16 March 2009—Last May, researchers at Hewlett-Packard stunned the electronics world with the demonstration of a fourth fundamental circuit element to add to the classical three. The memory resistor, or memristor, joined the resistor, the capacitor, and the inductor, closing a theoretical gap in the physics of electronic circuits. Now other researchers have created new types of memristors that rely on the magnetic properties of electrons, potentially leading to entirely new kinds of circuits that should be easy to integrate with existing electronics.
Current flowing through a memristor can alter its electrical resistance, and it retains that altered state even after the current is turned off, making it a natural for nonvolatile memory. The memristor promises the introduction of much tinier circuits, instant-on computers, and the ability to mimic the function of neurons in the human brain.
Shortly after the demonstration of memristance, researchers began looking for this property in spintronics—a relatively new branch of electronics itself. Spintronics is at the heart of recent advances in hard-drive data density and the niche nonvolatile memory known as MRAM. Whereas electronics works by manipulating the movement of electrons, spintronics works by manipulating a quantum mechanical property of electrons known as spin. (Imagine the electron as a spinning ball.) Spin is the property that is responsible for magnetism—materials are magnetized when a majority of their electrons have their spins pointing in the same direction. Melding memristors and spintronics yields devices whose resistance changes according to the spin of electrons passing through it, and those devices will remember that resistance.
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.