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Computing Unplugged
By Richard Butner
| |  What happens when the power goes out while you're typing on your computer? Unless you are connected to an uninterruptible power supply, you lose everything you were working on since you last saved the document. That's because your computer's random access memory (RAM), which stores information for fast access, can't function without power. The same goes for your cellphone and PDA. Both require a battery to keep the RAM intact with your phone numbers and personal data. But IBM researchers have developed a new form of RAM — magnetic RAM (MRAM) — that doesn't forget anything when the power goes out.
Unlike conventional RAM, which uses electrical cells to store data, MRAM uses magnetic cells. This method is similar to the way your hard drive stores information. When you remove power from your computer, conventional RAM loses memory, but the data on your hard disk remains intact due to its magnetic orientation, which represents binary information. Because magnetic memory cells maintain their state even when power is removed, MRAM possesses a distinct advantage over electrical cells.
Consider for a moment. . .
Here's how MRAM works. Two small magnetic layers separated by a thin insulating layer make up each memory cell, forming a tiny magnetic "sandwich." Each magnetic layer behaves like a tiny bar magnet, with a north pole and south pole, called a magnetic "moment." The moments of the two magnetic layers can be aligned either parallel (north poles pointing in the same direction) or antiparallel (north poles pointing in opposite directions) to each other. These two states correspond to the binary states — the 1s and 0s — of the memory. The memory writing process aligns the magnetic moments, while the memory reading process detects the alignment.
 Reading data- To read the bit of information stored in this memory cell, you must determine the orientation of the two magnetic moments. Passing a small electric current directly through the memory cell accomplishes this: when the moments are parallel, the resistance of the memory cell is smaller than when the moments are not parallel. Even though there is an insulating layer between the magnetic layers, the insulating layer is so thin that electrons can "tunnel" through it from one magnetic layer to the other.
- Writing data- To write to the device, you pass currents through wires close to (but not connected to) the magnetic cells. Because any current through a wire generates a magnetic field, you can use this field to change the direction of the magnetic moment. The arrangement of the wires and cells is called a cross-point architecture: the magnetic junctions are set up along the intersection points of a grid. Wires — called word lines — run in parallel below the magnetic cells. Another set of wires — called bit lines — runs above the magnetic cells and perpendicular to the set of wires below. Like coordinates on a map, choosing one particular word line and one particular bit line uniquely specifies one of the memory cells. To write to a particular cell (bit), a current is passed through the two independent wires (one above and one below) that intersect at that particular cell. Only the cell at the crosspoint of the two wires sees the magnetic fields from both currents and changes state.
The path of least resistance
Over the past four years, IBM researchers under the direction of IBM fellow Stuart Parkin have enhanced the tunneling process by developing a unique set of materials for the magnetic and insulating layers.
"When we started this program, the types of materials that were available were about 10 million times too resistive to be useful," Parkin says. "We've been able to reduce that resistance." Also, initially the difference in resistance between the parallel and antiparallel states was not large enough to be detected reliably. It turns out that a wide variety of ferromagnetic materials — substances exhibiting magnetic behavior — change resistance depending on the relative alignment of the magnetic moments' two layers. Much of the research has focused on engineering these ferromagnetic materials to have the other needed properties:
- Ability to rotate the magnetic moments using a very small magnetic field
- A smaller overall resistance
- An increased resistance differential between the two states — up from 10 percent to 50 percent. This differential makes distinguishing the two states of memory simple and reliable: you pass a small current through the device and monitor the voltage drop.
Setting the standard
MRAM combines many of the advantages of presently available forms of 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) — a faster, more expensive RAM used in memory caches. MRAM also has the potential to be extremely dense, packing more information into a smaller space. The 1,000-bit prototype is significantly denser than conventional static RAM.
The most important attribute of MRAM is its nonvolatility. In the absence of any electrical power, the magnetic moments maintain their alignment. Thus, the data is kept intact. This feature could enable instant-on computers, because the memory state would be maintained when you turned your computer off.
This instant-on ability doesn't just apply to desktop computers. "The most likely application for MRAM will be in pervasive computing devices," Parkin says. As portable wireless devices become universal, devices such as PDAs and cell phones will require the dense, fast, relatively inexpensive nonvolatile memory that MRAM can provide.
With four years of research completed, IBM is now partnering with Infineon, formerly a part of Siemens, to bring MRAM technology to market. IBM holds patents on both the crosspoint architecture of the magnetic junctions and the knowledge of the materials and physics involved. Infineon brings to the table their expertise in processing and process integration — the ability to integrate the magnetic devices with the conventional circuitry used by standard chips. Now that IBM has demonstrated the viability of MRAM with prototypes, work with Infineon will focus on developing a standard 8-inch wafer process so that MRAM will, in just a few years, roll off the assembly line and into the pervasive computing devices of the future.
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