Memory chips that rely on magnetic polarization have long promised the best of all worlds: a device that's durable, can read and write bits quickly, and doesn't require electricity to retain data. That's why there was great interest when the first commercialized magnetoresistive random access memory chip, or MRAM, hit the market last week.

MRAM combines magnetic technology with standard silicon-based microelectronics. It depends on so-called magnetic tunnel junctions that act like transistors. Data is stored in the form of resistance to current at these junctions. Magnetic polarization doesn't leak away, so data is retained when the power is off. And switching magnetic polarization doesn't involve movement among electrons or atoms, so the mechanism doesn't wear out.

The first MRAM chip, from Freescale Semiconductor, isn't a bargain. A 4-Mbit chip costs $25, compared with commodity integrated-circuit chips that run a few bucks each. MRAMs also are slower. Toshiba last year announced it clocked a dynamic RAM chip at 128 Mbits, for example.

Speed issues aside, MRAM has its supporters. "MRAM, in theory, is the holy grail of memory," says analyst Will Strauss of research firm Forward Concepts. "In essence, MRAM fits the bill for perfect memory."

The most common type of memory chip, volatile dynamic RAM, can't hold memory without being powered up. Neither can another type of fast memory chip, static RAM. Another device, the flash chip, has rewritable memory and holds its content without power. But MRAM reads and writes more quickly than flash. Also, unlike flash, MRAM's endurance seems limitless; it doesn't appear to deteriorate.

Initially, MRAM could be used in printers and computers to log data so the machines can be restored to their previous configuration when they lose power, says Saied Tehrani, Freescale's director of MRAM. Existing technology using static RAM, for example, would require not only the chip but a battery, battery holder, and control unit that regulates the flow of power to the chip.

Freescale will manufacture and license MRAM technology. Honeywell has agreed to a licensing deal that will let it develop MRAM for military and space applications.