate "universal memory" devices. Such devices would combine the performance attributes of DRAM with the nonvolatility of a disk drive. This paper examines the past and present magnetic memories from two perspectives: (1) processing and extendability to smaller dimensions, and (2) the memory hierarchy of the applications to which they are directed. From a processing requirement, magnetic memories must converge to the cell sizes of competitive memory technologies, which range from 8F~2 for DRAM to 0.5F~2 for disk drives, where F is the minimum processing feature. From a memory hierarchy perspective, magnetic memories must balance the advantages of nonvolatility with the market demands for performance and low cost, and must address the application shifts away from classical enterprise storage hierarchy to the now rapidly developing mobile storage needs. The success of magnetic memories will depend on their ability to fulfill the evolving memory hierarchy requirements of a large set of new and nontraditional applications and on their ability to approach the device sizes of competing DRAM and flash devices.
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