Nonvolatile Magnetoresistive Random-Access Memory Based on Magnetic Tunnel Junctions
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Nonvolatile
Magnetoresistive Random-Access Memory Based on Magnetic Tunnel Junctions
G. Grynkewich, J. Åkerman, P. Brown, B. Butcher, R.W. Dave, M. DeHerrera, M. Durlam, B.N. Engel, J. Janesky, S. Pietambaram, N.D. Rizzo, J.M. Slaughter, K. Smith, J.J. Sun, and S.Tehrani Abstract Magnetoresistive random-access memory (MRAM) is a new memory technology that is nearing commercialization. MRAM integrates a magnetic tunnel junction (MTJ) device with standard silicon-based microelectronics, resulting in a combination of qualities not found in other memory technologies. For example, MRAM is nonvolatile, has unlimited read and write endurance, and is capable of high-speed read and write operations. In this article, we will describe the fundamentals of an MTJ-based MRAM as well as recent important technology developments in the areas of magnetic materials and memory cell architecture. In addition, we will compare the present and future capabilities of MRAM to those of existing memory technologies such as static RAM and flash memory. Keywords: magnetic memory, magnetic switching, MRAM, MTJ, magnetic tunnel junctions, nonvolatile memory.
Magnetoresistive random-access memory (MRAM) technology combines a magnetic tunnel junction (MTJ) device with standard silicon-based microelectronics to obtain a combination of qualities not found in any other memory technology. MRAM is a highspeed, nonvolatile memory with unlimited read and write endurance. Because the data is stored in the magnetic state of the bit and is read out by sensing the resistance of the bit, MRAM is fundamentally different from commonly available commercial mem-
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ories, such as dynamic RAM and flash memory, that are based on stored charge. Table I shows a qualitative comparison of MRAM and four other commercial memories. Since only MRAM combines nonvolatility, read/write endurance, speed, and density, it has the potential to displace other memory technologies, particularly in applications that would otherwise require multiple memory types. An analysis of expected density and reduced system complexity shows that MRAM will also be
cost-competitive; overall system costs are reduced by eliminating the need for integrating multiple types of memory. MTJ-based MRAM makes use of quantum mechanical tunneling of spin-polarized electrons through a very thin (15 Å) dielectric.1,2 The thin dielectric, usually AlOx, separates two ferromagnetic layers and is thin enough to allow electrons to tunnel through it. The relative orientation of the magnetization in these two layers determines the resistance of the MTJ structure (Figure 1). MRAM devices are designed to have two stable magnetic states that correspond to high- and low-resistance states. Low resistance occurs when the two layers have the same magnetization orientation; high resistance occurs when the orientations are opposed to each other. The difference in resistance of the two states arises from the tunneling magnetoresistance effect. Tunneling magnetoresistance can be understood from a two-band model in which
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