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DESCRIPTION Unlike conventional RAM chip technologies, data is not stored as Electric Charge or current flows, but by Magnetic storage elements. The elements are formed from two Ferromagnetic plates, each of which can hold a magnetic field, separated by a thin insulating layer. One of the two plates is a permanent magnet set to a particular polarity, the other's field will change to match that of an external field. A memory device is built from a grid of such "cells". Data is written to the cells by creating an induced magnetic field in a grid of write lines above and below the cells. To write to a particular cell, one X and one Y write line is charged, and a transistor lying on one of the two lines switches the current on at that point. The resulting current creates an Induced Magnetic Field , which flips the polarity of the "writable" plate to match the induced field. To improve speed, writing is done in parallel by holding one X (or Y) line "high" and then powering all the Y lines along that row that need to be flipped. This pattern of operation is similar to Core Memory , a system commonly used in the 1960s. Reading is handled by yet another grid of wires passing through the insulators. Due to the Magnetic Tunnel Effect , the Electrical Resistance of the insulator between the plates is changed by the external magnetic field, in this case the field being held in the two plates in the cell. By measuring the resistance along the read lines, the state of any particular cell can be determined. Typically if the two plates have the same polarity this is considered to mean "0", while if the two plates are of opposite polarity the resistance will be higher and this means "1". There are two technologies for actually creating and holding the magnetic field. COMPARISON WITH OTHER SYSTEMS The main determinant of a memory system's cost is the density of the components used to make it up. DRAM uses small Capacitor s as a memory element with a transistor to control it. Capacitors basically consist of two small metal plates separated by a thin insulator, basically a single element that can be built as small as the current fabrication techology allows. This makes DRAM the highest density RAM currently available, and thus the least expensive, which is why it is the majority of RAM found in a computer. MRAM is physically similar to DRAM in makeup, consisting of metal plates and insulators, and could eventually be produced in higher densities, for roughly the same cost. MRAM has major advantages over DRAM, however, because it does not require power to "refresh" the memory, and thus has much lower power consumption (up to 99% less). Additionally MRAM is considerably faster than DRAM, largely due to the much lower current needed to store a bit into the cells. Another type of memory used in modern computers is Static RAM , or SRAM. SRAM is made of a series of Transistor s forming a Flip-flop , a circuit that holds one state or another. For this reason each bit stored in an SRAM is much larger than a bit in a DRAM made using the same fabrication technology, and it is thus much more expensive. Currently SRAM is typically used only for small amounts of high-speed memory like the CPU Cache , backed up by a much larger amount of DRAM as the computer's main memory. IBM researchers have demonstrated that MRAM can be six times faster than the industry standard's dynamic RAM (DRAM), and it is almost as fast as today's static RAM (SRAM). This is due to the much lower energy needed to "flip" the cells, as well as the lack of a "settling time" for the transistors. Moreover MRAM is higher density than SRAM, and thus lower cost. This combination of features makes MRAM particularly interesting for computer designers. The most common type of nonvolatile memory used today is Flash Memory , which can only be erased and rewritten in fairly large sectors (typically at least 256 bytes, but sometimes 64 kbytes or larger). Reading from flash memory is generally somewhat slower than either SRAM or DRAM, and erasing and writing flash memory is much slower (on the order of milliseconds). Flash memory also has limited write endurance, since the storage cells degrade each time it is erased and rewritten. MRAM is also non-volatile, meaning it can be used to replace Flash. Unlike Flash, MRAM does not degrade during writing, nor is it slower to write than to read. As the technology is developed it is expected to take over Flash roles first. With increasing densities, it appears possible that MRAM can take over from hard drives as well, producing machines that will turn on almost instantly. MRAM can therefore replace every type of memory currently being used. It has similar speeds to SRAM, similar density but much lower power than DRAM, and much faster and suffers no degradation over time as in Flash. It is this combination of features that some suggest make it the "universal memory", able to replace SRAM, DRAM and EEPROM and Flash. This also explains the huge amount of research being carried out into developing it. CURRENT STATUS In the summer of 2003 , a 128 kbit MRAM Chip was introduced, which was manufactured with 0.18 Micrometer technology. In June of 2004 , Infineon unveiled a 16-Mbit Prototype based on 0.18 micrometer once again. Honeywell International Inc. announced commercial radiation hardened 1 Mbit MRAM using 0.15 micrometer technology for use in aerospace and military systems in June 2005 . Freescale Semiconductors Inc., in December 2005, revealed MRAM that uses magnesium oxide, rather than an aluminum oxide, allowing for a thinner insulating tunnel barrier and improved bit resist during the write cycle, thereby reducing the required write current. Also in December of 2005, Sony Corporation announced the first lab-produced spin-torque-transfer MRAM, which utilizes a spin-polarized current through the tunneling Magnetoresistance layer to write data. This method consumes less power and is more Scalable than conventional MRAM. With further advances in materials, this process should allow for densities higher than those possible in DRAM . In February of 2006 , Toshiba and NEC announced a 16 megabit MRAM chip with a new "power-forking" design. It achieves a transfer rate of 200 MB/s, with a 34 ns cycle time - the best performance of any MRAM chip. It also boasts the smallest physical size in its class -- 78.5 square millimeters -- and a low power requirement of 1.8 volts. Proposed uses for MRAM include devices such as:
SEE ALSO EXTERNAL LINKS
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