March 21, 2009

Spintronic Versions of the Memristor

Yiran Chen and Xiaobin Wang, researchers at disk-drive manufacturer Seagate Technology, in Bloomington, Minn., described three examples of possible magnetic memristors this month in IEEE Electron Device Letters. In one of the three, resistance is caused by the spin of electrons in one section of the device pointing in a different direction than those in another section, creating a “domain wall,” a boundary between the two states. Electrons flowing into the device have a certain spin, which alters the magnetization state of the device. Changing the magnetization, in turn, moves the domain wall and changes the device’s resistance.

The different designs can be flipped between high- and low-resistance states at different rates, from picoseconds to microseconds, each preferable in different applications. For reading a hard drive, for instance, you’d want to sense changes in a magnetic field in a few picoseconds, whereas for something like a radiation sensor, you’d want a response time measured in microseconds.

And the devices are all relatively easy to construct. “We can easily integrate a magnetic device on top of a CMOS device,” says Chen.

Wang believes that a spin memristor can be more finely tuned and is more flexible than the device HP described, which was based on the movement of ions in a material. “It’s more broad in our opinion, more controllable,” he says. In part, that’s because of the variable switching rates, but it’s also because spin is not a binary condition—neither up nor down but rather existing along a continuum. So a device doesn’t need to make a complete change from magnetized to nonmagnetized to register a change in the resistance.

Stanley Williams, the HP resear who first described a memristor last May, says he’s glad to see other researchers getting into the area. “I am delighted that people are now playing the game of finding different physical representations of memristance,” he says. “In general, I think linking memristance and other phenomena such as spin transport is a very excellent path forward to putting a lot of functionality into a small package. The thing that really differentiates a memristor is the fact that it has and remembers a state. That is tremendously powerful for a passive device, and the implications of that have barely been explored.”

A University of Michigan electrical engineer has built a chip composed of nanoscale memristors that can store up to 1 kilobit of information.

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