MRAM
Categories: Computer memory | Non-volatile memory
Magnetoresistive Random Access Memory (MRAM) is a non-volatile computer memory (NVRAM) technology, which has been in development since the 1990s. Continued increases in density of existing memory technologies, notably Flash RAM and DRAM, have kept MRAM out of the market to date, but proponents believe that the advantages are so overwhelming that MRAM will eventually become widespread. More recently many of the companies working on MRAM have scaled back their efforts, as it appears the system has problems scaling down in size.
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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 capacitors 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 be produced in roughly the same density, 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. 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 transistors 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.
Current high-speed MRAM prototypes are about six times as fast as the best available SRAMs. 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 particularily interesting for computer designers.
The third common type of RAM used today is Flash RAM. Flash is similar to SRAM in construction, but adds a high-performance capacitor to hold the signal when power is removed. This means it has slighly lower density than SRAM, and has slightly higher costs. The capacitor makes Flash "non-volatile", similar to a hard disk for storage, maintaining its memory even when power is removed. Flash is used in a number of roles where small size and low power requirements make a hard drive too difficult to use, such as in digital cameras and MP3 players.
One problem with Flash is that in order to write to memory a high-power pulse must be passed through the capacitor to discharge it. This pulse degrades the capacitor slightly, and after a number of such writes the memory stops working properly. Generally devices last between 10,000 and 1,000,000 cycles, depending on the technology used. Writing is also quite slow as a result of this high power need.
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 is faster than SRAM, similar density but much lower power than DRAM, and much faster and suffers no degredation 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.
However it also appears that MRAM prototypes are already reaching their maximum theoretical densities, and that this density is much lower than DRAM. It appears that MRAM may still play a major role as a Flash and SRAM replacement, where it can complete in performance, but it may never be the "universal memory" the pundits first predicted.
Proposed uses for MRAM are in devices such as:
- aerospace and military systems
- Digital cameras
- Notebooks
- Smart cards
- Mobile telephones