Flipping Bits With Spinning Electrons

MINNEAPOLIS--Researchers have found a new way to flip the direction of minuscule magnets by running electrical currents through them. The technique, reported here Tuesday at the March meeting of the American Physical Society, could usher in smaller, more efficient memory for the computers of tomorrow.

Your computer is forgetful: Turn it off and all the information stored as millions of tiny voltages in its silicon-chip RAM memory vanishes as the electricity drains away. So materials scientists long to replace this system with devices consisting of tiny magnetic dots that will preserve the vital 0s and 1s even when the computer is off. The straightforward way to change a 1 to a 0, or vice-versa, would be to flip the direction of a magnet by running a current through a nearby wire to create a magnetic field. In that case, however, the magnetic bits could not be too much smaller and more closely packed than the bits on today's hard drives, or else several dots might flip together. So physicists Bob Buhrman, Jordan Katine, and their colleagues at Cornell University in Ithaca, New York, tried another bit-flipping method, one that exploits the fact that electrons spin like tops and act like little magnets.

The researchers made microchips decorated with small pillars of magnetic metal just 50 nanometers high and roughly 100 nanometers wide. In each pillar, a layer of copper was sandwiched between two layers of cobalt, a thin one on top and a thicker layer on the bottom. To write a bit, the researchers drew electrons through a pillar. When the current ran from bottom to top, only electrons whose magnetic orientation matched that of the lower layer passed into the thin upper layer. Because magnets generally try to line up with their poles pointing in the same direction, the electrons from below caused the thin upper layer to align itself magnetically with the lower layer, too. When the researchers ran the current from top to bottom, however, the effect reversed: The thin layer's magnetization flipped to oppose that of the thicker, immovable layer.

Later, to read the bit, the researchers passed a smaller current through the pillar and measured its resistance, which is higher when the magnetizations of the cobalt layers oppose each other.

A miniature colonnade of millions of such pillars would make an especially dense and energy-efficient memory chip, says Theodore Zhou, a physicist with Honeywell International in Minneapolis. Several hurdles remain: The change in resistance is small, making it technically difficult to tell the difference between a 0 and a 1, and the bits may not stay in one stat indefinitely. Still, "with this scientific breakthrough," says Zhou, "there's going to be a lot of engineering interest" in overcoming these obstacles.

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