Magnetoresistive Random Access Memory (MRAM) is a non-volatile computer memoryNVRAM) technology, which has been under development since the 1990s. Continued increases in density of existing memory technologies -- notably Flash RAM and DRAM -- kept MRAM in a niche role in the market, but its proponents believe that the advantages are so overwhelming that MRAM will eventually become dominant.
MRAM, the new breed of semiconductor memory uses magnetic properties to store data. This new kind of chip will compete with other established forms of semiconductor memories such as Flash memory and random access memory (RAM). Most engineers believe, that the technology called magnetoresistive random-access memory(MRAM) could reduce the cost and power consumption of electronics for cell phones, music players, laptops and servers.
The feature, that makes MRAM an alluring alternative to other forms of semiconductor memories, is the way it stores data. For example, flash memory and random-access memory(RAM) hold information as electric charge. In contrast, MRAM uses the magnetic orientation of electrons to represent bits.Using MRAM, reading and writing of data can be done unlimitedly with in nanaoseconds. MRAM can also hold the data with out a power supply.
Unlike conventional RAM chip technologies, in MRAM data is not stored as electric charge or current flows, but by magnetic storage elements. The elements are formed from two ferromagnetic
Data is written to the cells using a variety of means. In the simplest, each cell lies between a pair of write lines arranged at right angles to each other, above and below the cell. When current is passed through them, an induced magnetic field is created at the junction, which the writable plate picks up. This pattern of operation is similar to core memory, a system commonly used in the 1960s. This approach requires a fairly substantial current to generate the field, however, which makes it less interesting for low-power uses, one of MRAM's primary disadvantages. Additionally, as the device is scaled down in size, there comes a time when the induced field overlaps adjacent cells over a small area, leading to potential false writes. This problem, the half-select (or write disturb) problem, appears to set a fairly large size for this type of cell. One experimental solution to this problem was to use circular domains written and read using the giant magnetoresistive effect, but it appears this line of research is no longer active.
Another approach, the toggle mode, uses a multi-step write with a modified multi-layer cell. The cell is modified to contain an "artificial antiferromagnet" where the magnetic orientation alternates back and forth across the surface, with both the pinned and free layers consisting of multi-layer stacks isolated by a thin "coupling layer". The resulting layers have only two stable states, which can be toggled from one to the other by timing the write current in the two lines so one is slightly delayed, thereby "rotating" the field. Any voltage less than the full write level actually increases its resistance to flipping. That means that other cells located along one of the write lines will not suffer from the half-select problem, allowing for smaller cell sizes.
A newer technique, spin-torque-transfer (STT) or Spin Transfer Switching, uses spin-aligned ("polarized") electrons to directly torque the domains. Specifically, if the electrons flowing into a layer have to change their spin, this will develop a torque that will be transferred to the nearby layer. This lowers the amount of current needed to write the cells, making it about the same as the read process. There are concerns that the "classic" type of MRAM cell will have difficulty at high densities due to the amount of current needed during writes, a problem STT avoids. For this reason, the STT proponents expect the technique to be used for devices of 65 nm and smaller. downside is that, at present, STT needs to switch more current through the control transistor than conventional MRAM, requiring a larger transistor, and the need to maintain the spin coherence. Overall, however, the STT requires much less write current than conventional or toggle MRAM.
